). 1. Cluster Science no kenkyu
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1 .1
II ............................................................................................................................................... 11
1.1 .ll
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1.1.2 .n1.1.3 ##%#..................................................................................................................................... 12
1.2 .15
1.2.1 .15
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1.2.4 frAf...........................................................................................................17
1.2.5 ##£B..................................................................................................................................... 18
2 ztiztuDftmvmm....................................................................222.1. ................................................... 22
2.1.1 Progress towards resolving the mysteries of the CO and NO dimers................................... 22
2.1.2 FTMW spectroscopy of sodium chloride-water Complexes - A model system
for solvation..................................................................................................................................... 24
2.1.3 Spectroscopic studies of open-shell clusters: Clues to the nature of intermolecular
interactions....................................................................................................................................... 26
2.1.4 Electronic and vibrational spectra of benzene water ion...................................................... 27
2.1.5 Structure and dissociation energetics of van der Waals and hydrogen bonded clusters • • • • 29
2.1.6 Infrared depletion spectroscopy of aniline clusters -The hydrogen bond between the
NH2 and an aromatic ring-.......................................................................................................... 31
— iii —
2.1.7 Structure and dynamics of size-selected benzonitrile-(H20)n and
benzonitrile- (CH30H)n clusters investigated by IR-UV and simulated
Raman-UV double resonance spectroscopies............................................................................33
2.1.8 Solvent effects and chemical reactivity in small molecular clusters......................................34
2.1.9 Coupling of low frequency vibrations with proton transfer.................................................36
2.1.10 Rydberg state of rare gas-NO complexes...........................................................................38
2.1.11 Solvation of single alkali atom in water clusters.................................................................. 39
2.1.12 Laser spectriscopy of silver-ammonia complexes...............................................................40
2.1.13 Competitive coordination in M+(NH3)m(H20)n as studied by laser ablation-
molecular beam method: Experiment and simulation.............................................................. 42
2.1.14 Reactions of cluster ions by FTICR mass spectrometry.....................................................44
2.1.15 Collision induced dissociation of size selected Aluminum cluster.......................................46
2.1.16 Photoelectron spectroscopy of negatively-charged molecular clusters..................................48
2.1.17 High resolution spectroscopy of carbon containg molecules: From 13C to C13................ 50
2.2 .52
2.2.1 Clustering in the gas phase and solution: Preferential solvation of neutral
species and hydrohobic hydration of noble gases.............................................................52
2.2.2 Molecular clusters in electrolyte and non-electrolyte solutions............................................... 54
2.2.3 Process of cluster formation in liquid droplet expansion..........................................................56
2.3 h v .............................. 58
2.3.1 Structure and reactivity of small metal clusters (l-10nm) by HERTEM........................ 58
2.3.2 Stable bimetallic icosahedrons formed by annealing............................................................. 60
2.3.3 Catalytic properties of gold clusters deposited on metal oxides.........................................62
2.3.4 Au mltiply-twinnedparticle micelle.................................................................................... 64
2.3.5 CO interaction with size-selected, supported nickel clusters on thin MGO-film................ 66
2.3.6 Mesoporous silicate synthesis under low temperature and acidic medium condition
and its chemical modification.............................................................................................. 68
— iv —
2.3.7 Dynamics of clusters studied by matrix-isolation infrared spectroscopy combined with
pulsed-nozzle molecular-beam technique...........................................................................69
2.3.8 The "designer" optical resonances of metal nanoshells.......................................................71
2.4 ............................................................. 72
2.4.1 The effect of restraints on the radiation induced chemistry of cation clusters
and liquidclusters in zeolites........................................................................................................ 72
2.4.2 Photo excited state of Agl clusters incorporated into cage of zeolites.................................74
2.4.3 Metal and metal oxide clusters in zeolites: Growth and opto-electronic properties..............76
2.5 ..........................................................78
2.5.1 Studies on structure formation process by molecular dynamics.......................................... 78
2.5.2 Liquid water dynamics: Hydrogen bond rearrangement, phase space dynamics
and proton transfer........................................................................................................................80
2.5.3 Theory of Dullerene: A statistical model for growth............................................................ 81
2.5.4 Fullerene to Nanotube -Their difference and similarity in structure and
growth process-................................................................................................................................ 83
2.6 : ................................................................................... ................. 84
2.6.1 m............................................................................................................................. 84
2.6.2 7 - V (FTMS) ........................................................................ 85
2.6.3 f 86
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hi eaa .................................................................................................... 91
...........................................................................................................................................................................................................................................................................................................................................................................................93
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I Summary of Results
The cluster Science Group is dedicated to studies on clarifying the cluster properties
which play crucial roles in changes of materials status (condensation, crystalization, phase
separation and so on) and in chemical reactions (combustion, reaction in solution, catalysis, etc.)
and on exploiting the effective utilization of newly found properties. In order for materials to
exist stably in isolation, it is essential that more than certain number of atoms and/or molecules are
assembled. In case of cluster where the number of constituent atoms/molecules is limited, the
proportion of atoms and molecules located on the surface becomes greater than otherwise, and
enhanced reactivity may be expected. In fact cluster grows or disintegrates on collision with
other clusters, or undergoes reaction or transformation. The most significant feature of clusters is
their failure of existing in stable isolation. Clusters can exist in following four environments:
• Clusters in collisionless environments (molecular beams)
• Clusters in liquid or solution
• Clusters stabilized on surfaces or in matrices
• Clusters stabilized in a nanocage such as a zeolite.
The objective of this project is to clarify the characteristics of clusters in above
environments experimentally and theoretically for the understanding of various phenomena which
connect atom/molecule and bulk, and to contribute to the progress of materials science by finding
new noble properties of stabilized cluster.
There are two difficulties in cluster studies. One is that clusters are not stably isolable
and the other is that the amount of cluster with definite size is very small because clusters are
produced with broad size distribution. Therefore the development of new techniques are
inevitably required to promote the study on cluster. 1
(1) Clusters in collisionless environments (molecular beams)
The fundamental properties of clusters, structure and reactivity can be studied in
collisionless environments.
— 6 —
(a) Infrared Spectroscopic Studies on Clusters
As the information on the structure of electronic ground state of cluster is obtained in the
infrared spectra, the spectroscopic technique for this region was developed by combining REMPI
(Resonantly Enhanced Muplti-Photon Ionization) and IR double resonance with mass
spectrometric detection by which high sensitivity and mass selectivity are attainable, and clusters
of aniline (C6H5NH2) with various molecules have been studied to understand the interaction
between two molecules, hydrogen bond and other interaction. Not only the neutral clusters but
also cation of clusters can be studied by this technique. The details will be given in this report.
(b) NAIR Terahertz Spectrometer for High Resolution and High Precision
Measurements on Molecular Clusters
Since a highly sensitive terahertz spectrometer is a very important tool for the progress
in the cluster science, especially for determination of structures of clusters and of intermolecular
potentials, we decided to construct a terahertz spectrometer at our institute using a backward wave
oscillator (BWO). The details were described in the report of last year.
(c) Electronic Transition of Metal-molecule clusters
An electronic transition of a AgNH3 complex was observed for the first time. The
AgNH3 complex is produced with laser ablation of silver metal followed by a reaction with
ammonia in a free jet expansion. The spectrum was measured by using two-color resonantly
enhanced multiphoton ionization (REMPI) combined with mass selection by a time of flight mass
spectrometer. The 0-0 band of this transition at 21410 cm"1 is shifted from the corresponding
atomic transition of a silver atom by more than 8000 cm"1, indicating significant stabilization of the
complex in the electronic excited state. It suggests that the electronic excited state of this
complex is chemically bonded to the physical (van der Waals type) interaction in the ground state.
The vibrational progressions in the spectrum were assigned to the intermolecular stretching and
bending mode.
(d) FTICR study on cluster surface chemistry
— 7 —
The reactivity and its size dependence is an another interesting property of clusters to be
investigated. A Fourier transform ion cyclotron resonance mass spectrometer is a good tool with
which cluster chemical reactivity is being probed through by controlled introduction of various
molecular species. This new technique will be described in the section of 2.6 of this report
(2) Clusters in liquid or solution
To study the properties of liquid clusters an expansion liquid droplet source together with
a time of flight mass spectrometer (reflectron mode) was built at the begining of the project.
Direct observation of ionic clusters in electrolyte solution
Recently, measurement of mass spectra for ionic clusters in electrolyte solution succeeded by
means of specially designed electrospray mass spectrometer. When an electrolyte solution is
injected into the high electric field of the first chamber only the positively charged liquid droplets
are led to the downstream owing to the electric field and pressure balance. The positively
charged liquid droplets are exploded through adiabatic expansion and electrostatic repulsion,
which leads to the fragmentation of the liquid droplets into the clusters with single positive charge.
The resulting clusters with positive charge were analyzed by a quadrupole mass spectrometer
without any external ionization.
By this method, the difference of microscopic structure between aqueous nitric acid and
sulfuric acid solution, ion recongnition by crown ethers, solvation and crystallization of salt
solution, etc. were first observed directly.
(3) Clusters stabilized on surfaces or in matrices
Cluster Stabilization for Studying Cluster Properties
The novel stabilization and the structural observation techniques are essentially required
to study the cluster properties. Specially for the structural observation of very small clusters with
1-2 nm in diameter, it becomes very difficult to see their internal structures directly with the
transmission electron microscope (HRTEM) because of the inevitable irradiation effects. To get
out of this difficulty, the low-temperature cluster deposition technique along with the in situ x-ray
diffraction method were developed. The structure of small gold clusters were examined by the
- 8 -
novel apparatus and it was clarified that the gold cluster grows with keeping the decahedral
configuration in the gas phase. The same situation is also expected in the intermetallic clusters.
As a trial, the Au-Cu clusters were investigated with HRTEM. The structures of as-grown
clusters supported on an amorphous carbon film did not show any clear atomic configurations,
while those of clusters annealed under vacuum exhibited the icosahedral structures below 6 nm in
diameter and the bulk face-centered cubic structures above this diameter. The details will be
described in this report.
(4) Clusters stabilized in a nanocage such as a zeolite.
Preparation and optical properties of silver halide clusters in zeolites
Silver halides are very important semiconductor for photographic technology by making
use of their photochromic property. Furthermore, some of them show indirect transition or silver
ion conductivity in solid phase. There are several works on the properties of Agl clusters in
zeolites. In these works, however, it seems that the clusters are not truly formed in zeolite cages.
In the present work, Agl and AgBr clusters are successfully loaded into nano-size cages of LTA.
Optical absorption edge of Agl loaded Na type LTA show large blue shift compared with
the original bulk Agl. In X-ray diffraction pattern of this sample show several new reflection
peaks due to clusters in the cage. These results manifest that in our experiment Agl is loaded into
the cages of LTA as clusters resulting in the space group different from original bulk Agl. No
new functional property have been observed in this sample yet. More detailed works on this
clusters are under examination to understand the property of arrayed clusters by changing loading
density, types of zeolite, etc.
(5) Theoretical studies on clusters.
(a) Molecular dynamics simulation
Water clusters
Negatively charged water clusters formed after trapping an electron show peaks at 2, 6, 8,
etc. (no peaks at 3,4, and 5), which are explained by their larger dipole moment than those of other
size clusters. These peaks could be reproduced by MD simulation by cooling the water vapor and
counting the number of clusters with larger dipole moment than the certain value. TIP4P water
9-
potential was used. Clusters of 3, 4, and 5 water molecules are found as a ring structure
connected by the hydrogen bond showing small dipole moments. This was also confirmed with
path-integral molecular dynamics for a single electron bounded in the potential well produced by
water molecules.
Fcc-icosahedron structural transition of L-J clusters
According to electron diffraction measurement on Ar clusters, the structure of clusters
with more than 1,500 atoms are found as fee as the same as the bulk, and those with less than 1,500
are icosahedron which can not exist as a bulk crystal. MD simulation with evaporation carried by
us showed the transition size as ca. 1,500 and gave very similar structural factors as observation.
The results are much better than previous simulations which gave 3,000 atoms as a transition size
and should consider a special structure to obtain the similar structural factor.
(b) Mechanism for the carbon cluster formation (Statistical stochastic process)
The growth mechanism of fullerenes have been analyzed on the basis of stochastic
process of the formation probability of the 5-membered ring, P, and 6-membered ring, 1-P. The
Isolated Pentagn Rule (EPR) and no preferential growth in the direction were assumed. The
creation probability P(Cn) of fullerene Cn, is obtained as the function of P. The probability of P
was turned out to be large to form fullerenes. To explain the experimentally obtained relative
production ratio of higher fullerenes, however, the mutual interaction between two pentagon
should be considered. This was taken into account by introducing a little more preference (£ )
for the position of two relative pentagon form. The ratio of P(C60) and P(C?o) is set as P(C?o) /
P(Qo)=0.1 as typical one reported by many experiments. This gives a relationship between P and
£as£ = £0.1(P).
The higher fullerene C% has two isomers satisfying IPR; they are Td and D2-isomer. The Td-
isomer is found to be difficult to create as compared with the D2-isomer as shown by experiment.
The ratio P(C84(D2-(22))) / P(C84(D2d-(23))) obtained by the present model is a good
coincidence with the experiment. Furthermore, for C86 fullerene, two (^-symmetry isomers have
been observed by 13C NMR spectrum in the ratio about 4:1 which are identified by the present
theory to be Q>-(2) and CV(5) isomer as shown in the figure.
—10 —
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1) R.H.Page, Y.R.Shen, and Y.T.Lee: J. Chem. Phys. 88 (1988) 4621.
2) N.Mikami: Bull. Chem. Soc. Jpn. 68 (1995) 683; E±SS : fr%m% 46 (1997)
179.
3) : J. Mass Spectrom. Soc. Jpn. 44 (1996) 563.
4) T.Nakanaga, F.Ito, J.Miyawaki, K.Sugawara and H.Taken: Chem. Phys. Lett. 261
(1996) 414 ; K.Sugawara, J.Miyawaki, T.Nakanaga, H.Takeo, G.Lembach, S.Djafari,
H.-D.Barth, and B. Brutschy: J. Phys. Chem. 100 (1996) 17145; R.P.Schmid,
P.K.Chowdhury, J.Miyawaki, F.Ito, K.Sugawara, T.Nakanaga, H.Takeo and H.Jones:
Chem. Phys. 218, (1997) 291.
-12-
aniline monomer4—s—»—1—s—1—'—'—'—t-- \-- N-- 1.-- 1.—H—V-
Ar
aniline-CO
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aniline-ammonia
aniline-trimethylamine
aniline-triethylamine
aniline dimer
aniline-benzenet i 1 i. i. ■ i t
3300 3400
wavenumbers3500
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-13-
1. NH—X aniline - ammoniaaniline - pyrodine aniline - water aniline - TEA aniline - water cation aniline - CO cation
2. NH--N (aniline)
aniline dimer cation
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aniline - dimer aniline - benzene aniline - pyrrole
NH—jt aniline - benzene cationaniline - cation
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aniline - N2 aniline - CO aniline - Ar aniline - CH4
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1) B.Raoult and J.Farges: Rev. Sci. Instrum. 44 (1973) 430;
2) J. Farges, M.F. de Feraudy, B.Raoult and G.Torchet: J. Chem. Phys. 78 5067 (1983):
J.Farges, M. F. de Feraudy, B.Raoult and G.Torchet: J. Chem. Phys. 84 (1986) 3491.
3) T.Ikeshoji and G.Torchet: Gordon Research Conference on Molecular and Ionic
Clusters (1998).
4) S.Iijima and T.Ichihashi: Phys. Rev. Lett. 56 (1986) 616.
5) S.Yatsuya,S.Kasukabe and R.Uyeda: Jpn. J. Appl. Phys. 12(1973) 1675;S.Kasukabe,S.Yatsuya
and R.Uyeda: Jpn. J. Appl. Phys. 1 3 (1974) 1714.
6) J.Harada and K.Ohshima: Surf. Sci. 106 (1981) 51; K.Ohshima and J.Harada: J. Phys: C17
(1984) 1607.
7) K.Koga and H.Takeo: Rev. Sci. Instrum. 67 (1996) 4092.
8) S.Ino: J. Phys. Soc. Jpn. 21 (1966) 346; K.Kimoto and I.Nishida: J. Phys. Soc. Jpn. 22 (1967)
940.
9) D.K.Saha, K.Koga and H.Takeo: NAIR workshop '91 on Cluster Science (1997).
# 30 %© ©#*§£> ft £o Cft j; D f 5VXc##U%^@
—18 —
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—21 —
2 -tft-etboawoMM 2.1. (MQtp)2.1.1PROGRESS TOWARD RESOLVING THE MYSTERIES OF THE CO
AND NO DIMERS
A.R.W. McKellar. M.J. Brookes, and J.K.G. Watson
Steacie Institute for Molecular Sciences, National Research Council of Canada,Ottawa, Ontario K1A 0R6, Canada
CO DimerThe CO dimer represents an outstanding unsolved problem in the spectroscopy
of Van der Waals molecules. It was first detected nearly twenty years ago by Vanden
Bout et al. [1] using molecular beam electric resonance spectroscopy; five lines were
observed (one at 1.5 GHz and a quartet at 16 GHz) which could not be assigned. In
1994, Havenith et al. [2] published an analysis of a portion of the CO dimer infrared
spectrum around 2146 cm-1, recorded using a slit jet and a double modulation
technique. However, there were difficulties with their assignment of the observed
lines to a K = 2 - 1 subband.
The problem in interpreting the spectrum of the CO dimer is the absence of
regular spectral patterns, a consequence of its Soppiness and the large number of
low-lying rovibrational levels. There is a further complication due to vibrational
degeneracy in the upper state of the IR spectrum, v(CO)=l. It is clearly desirable to
achieve the coldest possible supersonic expansion in order to simplify the spectrum.
■ In the present work, we have used a rapid-scan diode laser system together with
pinhole and slit supersonic expansions to study the IR spectrum of the CO dimer. By
probing the jet at varying distances downstream, we observed spectra over a range
of effective temperatures from about 15 down to 1 K. The temperature dependence
of the transitions enabled us to identify three subbands arising from the lowest
rotational levels of the dimer. The assigned transitions account for almost all of the
strongest lines in our coldest jet spectra. The effective intermolecular separation for
(C0)2 was found to be a surprisingly large 4.35 A. Our analysis, which involves thesame spectral region and some of the same transitions, is incompatible with that of
Havenith et al. [2],
NO DimerThe propensity of the NO radical to dimerize has long been known. From pure
rotational [3] and infrared [4] spectra, we know that (N0)2 is a planar C2u molecule
with an N-N distance of about 2.26 A and a singlet ground electronic state. One
mystery of the NO dimer involves the nature and location of the many other dimer
electronic states (both singlet and triplet) that are formed by bringing together two
-22-
NO monomers. Another mystery involves the locations of the four low frequency
(intermolecular) vibrational modes of the dimer, which have never been observed in
the gas phase before.
We have now solved the second mystery by means of observations with a long-
path (180 m), low-temperature (90 K) absorption cell and a Bomem FTIR spec
trometer in both the mid- and far-infrared regions. The ax vibrations v2 (symmetric
bend) and uz (intermolecular stretch) have been unambiguously assigned at 239.36
and 134.50 cm-1, respectively. We also have strong indications that v4 («2 torsion)and Uq (b2 asymmetric bend) are located at about 117 and 429 cm-1.
These values for the intermolecular modes of (NO)a agree poorly with previous
“accepted” [5] values based on spectra of condensed NO or matrix-isolated NO
dimer. However, they are reasonably consistent with a force field which we had
derived [6] from centrifugal distortion parameters. The NO dimer poses a challenge
for quantum chemists, as will be shown by comparing our results with some recent
Cl and DFT calculations.
References
[1] P.A. Vanden Bout, J.M. Steed, L.S. Bernstein, and W. Klemperer, Astrophys.
J. 234, 503 (1979).
[2] M. Havenith, M. Petri, C. Lubina, G. Hilpert and W. Urban, J. Mol. Spectrosc.
167, 248 (1994).
[3] C.M. Western, P.R.R. Langridge-Smith, B.J. Howard, and S.E. Novick, Mol.
Phys. 78, 55 (1981); S.G. Kukolich, J. Mol. Spectrosc. 98, 80 (1983); M.D.
Brookes, A.R.W. McKellar, and T. Amano, J. Mol. Spectrosc. 185, 153 (1997).
[4] Y. Matsumoto, Y. Ohshima, and M. Takami, J. Chem. Phys. 92, 937 (1990);
B.J. Howard and A.R.W. McKellar, Mol. Phys. 78, 55 (1993); J.K.G. Watson and
A.R.W. McKellar, Can. J. Phys. 181, 181 (1997).
[5] E.M. Nour, L.-H. Chen, M.M. Strube, and J. Laane, J. Phys. Chem. 88, 756
(1984); V. Menoux, R. Le Doucen, C. Hausler, and J.C. Deroche, Can. J. Phys. 62,
322 (1984).
[6] A.R.W. McKellar, J.K.G. Watson, and B.J. Howard, Mol. Phys. 86, 273 (1995).
-23-
2.1.2FTMW SPECTROSCOPY OF SODIUM CHLORIDE - WATER
COMPLEXES — A MODEL SYSTEM FOR SOLVATION
Yasuki Endot Yasuhiro Ohshima and Asao Mizoguciii
Department of Pure and Applied Sciences, The University of Tokyo, 3-8-1 Komaba, Meguroku, 153 Tokyo, Japan
A series of molecular complexes, NaCl-(H20)n (n = 1...3), have been observed
by Fourier-transform microwave spectroscopy. The presicely determined molecular
constants for these complexes including their isotopomers yielded the first experi
mental determination of the structures of the complexes containing a molecule with
an ionic bond. It was found that both NaCl-H^O and NaCl-fllgO^ have almost
planar structures with the oxygen atom attatched to Na a nd one of the hydrogen
atoms to Cl, where NaCl-(H20)2 has a axis along the NaCl bond. On the other
hand, NaCl-(H20)3 is a symmetric top with a C$ symmetry axis. It was concluded
that bond lengths between Na and Cl become substantially longer when the number
of water molecules attatched is increased.
IntroductionIt has been well established that FTMW (Fourier-transform microwave) spec
troscopy combined with a Fabry-Perot type cavity is a powerful experimental tech
nique to study intermolecular interactions in small sized complexes. Recently, we
have applied the method to study structures of sodium chloride-water complexes,
becasuse they are expected to be an interesting model system to understand de
tailed mechanisms of soluvation of salts into water. It has been postulated that
when the number of water molecules attatched to NaCl is smaller, ionic bond be
tween sodium and clorine still exists, while the bond length becomes longer as the
number of water molecules is increased. Although recent ob initio calculations con
firmed the elongation of the bond length, no experimental data has been obtained for this system.
ExperimentalThe spectrometer used is a Balle-Flygare type FTMW spectrometer. A sodium
chloride rode is irradiated by a fundamental output of a Nd-YAG laser to produce
free NaCl molecules. The vaporized NaCl was expanded into vacuum with Ar as
a carrier gas, where a small amount of water is mixed with Ar to form sodium
—24—
chloride-water complexes. A frequency region from 5 to 25 GHz was observed al
most continuously, and many lines were picked up. Each of the observed lines was
checked whether it contained NaCl and/or water molecules. By referring to a struc
ture predicted by ab initio calculations, transitions of NaCl-H20 were picked up and
fitted to an asymmetric top Hamiltonian. Analysis of the hyperfinc splittings due
to eQq couplings of Na and Cl helped the rotational assignment.
After the successful assignment of NaCl-H20, it was rather easy to pick up tran
sitions of NaCl-(H20)3, since the complex was a symmetric top. Finally, transitions
of NaCl-(H20)2 were picked up among the remaining lines. Most of the observed
lines were thus assinged to the transitions of these three complexes and their iso-
topomers with 3'C1. For NaCl-H20, deuterated species were also observed using
enriched samples.
ResultsFrom the rotational constants of the Na35Cl-H20 species and its isotopomers,
it was possible to unambiguously determine the molecular structure by assuming the structure of water to be unchanged upon complex formation. It was found that
NaCl and one of the OH bond form a cyclic structure in the complex. Furthermore,
the complex is almost planar with a possibility that the non-bonded hydrogen lies
slightly out of the plane. Similarly, NaCl-(H20)2 also was concluded to have an
almost planar structure. Structure of NaCl-(H20)2 was thus determined by as
suming that the second water molecule was attatched on the other side of NaCl.
The O-Na-Cl angle was found to be decreased by about 5 degrees for this complex.
Since NaCl-(H20)3 is a symmetric top, it is reasonable to assume that three water
molecules are attached symmetrically around NaCl. By transferring the structural
parameters of the smaller complexes, it was concluded that the bond length between
Na and Cl is considerably elongated for this complex, by about 50 pm.
We were thus able to detect complexes representing an early stage of solvation
of NaCl into water, where charges of sodium and chlorine begin to separate toward
solvated Na+ and Cl". It is expected that fourth water is attatched either symmet
rically forming a complex with C.\ symmetry or on one side of NaCl breaking the
symmetry. At present, most of the observed lines have been assigned to the above
mentioned three species. Transitions of larger complexes arc thus expected to be
much weaker in intensity.
—25 —
2.1.3
SPECTROSCOPIC STUDIES OF OPEN-SHELL CLUSTERS: CLUES TO THE NATURE OF INTERMOLECULAR INTERACTIONS
Brian J. Howard and Christopher J. Whitham
Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford 0X1 3QZ, United Kingdom.
The high resolution infrared and microwave spectroscopy of weakly bound clus
ters is a valuable source of information on the nature of intermolecular forces. It
provides detailed information on the preferred relative orientation of molecules and
in some cases details details of the intermolecular potential energy surface. What is
less apparent is the effect of complex formation on the electron distribution of the
molecules involved. Is there any incipient chemical bond formation?
In this talk I shall present recent results on the microwave fourier transform
and infrared diode laser absorption spectroscopy of clusters containing,the stable
open-shell molecules NO, NOg and Og.
The spectra of the complexes Ar-NOg and Xe-NOg exhibit well resolved hyper-
fine structure. In the case of the argon complex the spectral constants can all be
explained in terms of the projections of monomer constants. In the xenon complex
serious perturbations are observed. There are two magnetic nuclei of Xe, namely
129Xe and 131 Xe. These shown Xe magnetic hyperfine structure which indicates sig
nificant unpaired electron transfer between the molecules.
Spectra of the complex NO-HF have also been analysed. These show that the
HF perturbs the orbital angular momentum of the NO molecule. The barrier to
free motion is about 300 cm-1, with the electron preferring to be in the plane of the
complex. Hyperfine structure due to the hydrogen and fluorine magnetic nuclei also
indicate significant unpaired electron transfer from NO to HF.
The oxygen complexes OCS-Og and NgO-Og have been observed and analysed.
Models for the electron transfer and correlation effects observed will be pre
sented.
—26 —
ELECTRONIC AND VIBRATIONAL SPECTRA OF BENZENE TRIMER ION
Kazuhiko Ohashi
Institute for Molecular Science, Myodaiji, Okazaki 444, Japan
We have studied electronic structures of cluster ions through photodissociation
spectroscopy. The method employs tandem mass filter arrangements which isolate
specific cluster ions for photodissociation and analyze the mass number of the resultant
fragments. The photodissociation spectra are obtained by recording the yields of the
fragment ions as functions of the photodissociation wavelengths. We have alreadyreported the spectra of (C&H^z"*" and (CgH6)3+ in the visible to near-infrared region (solid curves in Fig. I).1,2 The timer spectrum displays local excitation (LE) bands due
to a 7Z<-iz transition at 430 nm and a it<r-a transition at 620 nm. The position of the
Jt <-<T band shifts gradually to the red with increasing cluster size, while that of the %<r-jt band is almost independent of cluster size. These spectral shifts are consistent with a
triple sandwich structure for (CgHg)]"*". The most intense band at 960 nm of the timer
spectrum arises from intermolecular charge resonance (CR) interaction. A weaker CR
band is seen at 1220 nm. The CR bands characteristic of (CgHgk"1" remain essentially
intact in (CgHfOgt Therefore, we propose that the positive charge is localized on a dimer
unit, namely dimer core, which acts as a chromophore for the CR transition in (CgHgte"1-,
and that the third benzene molecule behaves like a neutral solvent. The main purpose of
the present study is to confirm the dimer core structure and to obtain further information
on the electronic and geometric structures of (CgHeh"1".To begin with, we performed hole-burning experiments3 in order to ascertain
whether two isomers of (CgHels4" are responsible for the appearance of the two CR
bands. Intensities of the two CR bands monitored by the probe laser were found to be
reduced by the same amount with the introduction of the hole-burning laser. The
behavior proves that the two CR bands are attributable to the same species.
Charged benzene molecules in (CeHete-1" are responsible for all the absorption
bands observed in the visible to near-infrared region. It is worthwhile to probe the third
benzene molecule in order to confirm the dimer core structure. To this end, we measured
the photodissociation spectra of (CgH6)2+ and (CgH6)3+ in ultraviolet region (closed
circles in Fig. 1) where neutral benzene molecules show the Si<—Sq absorption (n*<—tv transition). The dimer spectrum exhibits a band with its onset at 230 nm. We assign this
band to a K*<—7t transition of the charged benzene molecules, because the two molecules
in (C6H6)2+ share the positive charge. In addition to this band, the timer spectrum
2.1.4
-27-
displays a characteristic band around 250 nm. We attribute this band to the third benzene
molecule with a neutral character, because the position of the band is in agreement with
the Si*-So spectra of neutral benzene. The appearance of this band provides further
evidence for the dimer core structure of (CeHeb"1".
Finally, we turn our attention to the vibrational spectroscopy of (C6H6)3+. Figure
2 shows the photodissociation spectrum of (CgHeb"1" in the region of C-H stretching
vibrations. Two distinct maxima are seen at 3050 and 2980 cm-1. We assign the 3050-
cm-1 band to the third benzene molecule of a neutral character, because the band position
is close to those observed in vibrational spectra of neutral benzene. The 2980-cm-1 band
is attributed to the charged benzene molecules. The shape of this band is asymmetric and
the intensity is almost one order of magnitude larger than that of the 3050-cm-1 band.
The intensity is considered to be enhanced by the CR interaction between the two molecules in the dimer core.1 2 3 4 Analysis of the spectra of isotopically mixed trimer ions
with benzene-^6 molecules reveals that the frequencies of the C-H stretching modes vary
with the position of the chromophoric molecule in the triple sandwich. The absorption
maximum of CgH^ in the center of the sandwich is located at a frequency lower than that
of C6H6+ at the end. The asymmetric shape of the 2980-cm-1 band is due to the overlap
of the two components (broken curves in Fig. 2).
trimer
this work trimer
center
dimer
2900 2950 3000 3050
Wavelength / nm Frequency / cm
Fig." 1 Electronic spectra of benzene dimer and trimer ions. Fig. 2 Vibrational spectrum of benzene Solid curves represent the results of our previous studies. trimer ion. Solid curve represents the Closed circles stand for the data obtained in this work. sum of the two components.
1 K. Ohashi, Y. Nakai, T. Shibata, and N. Nishi, Laser Chem. 14, 3 (1994).2 T. Shibata, K. Ohashi, Y. Nakai, and N. Nishi, Chem. Phys. Lett. 229, 604 (1994).3 K. Ohashi, Y. Inokuchi, and N. Nishi, Chem. Phys. Lett. 263, 167 (1996).4 H. Matsuzawa, H. Yamashita, M. Ito, and S. Iwata, Chem. Phys. 147, 77 (1990).
-28-
2.1.5STRUCTURE AND DISSOCIATION ENERGETICS OF VAN DER
WAALS AND HYDROGEN BONDED CLUSTERS
R.M.Helm, Th.L.Greener, J.Braun, and H.J.Neusser
Institut fur Physikalische und Theoretische Chemie, Technische Universitat Miinchen, Lichtenbergstr. 4, D-85748 Garcbing, Germany
Structural and energetic data of clusters are important for the understanding of
the intermolecular interaction on a microscopic scale. Here we present new results on
the structure, intermolecular dynamics, and dissociation energy of systems ranging
from benzene-Ar to the complex of the amino acid chromophor indole (CgHyN) with
water.
Highly resolved sub-Doppler spectra of clusters with rotational resolution have
been measured with mass selection in a resonance-enhanced two-photon excitation
process. The first photon originates from a nearly Fourier-transform limited laser
pulse with a frequency width of typically 70 MHz in the UV. The pulses are pro
duced by pulsed amplication of cw single mode dye laser light and frequency dou
bling. Recently we extended our investigations from van der Waals clusters [1] to
hydrogen-bonded aromatic molecule-water systems [2, 3]. To analyze the complex
rotational structure in the vibronic bands of these systems we developed a new
automated fitting procedure yielding accurate values for the rotational constants
by optimization of the cross correlation of a theoretical spectrum with the experi
mental spectrum (Correlation Automated Rotational Fitting: CARF[4]). Fitting
of the rotational constants is possible without a preceding analysis and assignment
of the rotational transitions. We present the structure of two prototype systems, benzonitrile-water and indole water, and the intermolecular dynamics of hydrogen
bonding in phenol-water. In benzonitrile-water (C6H5CN'H20) the water is found
with its oxygen nearly in the plane of benzonitrile, nested between the cyano group
and the ortho hydrogen [2]. In indole-water the water is in a hydrogen accepting
position attached to the amino hydrogen with its oxygen oriented towards the NH
hydrogen [5]. In phenol-water (CgHgOH-HgO), we investigated the intermolecular
dynamics by high resolution spectroscopy [3]. An autocorrelation procedure yields
directly the torsional splitting of the individual vibronic bands prior to a rotational
analysis. In this way we were able to assign several intermolecular vibrational states
and investigate the tunnel splittings caused by the hindered rotation of the water
moiety. This is an important step towards an understanding of the intermolecular
potential surface of the hydrogen bond.
—29 —
Mass analyzed pulsed field threshold ionization (MATI) of optically excited high
Rydberg states is a powerful method for the production of state-selected molecular
and cluster ions. Its mass selectivity allows us to detect the threshold for cluster
dissociation with high precision [6]: At the dissociation energy the threshold ion
signal at the parent mass breaks down and at the same time threshold ion signal appears at the fragment mass. We have used the MATI technique to study a series of
van der Waals complexes of aromatic molecules with increasing number of aromatic
rings with noble gas atoms attached to these microsurfaces. The dissociation energy
of the Ar atom is found to depend mainly on the number of the aromatic rings (i.e.
the size of the microsurfaces) and approaches the corresponding value of Ar attached
to a graphite surface [7]. Recently we were able to measure the first threshold ion
spectrum of a complex with water and hydrogen bonding: indole-water [8]. The
so found large binding energy (Do=1630 cm-1) of the neutral indole-water complex
supports the structural data found from high resolution spectroscopy and points to
a hydrogen bonding between the hydrogen-donating indole and the oxygen atom of
the water molecule.
The data on isolated complexes found in this work is supposed to provide basic
information on the nature of hydrogen bonding of 0-H and N-H groups which is
important in biology and solvation processes. 1
[1] Th. Weber, A. von Bargen, E. Riedle, H.J. Neusser, J. Chem. Phys. 92, 90 (1990).
[2] R.M. Helm, H.-P. Vogel, H.J. Neusser, V. Storm, D.Consalvo, H.Dreizler, Z. Naturforsch. 52a, 655 (1997).
[3] R.M. Helm, H.-P. Vogel, H.J. Neusser, J. Chem. Phys., in press.
[4] R.M. Helm, H.-P. Vogel, H.J. Neusser, Chem. Phys. Letters 270, 285 (1997).
[5] R.M. Helm, M. Clara, Th.L. Grebner, H.J. Neusser, J. Phys. Chem., submitted.
[6] H. Krause, H.J. Neusser, J. Chem. Phys. 97, 5923 (1992).
[7] Th.L. Grebner, R. Stumpf, H.J. Neusser, Int. J. Mass Spectrom. Ion Proc. (1997), in press.
[8] J.E. Braun, Th.L. Grebner, H.J. Neusser, J. Phys. Chem., submitted.
-30-
2.1.6 Infrared depletion spectroscopy of aniline clusters -The hydrogen bond between the NH2 and an aromatic ring-
Taisuke NakanagaNational Institute of Materials and Chemical research
1-1 Higashi, Tsukuba, Ibaraki 305, Japan
Introduction
The hydrogen bond interaction through NH bond has been studied using the
aniline cluster as a model system. It has been found from the spectroscopic study that
there are two kinds of hydrogen bond which play important roles in the aniline clusters.
One is the usual hydrogen bond between the NH bond and a non-bonding electron pair.
This interaction gives a similar spectroscopic effects as are observed in bulk systems.
The other is the interaction between the NH bond and the %-electron of the aromatic ring.
In this case, the spectroscopic feature is quite different from that of the former one.
The vibrational spectra of aniline dimer, aniline-benzene, and aniline-pyrrole
clusters in the NH stretching vibration region have been measured by infrared depletion
spectroscopy. The structures and frequency shifts of the NHz stretching vibrations of
the clusters have been discussed considering the results of ab initio MO calculations.
Experimental
The infrared spectra of aniline
clusters have been measured by infrared
depletion technique. The infrared ab
sorption was measured as the depletion
of the ion signal of REMPI-TOF mass
spectrometry, which was induced by the
vibrational excitation or vibrational pre-
dissociation by infrared laser. Details
of the experimental setup are given
elsewhere.
Fig. 1 compares the infrared spectra of aniline dimer, aniline-benzene, and
• Aniline monomer / .
•Aniline dimer V :- ■ - - •_______ _________ ___ V ■ ___
Aniline benzene
f
■ Aniline pyrrole
3300 3 400
y pyrrole •
3500wavenumbers
Fig. 1. Infrared depletion spectra of aniline, aniline dimer, aniline benzene and aniline pyrrole.
-31-
aniline-pyrrole with that of aniline monomer in the NH stretching vibration region. It is
interesting that these clusters give similar amplitudes of the frequency shifts. Since the physical and chemical properties such as pKa or proton affinity of these three molecules
are different from each other, we can expect various values of the frequency shifts.
Discussions Table 1. Ab initio MO calculations of anilineThe structures of the clusters (in cm-1).
clusters have been estimated using
ab initio MO method(Gaussian 94). MP2/6-31G**
anilinedimer
anilinebenzene
anilinepyrrole
In the calculations, we tried to find AE 3389 2025 2922.6Vsym(Av) 3393(17) 3394(16) 3395.1(15)
the isomer which reproduce the _ Va-6ym(Av)experimental
3492(27) 3492(27) 3490.4(29)
experimental frequency shift and the Vsym(Av) 3394(27) 3393(28) 3393(28)
ratio of the shifts AVsyn/Ava-sym-Va-sym(Av) 3466(42) 3467(41) 3464(44)
, ' AE: stabilization energy at equilibrium position.Table 1 and Fig. 2 shows the results
of the calculations using the basis Table 2. Structure parameters of the NH2 groupset of MP2/6-31G* *. When the monomer aniline aniline anilinebasis set of RHF or DFT was used, r(NH) 1.0103
dimer1.0123
benzene1.0122
pyrrole1.0124
the frequency shift could not be a(HNH) 109.40 107.49 107.36 107.077t(JNH2) 46.75 50.72 50.88 51.54
reproduced well. The isomers in d(NH2-7t) 2.82 2.90 ■ 2.95Fig. 2 are the most stable onesexcept the aniline-pyrrole cluster. In the case of the aniline-pyrrole cluster, the isomer
which gave the best frequency shifts was the second stable one. The most stable one
gave a set of the frequency shifts which cannot be accepted.
The frequency of the NH2 stretching vibration is influenced by the distance
between the NH bond and the counter molecule. The distance between the two mole-'
cules is determined by the interaction between two aromatic rings. Several calculated structural parameters of theclusters are given in Table 2. This seems to be the reason
for the constant frequency shifts of the clusters.
of£**S>o
Aniline dimer Aniline benzene Aniline pyrrole Fig.2 The calculated structure of aniline clusters.
-32-
2.1.7Structure and dynamics of size-selected benzonitriIe-(H20)n and
benzonitrile-(CH3OH)n clusters investigated by IR-UV and stimulated Raman-UV double resonance spectroscopies
Takavuki EBATA. Seiichi ISHIKAWA and NaoMko MIKAMI Department of Chemistry, Graduate School of Science, Tohoku University
Sendai 980-77, Japan
Structures and dynamics of the vibrational relaxation of the benzonitrile(BN)-(H20)n=M and BN-(CH3OH)n=w clusters have been investigated by use of very sensitive vibrational spectroscopies (IR-UV and stimulated Raman-UV double resonance spectroscopies) and by ab initio
calculations. Benzonitrile has a large dipole moment (4.14 Debye) and iteasily forms clusters with other molecules. Especially, for the cluster formation with the molecules having OH group, such as HgO or CH3OH, the hydrogen-bond (H-bond) interaction is thought to play important role in addition to the dipole-dipole interaction.
In the present work, we applied fluorescence detected stimulated Raman spectroscopy (FDSRS) and fluorescence detected IR spectroscopy (FAIRS) to observe the vibrational spectra of the size-selected clusters, where a population depletion induced by stimulated Raman pumping or IR absorption by a tunable IR laser is monitored by LIF.We observed Raman spectra of v12(C-C stretch), vcn(C-N stretch) and vch(C-H stretch) vibrations of the BN site and IR spectrum of v0H of the HjO (or CH3OH) site.Among the four vibrations, vCN and v0H showed drastic change upon the cluster
formation and it was concluded that the structure of the clusters are essentially of ring form as is shown for BN-(H20)2 in Fig. 1.
The relaxation of after the vibrational excitation was also investigated by measuring the electronic transition from the relaxed levels or dissociation product. It was found that IVR is the main channel for the v12 excitation, while the vibrational predissociation (VP) is the main channel for the vCN excitation.
-33-
2.1.8SOLVENT EFFECTS AND CHEMICAL REACTIVITY IN
SMALL MOLECULAR CLUSTERS
D. Solsadi. C. Jouvet, S. Martrenchard-Barra, C. Dedonder-Lardeux,G. Gregoire
Laboratoire de Photophysique Moldculaire du CNRS, University Paris-Sud, 91405 Orsay, France
Solvent assisted intramolecular charge transferWe have studied the role of the solvent (water, acetonitrile ...) on the
intramolecular charge transfer in the case of Donor-Acceptor molecules such as
substituted aniline derivatives (the model compound being the dimethyl-amino
benzonitrile DMABN).
When in polar solutions, they are excited, these molecules show an
intramolecular charge transfer with a torsion of the donor moiety with respect to the
acceptor moiety. This twisted intramolecular charge transfer (TTCT) is characterized by
a dual fluorescence, a blue shifted band corresponding to the non twisted conformation
and a red one corresponding to the twisted charge transfer state.
hi solution, the factors governing this photochemical behavior are :
■ the polarity of the solvent which leads to a stabilization of the charge transfer
state;
■ the torsion angle in the ground state : if the molecule is twisted, the torsion of
the excited molecule will be easier;
■ the energy gap between the first and the second excited states corresponding
to the locally excited state and to the charge transfer state.
In small clusters we have shown that for dimethyl-amino benzene methylester
(DMABME) the transfer to the TTCT state can be induced by one molecule of a polar
solvent only (water or acetonitrile). When the cluster size is increased, the red
fluorescence also increases.
A kinetic study of this process on the picosecond time scale shows that the
crossing towards the TICT state is faster when the number of solvent molecules
increases. El this pump-probe experiment a first photon excites the complex and the
second one delayed in time, leads to the ionizing continuum. The biexponandal decay
—34—
of the ion signal has been assigned to the delocalization of the wavepacket initially
prepared on the locally Si excited state to a mixed TICT/Si intermediate state. When
the number of solvent molecules is increased, the wavepacket is more displaced towards
the TICT state.
Chemical reactivity in small molecular clustersAn important effect of solvation on chemical reactions is to lower the barriers
along the reaction coordinate. We have studied this kind of effect in clusters as a
function of size.
For example, we have studied recently the polymerization reaction in vinyl
chloride (CaHgCOn clusters and the proton transfer in the Phenol(ammonia)n clusters by
Threshold PhotoElectron Photoion Coincidence (TPEPICO) experiments using the
synchrotron radiation at LURE in Orsay.
The ionized vinyl choride dimer reacts leading to C4H5CI"1" with loss of one HC1 molecule. This reaction occurs after crossing a barrier of 0.2 eV. However, the energy
content measured in the C4H5CI reaction product indicates a more complex mechanism
involving a second barrier on the reaction path. Trimer and tetramer also polymerize.
However for larger clusters the react products are no longer detected. This can be
interpreted as an effect of thermal bath in the clusters where the reaction is in
competition with energy redistribution in the vibrational modes of the system.
A second example is the study of the acid-base reaction occuring in the ground
state of phenol-ammonia clusters. By measuring the ionization energies of the clusters
we have been able to determine that at least 6 molecules of ammonia are necessary to
allow the proton transfer. This can be directly correlated with the proton affinities of
ammonia clusters which increase with the size and with the acidity of the phenol
molecule.
-35-
2.1.9 COUPLING OF LOW FREQUENCY VIBRATIONSWITH PROTON TRANSFER
Hiroshi Sekiva. Kaori Nishi, Tetsuro Fukuchi, and Nobuyuki Nishi
Department of Chemistry, Faculty of Science, Kyushu University,Higashi-ku, Fukuoka 812-81, Japan
We have studied proton (or hydrogen atom) tunneling in jet-cooled tropolone (TRN),
9-hydroxyphenalenone, and van der Waals clusters of tropolone with solvent atoms or
molecules. The results unambiguously indicate that proton tunneling in polyatomic
molecules can not be described with one-dimensional model along the proton transfer
coordinate. Therefore, proton tunneling in polyatomic molecules should be described by
multidimensional coordinates. We noted that low-frequency inteimolecular or
intramolecular vibrations may play important roles in proton tunneling in the condensed
phase. Because low frequency vibrational modes can be thermally populated at room
temperature, whereas high-frequency modes are thermally inaccessible. However, it is
difficult to investigate the influence of low-frequency modes on proton tunneling in the
condensed phase due to broadening of the energy levels. Thus, we considered to
investigate the effects of low frequency vibrations on proton tunneling by preparing van
der Waals clusters involving TRN as a chromopher in a free jet.
Previously, we have reported the Si-So fluorescence excitation spectra of TRN-Mn
(M=Ar, Kr, Xe, N%, CO, CH4, etc.; n=l, 2) clusters in the longer wavelength region of the electronic origin band of TRN.1'2) In the present work, we have successfully applied
hole-burning spectroscopy to separate the transitions between lower-wavenumber and
higher-wavenumber tunneling doublet components, and to distinguish structural isomers.
A lot of additional vibronic bands have been observed in the hole-burning spectra which
are absent in the fluorescence excitations spectra. This suggests that the vibrational
relaxation in TRN-Mn clusters is very fast. New information about the effects of the
intermolecular interactions have been obtained by analyzing the hole-burning spectra.
The dominant vibronic bands observed in the fluorescence excitation spectrum of
TRN-Kri are byj, byJ H|, byj, byJ Hj, szJ, and szJ Hj, where x and y are principal
axes for the intermolecular vibrations approximately along the short and long in-plane
axes of TRN, z is perpendicular to the xy plane. The notations by and sz indicate the bending and stretching modes, respectively. In addition to these transitions, the bxJ Hj,
bXQ Hj, and bxJbyjHj transitions have been clearly observed in the hole-burning
spectrum obtained by probing the 0% Hj transition. The observation of the bxJ H|
—36 —
transition implies that the y principal axis is not parallel to the C% axis of TRN in the
transition state and the positions of Kr before and after proton transfer are inequivalent.
However, the position of Kr may depend on the time.
In TRN-(CH4)i the tunneling splitting in the zero-point level of the Si state (16 cm'1)
decreases with increasing the vibrational quantum number of an intermolecular vibrational
mode. This intermolecular mode may involve the torsional motion of CH4. We
measured the hole-burning spectra of TRN-(N2)i and TRN-(CO)i. The N% and CO molecules are isoelectronic. But CO has a small electric dipole moment (0.1 D). The 0q
tunneling splitting of TRN-(N2)i is about a half of that of TRN. A lot of low-frequency
intermolecular vibrations have been observed in the hole-burning spectrum of TRN-
(N%)i. It has been found that two types of intermolecular vibrations exist. One type of
vibration decreases the tunneling splitting, while the other type vibration does not change the tunneling splitting. The 0° tunneling splitting of TRN-(CO)i is very small (0.5 cm"1)
comparing with the corresponding value (9.5 cm"1) of TRN-(N2)i. The difference in the
magnitude of the tunneling splittings may be due to polar interactions between TRN and
CO which increases the height of the potential energy barrier to tunneling.
A question is what kind of intramolecular modes are perturbed due to the attachment
of M on the molecular plane of TRN. We observed the hole-burning spectra of TRN- (CH4)i in the region up to 430 cm"1. The tunneling splitting of a tunneling promoting
mode V14 (0...0 stretch) is similar to that of the bare molecule, suggesting that the
attachment of CH4 on the molecular plane does not change the normal coordinates of the
in-plane modes. In contrast, a prominent progression of the V26 out-of-palne bending
mode has not been observed in TRN-(CH4)i.
The result of TRN-(CH4)i has been compared to those of 5-methyltropolone, in
which two large amplitude motions, proton transfer and internal rotation of the methyl
group, coexist. It has been found that the internal rotation of the methyl group strongly couples with the proton transfer coordinates. The 0° tunneling splitting of 5-
methyltropolone is about one order smaller than that of TRN. The strong coupling of the
rotation of the methyl group with the proton transfer has been ascribed to asymmetry of
the double-minimum potential well along the tunneling coordinate. The introduction of
asymmetry is a prominent feature in 5-methyltropolone, whereas the degree of
asymmetry is much smaller for the TRN-Mi clusters.
References
1) H. Sekiya, H. Hamabe, T. Nakajima, A. Mori, H. Takeshita, and Y. Nishimura,
Chem. Phys. Lett. 224, 563 (1994).
2) H. Sekiya, H. Hamabe, H. Ujita, N. Nakano, and Y. Nishimura, J. Chem. Phys. 103, 3895 (1995).
-37-
2.1.10RYDBERG STATES OF RARE GAS-NO COMPLEXES
Kazuhiko Shibuya. Kazuhide tsuji and Kinichi Obi
Department of Chemistry, Tokyo Institute of Technology,■ 2-12-1 Ohokayama, Meguro, Tokyo 152, Japan
The Rydberg states of Rare gas-NO complexes have been studied using resonance
enhanced multiphoton ionization (REMPI) techniques combined with a time-of-flight
(TOF) mass spectrometry. The Rydberg excited states of bare NO are well understood in
terms of spectroscopy of the diatomic molecule. The spectroscopic characterization made
for the Rydberg and valence excited states of NO allows us to give an insight into the roles
of rare gas in the excited potential surfaces and excited state dynamics of the complexes,
Rg-NO (where Rg = Ne, Ar and Kr).
The complexes were prepared in the molecular beam. We pumped the complexes
to a number of the excited states, the potential surfaces of which and the reaction dynamics
on which were probed using the photoionization. The geometry of Ar-NO(X2II) is reported
by Mills et al. [1] as Rat-no=371 pm, rN-o=H5 pm and 0=84.8°, while that of ionic Ar-
N0+(X12) is reported by Takahashi [2] as Rat-no=268 pm, rN-o=106 pm and 0=97.0°. The
initial Ar-NO(X2II) and final Ar-N0+(X12) complexes are basically T-shaped, and the
intermediate Rydberg states, Ar-NO*, of interest are, therefore, are prepared only in the T-
shaped Franck-Condon region. The binding energies of the excited states were determined
and considered how the values vary against the Rydberg states pumped. The floppy
vibrational motions were analyzed so to give the measured frequencies. The dissociation of
the complexes are evident in some of the Rydberg states, and the rates are controlled by the
electronic state, the vibrational quantum number of the NO moiety, and the intermolecular
floppy motion as well. The half-collision process could be compared with the reported full-
collision event if available. Our objective is to apply the spectroscopic procedures to the
understanding of Rg-NO in the Rydberg excited states and to explore the excited state
dynamics of the floppy complexes.
[1] P. D. A. Mills, C. M. Western, and B. J. Howard, J. Phys. Chem. 90,4961 (1986)
[2] M. Takahashi, J. Chem. Phys. 96, 2594 (1992)
-38
Solvation of Single Alkali Atom in Water Clusters
Kiyokazu Fuke, Ryozo Takasu, and Kenro Hashimoto*
Department of Chemistry, Kobe University, Nada-ku, Kobe 657 Japan ^Computer Center, Tokyo Metropolitan University, Minami-Ohsawa, Hachioji-shi, Tokyo 192-03 Japan
Although many efforts have been paid to clarify the structure and localization
modes of electrons in fluid, especially in water, its intrinsic properties are still the subject
of intensive studies. Advances in molecular beam technique permit to prepare the clusters
consisting of alkali atom and water molecules, which enable us to explore the fundamental
questions whether a single alkali atom is dissolved in finite clusters and how many
solvent molecules are required to form the bulk-like solvated electrons. In the alkali atom-
solvent clusters, the valence electron of alkali metal atom is expected to be transferred to
the solvent molecules at a certain cluster size, and the ground state may have an ion-pair
character as in the case of bulk fluids. In order to probe this transition, we have examined
the photoionization processes of the water clusters containing alkali atoms such as M= Li
and Cs. The ionization potentials (IPs) of M(H20)„ have been found to become constant
for ri>4 and its limiting values (n—»°°) are consistent with an estimated photoelectric
threshold of ice (3.2 eV).1 To explore further on the solvation state of metal atom in
clusters, we examine the photoelectron spectra (PES) of Li (H20)„ andNa (H20)„ as well
as the electronic and geometrical structures of these clusters using the ab initio
calculations. For the Na (H^O),, clusters, the 2S—lS and 2P—*5 transitions, which
correspond to the photodetachment transitions to the neutral ground and first excited
states, exhibit monotonous increase in vertical detachment energy with increasing n. On
the other hand, we observe striking results for the Li (H20)„ clusters that the 2S—‘5 type
transition shifts to the red with respect to that of Li in addition to the rapid decrease in the
2P—2S energy separation for n up to 4. For n>5, both the 2S—'S and 2P—'5 type
transitions are found to shift to the blue and their bandwidths become much broader. On
the basis of these results as well as the theoretical results, we discuss the geometrical
structures of the hydrated alkali atoms. We also discuss the localization mode of the
valence electron on the metal atom in relation to the early stage of the solvated-electron
formation in water clusters.
1 R. Takasu, F. Misaizu, K. Hashimoto, and K. Fuke, J. Phys. Chem. 101, 3078,
(1997).
2.1.11
-39-
Laser spectroscopy of silver-ammonia complexes
Jun Miyawaki
National Institute for Advanced Interdisciplinary Research, AIST, MITI
Higashi 1-1-4, Tsukuba, Ibaraki 305, Japan
Clusters consist of metal atoms and small molecules serve fundamental and
interesting model systems for the understanding of metal-ligand bonding. Silver-
ammonia complexes are of such clusters, which we are investigating by using laser
spectroscopic technique.
The complexes of silver and ammonia were produced with the laser ablation of
silver followed by collisions with ammonia seeded in carrier gas in a free jet expansion.
The spectra of complexes were observed by using resonantly enhanced multiphoton
ionization (REMPI) spectroscopy combined with a mass selection by a time of flight
mass spectrometer.
Figure 1 shows the REMPI spectrum of AgNHg complex. The peak located at
21410 cm'l is assigned to the 0-0 band, which is red shifted by more than 8000 cm'l
from a transition of a silver atom between its ground state and the lowest excited state (Ag
5s2S -> 5p2Pi/2).
There are long progressions having energy spacings of about 370 cm~l. They are
assigned to those of the intermolecular stretching mode. The peaks at 185, 805 cm'l
higher from the 0-0 band are assigned to the intermolecular bending and inversion of
ammonia modes, respectively.
2.1.12
—40 —
The significant stabilization of this complex in the electronically excited state is
suggested by the following facts observed in the spectrum; the large energy shift of the 0-
0 band, long progressions of the intermolecular stretching mode, and strongly blue-
shaded rotational contour. It may be concluded that this complex is chemically bonded in
the electronic excited state in contrast to the weak bonding with a physical (van der Waals
type) interaction in the ground state.
21200 22000 22800wavenumber (cm'1)
Fig.l REMPI spectrum of AgNHg
-41
2.1.13COMPETITIVE COORDINATION IN M+(NH3)/n(H20)„ AS
STUDIED BY LASER ABLATION-MOLECULAR BEAM
METHOD: EXPERIMENT AND SIMULATION
Hiroyasu SATO, Akiyoshi MATUZAKI, Satoru NISHIO, Osamu ITO,
Koji FURUKAWA and Takashi KAWASAKI
Department of Chemistry for Materials, Faculty of Engineering, Mi'e University, 1515
Kamihamacho, Tsu 514-8504, Japan
We have been studying reactions of laser ablated monopositive metal ions with a
variety of molecules and clusters, either organic, inorganic or organometallic, in a
molecular beam injected near by. Product ions are detected by a quadrupole mass
spectrometer. We named this very simple and versatile technique "laser
ablation-molecular beam (LAMB)" method.1,2 So far we have studied reactions of a
variety of monopositive metal ions with amines,3 benzene4, benzene clusters,5
chromium hexacarbonyl,6,7 dimanganese decacarbonyl,8 ammonia clusters,9
methanol clusters,10 ammonia-methanol’1, methanol-water12,13, ammonia-water13,14
and ammonia-acetone clusters.15
In the present presentation16, ammonia-water mixed-ligand complexes of
monopositive metal ions Mf (M=Mg, Al, Mn and Co) were prepared by the LAMB
method using ammonia-water binary clusters. Relative abundances of
M*(NH3)m(H20)„ are characterized with intensity gaps which indicate limited
(typically 2 or 3) coordination numbers in the first coordination sphere. Three
—42 —
patterns of competitive coordination, i.e., selective, nonselecdve and
magic-number-like, are observed. The patterns are metal-specific and relatively
independent of stagnation ratios of two component gases. The coordination numbers
as judged from the intensity gaps remain the same throughout the stagnation ratios
studied. A model simulation of the dynamic processes involved was made under
simple-minded assumptions: (1) the ensemble of metal complex ions starting from the
reaction region is characterized with a temperature Tstait (its value being taken as an
adjustable parameter), (2) only evaporation of component ligands one by one occurs
after metal complex ions start from the reaction region into the quadrupole, (3)
activation energy of each evaporation step is determined by binding energy of the
leaving ligand, and (4) temperature drop rate of complex ions per one microsecond is
constant (its value being taken as an adjustable parameter). Such a simulation
procedure is found successful in reproducing the positions of intensity gaps, together
with the qualitative features of the metal-specific coordination patterns observed.
References
lH.Salo,Res.Chem.Intenned.,19, 67 (1993); 2H.Sato,Photochemistry (Japanese PhotochemistryAssociation) ,2 0,54 (1995); 3H.Sato, M.Kawasaki, K.Kasatani andT.Oka, Nippon KagakuKaishi, 1240( 1989); 4H.Higashide,T.Oka,K.Kasatani,H.Shinohara and H.Sato,Chem.Phys.Lett.,163, 485
(1989); 5H.Higashide,T.Kaya, M.Kobayashi, H.Shinohara and H.Sato,Chem.Phys.Lett.,171, 297
(1990); *T.Oka, K.Kasatani,H.Shinohara and H.Sato,Chem.Lett.,9il (1991); 7H. Sato, K. Kasatani,
T.Oka, A.Matsuzaki, S.Nishio, K.Furukawa, T. Wada, T. Yada, S. Hayashi, H. Kobayashi and T.Yamabejtppl.Organometal.Chem.,11,913(1997); 8T.Oka,K.Toya, K.Kasatani, M.Kawasaki and
H.Sato,Chem.Lett., 1865 (1988); 9T.Kaya,M.Kobayashi,H.Shinohara and H.Sato,Chem.Phys.Lett.,
186,431(1991); iaT. Kay a, Y. Hori ki, M.Kobayashi ,H. S hi nohara and H.Sato, Chem.Phys.Lett., 200,
435 (1992); "Y.Horiki, S.Nishio, H. Shi nohara and H. S aloJ.Phys. Chem., 98, 10436 (1994);
"H.Sato,A.Matsuzaki,S.Nishio,Y.Horiki and O. I to, Surf.Rev.Lett.,3,799 (1996); 13H.Sato,Y.Horiki
andH.SatoJ.Photochem.Photobiol. A Chem.,92,17 (1995); l4O.Ito,Y.Horiki,S.Nishio, A.Matsuzaki and H.Sato,Chem.Lett., 9 (1995); lsO.Ito,K.Furukawa, Y.Horiki,S.Nishio,A.Matsuzaki and
H.Sato,Appl.Surf.Sci., 106 , 90 (1996); lfiH. Sato, A. Matsuzaki, S. Nishio, O. Ito, K. Furukawa
andT. Kawasaki, J. Chem. Phys., in press.
—43 —
2.1.14
REACTIONS OF CLUSTER IONS BY FTICR MASSSPECTROMETRY
K. Sugawara, E. M. Markin, and A. B. Vakhtin
National Institute for Advanced Interdisciplinary Research 1-1-4 Higashi, Tsukuba 305, Japan
Fine metal particles are widely used as heterogeneous catalysts in chemical
industry. In some cases, smaller particles have higher reactivity and selectivity, which
may be understood not only by larger surface area but by special structure of particular
particle sizes. In spite of enormous studies on heterogeneous catalysis, detailed
understanding of actual processes has not yet given. Therefore, it is quite reasonable to
study the reactions of metal clusters isolated in vacuum. One may demonstrate the high
reactivity and selectivity at particular sizes, and moreover one may find specific
processes, which have never observed, by decreasing size from fine particle to cluster.
Actually there are many works on the reactions of isolated metal clusters with simple
molecules by using mass spectrometry, and it is well known that the reactivity is very
sensitive to cluster size. However, such interesting reactivity is poorly understood,
especially for clusters made from transition metal elements, which are mainly used for the
catalysts. To consider such specific reactivity in detail, structural information should be
required. We adopted infrared spectroscopy to observed adsorbed molecules on metal
cluster surfaces in the present project. Molecular vibration is very sensitive to binding
site structure. One can observe difference between vibrational bands of adsorbed
molecules and those of free molecules. By comparing infrared spectra with huge data
obtained in the fields of surface science and gas phase molecular spectroscopy, we may
get insight into the structure and reactivity on metal cluster surfaces, and further into the
detailed processes of the catalytic reactions.
To study wide size dependence of clusters, a mass spectrometric technique with
high mass resolution is combined with laser spectroscopic method. The present
experimental system is based on Fourier transform ion cyclotron resonance (FTICR)
mass spectrometry, whose advantages are (1) very high mass resolution m/Am ~ 106, (2)
selective acceleration and removal from a trap cell, (3) ability to follow sequential processes, and so on. We can trap cluster ions with a single component for a long time
(sec-min) without collisions. Slow reactions and decomposition of clusters and
molecule-adsorbed clusters can be observed.
Metal cluster ions were produced in a helium jet by laser ablation of a metal rod
and then transferred into a trap cell put in a static magnetic field. The trapped cluster ions
—44 —
were thermalized by collision with argon. After thermalization the clusters were transferred into the next trap cell which was filled with reactant molecules. The numbers
of the cluster ions and reaction products were counted as a function of reaction time.
First we selected Nb clusters to check the constructed apparatus since Nb has only one
isotope (to get intense signal) and laser ablation of the metal was well studied. We
studied the reactions of Nb cluster ions with simplest molecule In Fig. 1 typical time
variation for the reaction of Nb cluster ions with hydrogen molecules is shown. It was
found that Nbio+ and Nbi2+ are not reactive to hydrogen, while the others react fast with
the molecule. In the case of Nbg+, the first addition of hydrogen is fast but the second
addition is very slow.
Metal carbonyl was also adopted as a cluster source. Sequential ion-molecule
reactions were started by the irradiation of Cr(CO)$ with an electron beam, and then only
one component of Crn(CO)m clusters was maintained in the trap cell. Binding energies of
CO with Cr clusters were estimated from the probability of decomposition induced by
collisions with He. We are trying to introduce laser light to estimate binding energy of
molecules on cluster surface and to find infrared absorption band originated from the
vibration of adsorbed molecules.
Nb, +H,r ioo.o-1| 3710 H
f 3540-j
I 1310-1
740-0 7MO 7840 8000 6200 0440
|icu-Nb' + H,
t-o
*330 #440 #600 9800 tOOOO K200 10449
f 5000-1 | 3710-j
\ 2340-j
7440 - 7600 TWO 8000 8300 #200 #440 #640 $840 16000 16240 10440
7400 7640 7800 8040 8200 6*40
3 t« 0.6 s
*200 *440 #600 *800 tOOOO 10200 10449
| 5000-1
I3™!8 2300-1! 12104
00-
t-O.ds
7440 7640 7840 8640 8200 6400
1-0.9S
JIjW*200 #440 9640 *840 16040 10200 10440
t»1.2s
7400 7640 7840 8040 8200 6440 *240 #440 #840 #800 10040 10340 10*40JUs—s
Fig. 1. Mass spectra obtained in the reactions of Nbn+ (n=8-l 1) with H2. Reaction times are 0, 0.3, 0.6, 0.9, and 1.2 sec.
—45 —
2.1.15Collision induced dissociation of size selected Aluminum cluster
Oddur Ingolfsson and Harutoshi Takeo National Institute for Advanced Interdisciplinary Research
Shinji NonoseGraduate School of Science and Technology, Kobe University
Collision induced reactions of cluster ions can be used to access quantities such as ionization and binding energies, to provide insight into the relationship between structure and reactivity and to help to understand reaction mechanisms [1]. This is not only of interest in understanding the physical properties of the cluster as a transition between the gaseous and condensed phase, but is also of great interest in the case of the transition metals to enhance our understanding of their catalytic properties.Here we report the first results obtained with an new apparatus constructed in order to study the energy dependency of collision induced reactions of size selected metal clusters. The Apparatus (Fig I) is equipped with a secondary ion source, where metallic clusters are produced by fast ion bombardment of an appropriate target (T). For cooling, the produced metallic cluster ions are guided with an octopole ion guide (OPIGI) through a He filled collision chamber (CHI). A single cluster size is then selected by a quadrupole mass filter (QMS I), injected into a quadrupol deflector (QDF) and deflected 90° around the comer before entering a second octopole ion guide (OPIG II). This narrows the energy distribution of the beam and prevents fast neutral and ionic Argon (from the primary ion source) from entering the collision chamber. The OPIG II then guides the size selected reactant cluster ions through a reaction chamber (CH IT) containing 10'5 Torr of a neutral target
gas. The reaction products are analyzed by a second quadrupole mass filter (QMS II).
J He inlet
3 __------ \
primary \ion source \L \
31111 =*1111
^ ion detector I
. —retarding field analyser l|II5’yl .qdf
T OPIG I CHI QMSHSin / opig n
CHH*target gas inlet
QMSH
Figure I Scematic of the apparatus ^ ion detector II
As a test system we have chosen to study the energy dependency of the fragmentation of Aln+ in collision with Ar. We observe Aln+ (n = 2 - 14) with a step like drop in intensity after AI3+ and AI74*. A similar behaviour has been observed after Al?+ and Alpf*" for Aln+
clusters produced by sputtering [2,3]. In the case of laser ablation those cluster sizes (n=7 and n=14) appear with enhanced intensity [4]. This correlates well with the electronic shell
46 —
model, as AI3+, Aly+, AI14+ all have, or
are close to a closed shell structure (8,20 and 41 valence electrons respectively).
Figure 11(a) shows a typical mass spectrum recorded in the range of 30 to 200 amu using QMS I as an ion guide only (DC off). In Figure 11(b) the first quadruple is set on a fixed mass (81 amu), so that, from the initial cluster distribution, only the trimer is transmitted. Figure 11(c) displays a mass spectrumrecorded under the same conditions as in 11(b), but now with an Ar gas inlet into the second collision chamber. Beside the selected tetrameter reactant cluster ion we additionally observe the product ions Ah"*" and Al+ representing the reaction channels
Aln+—> Al+ + Aln-1 ■ (1)Aln+ —> Aln-1+ + A1 (2)
respectively. Finally figure IH shows the collision energy dependency of the dissociation cross sections for the different product formation for AI3 (a) and AI4 (b). In the case of the trimer, where reaction (1) and (2) are the only open channels, reaction (2) dominates. The onset of reaction (1), however, is not shifted toward higher energies. The tetramer shows a different behaviour as channel (1) dominates and the onset of the much less abundant reaction (2) is shifted more than one eV toward higher energies. In addition, a third reaction channel leading to the observation of the dimer ion opens at the same energy as reaction (1) and steadily increases in intensity. 1
M/z (amu)
Figure II Mass spectra recorded in the range of 30 to 200 amu (a) DC off on QMS I (b) QMS I fixed on 81 amu (c) Ar gas inlet into CH H
■ (a) Al-
©O 0 0 o o
□□□□□noo o o
8 □ □ □
Collision Energy (CM, eV)
Figure IDE Energy dependency of the cross sections for the different product formation for (a) Al3+ and (b) Alf. 0 AI^O Al£ □ A$
[1] D.C. Parent and S.L. Anderson, Chem. Rev. 92 (1992) 1541[2] F.M. Devienne and J-C Roustan, Org. Mass Spectrom. 17 (1982) 173[3] L. Hanley, S.A. Rutta and S.L. Anderson, J. Chem. Phys. 87(1) (1987)260[4] M.F. Jarrold, J.E. Bower and J.S. Kraus, J. Chem. Phys. 86(7) (1987)3876
-47-
2.1.16 PHOTOELECTRON SPECTROSCOPY OF NEGATIVELY-CHARGED MOLECULAR CLUSTERS
Takasiii Nagata
Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Department of Basic Science, Graduate School of Arts and Sciences, The
University of Tokyo, Iiomaba, Meguro-ku, Tokyo 153, Japan
One of the fundamental issues regarding molecular cluster anions concerns their
electronic properties: namely, how the aggregates of molecules bind the excess elec
trons and how the electronic structures evolve with cluster size. This question is not
always easy to give an intuitive answer, even for the simple molecular clusters such as
(C02)", (CS2)", (NO)“..... etc. To address this issue we have measured photoelec
tron spectra to probe the electronic structures of the cluster anions. Of particular
interest are (1) the extent of charge localization/delocalization in the clusters, and (2) the existence of ’’electronic isomers” having different electronic structures.
Core switching in (C02)“ cluster anionsThe cluster anions of carbon dioxide, (C02)“, with n>3 are readily formed in
the collisions between neutral (C02)yv clusters and slow electrons, while a bare C02
molecule does not bind an excess electron as long as it retains the linear equilibrium
geometry. We have measured photoelectron spectra of (C02)~, from which the
vertical detachment energy (VDE) is determined as a function of cluster size 2 <
n < 16 [1], The n-dependcnce of VDE exhibits sharp discontinuities between n = 6 and 7, and between n = 13 and 14. This indicates that the anionic core of
(C02)“ changes from COj to C20^ at n = 7, and from C20^ to CO.J at n = M. A
hole-burning type of photoelectron measurement has been performed revealing that
(C02)g is flopping between the two isomeric structures, CO-J (C02)r, and C2Oj
(COs),.
We have also demonstrated for the first time the chemical reactivity of (C02)~
as a nucleophile in the gas-phase reactions; (C02)~ reacts with CII3I leading to
the formation of an acetyloxy iodide anion, CrT3C02I- [2, 3]. The overall reaction
process can be regarded as carboxylation by the reductive activation of C02, which
opens up a possibility of studying elementary processes of electrochemical reactions,
not in the condensed phase but in the gas phase with a restricted number of solvent
molecules.
-48-
Coexistence of electronic isomers in (CS2)“Recent ab initio MO calculations predict two types of stable structures for
(CS2)2-; one is an ion-neutral complex CS2 (CS2) having a bifurcated structure and
the other is a C2S4 molecular anion containing a dithiabutane 4-mcmbevd ring [•!].
We have measured the photoelectron spectra of (CS2)7 to explore these isomers
with different electronic structures [5]. The lowest-energy photoelcctron band of
(CS2)“ with 1 < n < 6 shows a spectral shift, which lies in the range expected for
typical ion-molecule interactions. The result indicates that the electronic structure
of (CS2)“ evolves as an ion-neutral complex, CS2 (CS2)„_i, with increase in the
cluster size. A closer examination revealed that the spectra for n >2 possess a high- energy band ascribed to the C2S,7(CS2)„_2 structure. These experimental findings
demonstrate the coexistence of “electronic isomers” represented as CS7(GS2)„_i and
CzS7(CS2X_2.
Evolution of electronic structures of (NO)7Negatively-charged clusters of nitric oxide, (N0)7, are known to exhibit in
triguing mass spectral distributions with an even/odd alternation; odd-sized (NO),7
predominate over even-sized cluster ions. The photoelectron spectra of (NO)7 show
that the spectral features of (NO)7 change abruptly on going from n = 2 to 3. This
spectral change indicates that an (N0)7 molecular anion is formed at n = 3, and
that it behaves as an anionic core for the larger analogues of n>4. Ab initio cal
culations were also carried out on the geometrical structure of (NO)J at tlic MP2
level of theory. The calculations provide an open-chain N3O3 structure as a most
stable geometry. Based on these experimental and theoretical results, we conclude
that the electronic structures of (NO)7 can be expressed as N3Oj (NO)n_3. The
overwhelming stability of the odd-sized (N0)7 is due probably to the spin pairing
in dimeric NO solvents surrounding the N3Oj core.
References [1] T. Tsukuda, M. A. Johnson and T. Nagata, Chcm. Phys. Lett. 268
(1997) 429.
[2] T. Tsukuda, M. Saeki and T. Nagata, Chcm. Phys. Lett. 251 (1996) 309.
[3] T. Tsukuda, M. Saeki, S. Iwata, T. Nagata, J. Phys. Chem. 101 (1997) 5103.
[4] K. Hiraoka, S. Fujimaki, K. A ruga, and S. Yamabc, J. Phys. Chcm. 98 (1994) 1802.[5] T. Tsukuda, T. Ilirosc and T. Nagata, Chcm. Phys. Lett. 279 (1977) 179.
—49 —
HIGH RESOLUTION SPECTROSCOPY OF CARBON CONTAINING MOLECULES:
FROM 13C TO C13
Thomas F. Giesen and Gisbert Winnewisser
I. Physikalisches Institut, University of Cologne, Zulpicher Strasse 77, 50937Cologne, Germany
Characterizing carbonaceous matter is of direct relevance in establishing the
nature of interstellar dust and the dynamics governing its evolution. The carbon
bond furnishes the backbone of a large amount of structures, like the linear and
branched chains, the cyclic and polycyclic rings, and the three dimensional cage molecules (e.g. Ceo)- Beyond this, it is unique among the abundant elements in its
ability to condense into solid grains.
The isotopic abundance of atomic carbon (12C / 13C) is of great astrophysical in
terest. It is astonishing that the pure 13C has been detected only recently by Keene
et al, and undoubtedly identified by precise high resolution measurements in our
laboratory. The 3P% <—3P% fine structure transitions of atomic carbon (Cl) 12C and
13 C will be presented.
The distribution of interstellar CO has been studied extensively. It is most likely
that also the (CO)g dimer is quite abundant in the cold interstellar clouds. We have
recently detected some transitions of this Van der Waals complex in the laboratory
and a search for its interstellar presents will be conducted soon.
A long list of carbon chain molecules (e.g. HC„N, CnH, with n as large as 11)
are known by astrophysical observations. Conspicuously absent from this list are
pure carbon clusters (Cn), which could, in principle, account for a significant frac
tion of the total galactic carbon budget [1]. Pure carbon chain molecules have no
rotational spectrum but can be observed by their extraordinarily low bending vi
brations which are excited at 100 K. For the first time we have detected C3 at 63.1
cm-1 in the cold interstellar clouds [2] using the Ivuiper Airborne Observatory and
the far-IR heterodyne spectrometer of Betz et al.
We have started a systematic search for the high resolution spectra of carbon chain
molecules in the gas phase. For this purpose a frequency stabilized tunable IR-diode
laser spectrometer has been combined with a laser ablation carbon cluster source.
2.1.17
—50 —
Currently the infrared spectra of carbon chain molecules up to C13 [3] have been
studied. The asymmetric stretching modes and combination bands of the asym
metric stretch with bending modes have been obtained. The latest results and the
spectroscopic technique will be presented.
We also have started to study silicon containing carbon clusters of the type SiCn
and SiCnSi which might exist in cyclic or three dimensional structures. The infrared
spectra of SiC4 [4] and SiCsSi [5] will be presented. For both molecules we found
that the linear isomers is the most stable.
References[1] J.R. Heath, R.J. Saykally Space Carbon: Neutral Pathways?
Science 274, 1480-1481 (1996)
[2] A. Van Or den, J.D. Cruzan, R.A. Provencal, T.F. Giesen, R.J. Saykally, R.T. Boreiko, A.L. BetzProceedings of the Airborne Astronomy Symposium on the Galactic Ecosystem; The Astronomical Society of the Pacific: San Francisco, 73 (1995)
[3] T.F. Giesen, A. Van Orden, H.J. Hwang, R.S. Fellers, R.A. Provencal, R.J. Saykally Science 265, 756 (1994)
[4] A. Van Orden, R.A. Provencal, T.F. Giesen, R.J. SaykallyCharacterization of silicon-carbon clusters by infrared laser spectroscopy - the u\ bandof SiC4Chem. Phys. Lett. 237, 77-80 (1995)
[5] A. Van Orden, T.F. Giesen, R.A. Provencal, H.J. Hwang, R.J. SaykallyCharacterization of silicon-carbon clusters by infrared laser spectroscopy - the 7/3(0"*) band of linear SigCsJ. Chem. Phys. 101, 10237 - 10241 (1994)
— 51 —
2.2
CLUSTERING IN THE GAS PHASE AND SOLUTION:PREFERENTIAL SOLVATION OF NEUTRAL SPECIES AND
HYDROPHOBIC HYDRATION OF NOBLE GASES
Alessandro Bagno, Marco Campulla, Matteo Pirana, Gianfranco
SCORRANO AND STEFANO STIZ
Centro CNR Meccanismi Reazioni Organiche, Dipartimento di Chimica Organica,via Marzolo 1, 35131 Padova (Italy)
Two aspects of clustering will be covered in this talk: the preferential solvation
of neutral and charged organic species in mixed solvents, and the structure of gas-
phase neon-water clusters from db initio calculations as models of the hydrophobic
hydration shell.
When a species is dissolved in a mixed solvent, the composition of its solvation
shell may be different from that of the bulk solution. This situation is known as
preferential solvation (PS). It affects several chemical properties, e.g. the structure
and stability of micelles, polymers and proteins, and transport phenomena through
membranes. Most efforts to detect PS in specific systems have been devoted to
electrolyte solutions, whereas simple neutral organic molecules have received less
attention. Therefore, we have investigated the possibility to detect the PS of neu
trals in binary solvent mixtures by means of 2-D NMR measurements (intermolecular
1H NOESY). The integrals of NOESY crosspeaks due to intermolecular interactions have been related to the composition of the solvation shell by means of a modified
Macura-Ernst formalism. Thus, the volumes of intermolecular crosspeaks depend
on the solvent composition, on the mixing time and on the relaxation matrix, whose
elements have been calculated from the equations for intermolecular dipolar relax
ation. Some results obtained with this technique will be presented, i.e. (a) PS
of phenol and tetraalkylammonium ions in various binary mixtures; (b) PS of ni-
troanilines in cyclohexane/THE; (c) dependence of the PS of phenol on the relative
concentration of the two cosolvents; (d) PS in water/alcohols systems.
The comparison of such results with ob initio calculations on solute-solvent
hydrogen-bonded complexes shows that PS originates mainly from the micro-aggregation
of solvent molecules rather than from individual solute-solvent interactions.1
The interaction of nonpolar solutes with water is a fundamental aspect of sol
vation. Although the name of this interaction (hydrophobic) and the examples that
readily come to mind (oil does not dissolve in water) carry the idea of being re
pulsive in nature, it is not necessarily so. What is more, its impact goes much
beyond the fact that nonpolar substances do not dissolve in water, as most biologi-
2.2.1
-52-
cally important molecules possess hydrophobic portions coexisting with hydrophilic
ones in a largely aqueous medium. The neon-water system was investigated by ah initio quantum chemical calculations. The structure of 1:1 clusters, as well as of
the clusters Ne(H20)i6, Ne(H20)si, Na(H20)i"7 and Na(H20)j^, was investigated
at the HF/ST0-3G and 3-21G levels, and their radial distribution functions were
calculated. The solvation shell of Ne features a tangential arrangement of water
molecules, whereas Na+ is solvated via the oxygen atom. The comparison of the
data obtained for 1:1 complexes and larger clusters shows that tangential solvation
is not caused by a preference of the apolar solute and water to arrange themselves
in that way, but rather by the overall interaction being dominated by the much
stronger tendency of water to establish and retain its hydrogen bonding network.2
1. Bagno, A.; Scorrano, G.; Stiz, S. J. Am. Chem. Soc. 1997, 119, 2299.
2. Bagno, A. J. Phys. Chem., submitted.
—53—
2.2.2 Molecular Clusters in Electrolyte and Non-electrolyte Solutions
Akihiro Wakisaka. Hassan Abdoul-Carime, Hitomi Kobara and Shunsuke Mochizuki National Institute for Advanced Interdisciplinary Research, Higashi 1-1-4, Tsukuba,
Ibaraki 305, Japan
In solution, intermolecular distance is very short and orientation of molecules is
free; therefore, various intermolecular interactions such as electrostatic, hydrogenbonding, hydrophobic interactions, etc. lead inhomogeneous microscopic structure which
seems to be a kind of cluster. In this sense, chemical reactions in solution should proceed
through some form of clusters, and the study on cluster structure in solution is essential
to the chemistry in solution. As one of the way to observe the clusters in solution, mass
spectrometric analysis of clusters isolated from liquid droplets were carried out.
Clusters in non-electrolyte solution: Fig. 1 shows a schematic illustration of the
mass spectrometer designed for the neutral clusters in solution. This is composed of a
heated nozzle, a quarupole mass-filter and a four-stage differentially pumped vacuum
system. A sample solution is injected into the first chamber through the heated nozzle.
When a part of solution is vaporized in the nozzle, the resulting gas-liquid mixture forms
a flow of liquid droplets. Owing to the pressure balance, the liquid droplets enter the
second and the third chamber with lower pressures and they explode through adiabatic
expansion, which leads to fragmentation of the liquid droplets into clusters. The mass
spectra of the clusters are measured by a quadrupole mass filter after electron impact
ionization.
As a typical example of this experiment, the interactions of a carboxylic acid with an
aromatic base will be presented. The effects of pKa, size of alkyl group and solvent on
this acid-base interaction are discussed on the basis of the molecular clustering.
Clusters in electrolyte solution: The mass spectrometer specially designed for
ionic clusters in electrolyte solution is shown in Fig. 2. This is a kind of electrospray
mass spectrometer. When an electrolyte solution is injected into the high electric field of
the first chamber, as shown in Fig. 2, only the positively charged liquid droplets are led
to the downstream owing to the electric field and pressure balance. The positively
charged liquid droplets are exploded through adiabatic expansion and electrostatic
repulsion, which leads to the fragmentation of the liquid droplets into the clusters with
single positive charge. The resulting clusters with positive charge were analyzed by a
quadrupole mass spectrometer without any external ionization.
—54 —
By this method, it was observed that the cluster structure of aqueous nitric acid was
quite different from that of aqueous sulfuric acid. The relation of the observed cluster
structure with physical and chemical properties of these acids will be presented.
Fig. 1 Schematic of the specially designed mass spectrometer for neutral clusters isolated from non-electrolyte solution.
4kV 180V 160 V 140V
TOMS760 Torr ,
heater
PUMPPUMPPUMP PUMP PUMP
Fig. 2 Schematic of the specially designed mass spectrometer for ionic clusters isolated from electrolyte solution.
1. "Preferential Assotition of 7-Azaindole Dimer in Acetonitrile Studied by Mass Spectrometry," T. Arai, T. Koyama and A. Wakisaka, Chem. Letters, 1997,123-124.2. "Clustering of a Hydrogen-bonding Complex between Indole and Isoquinoline: Correlation with Nucleation of Intermolecular Compound," Y. Yamamoto and A. Wakisaka, J. Chem. Soc. Faraday Trans., 1997, 93, 1405-1408.3. "Molecular Self-Assembly composed of Aromatic Hydrogen-bond Donor-Acceptor Complexes," T. Koyama and A. Wakisaka, J. Chem. Soc. Faraday Trans., 1997,93,3813-3817.4. "Direct Observation of Acid-Base Interaction by means of Mass Spectrometry for Clusters," S. Mochizuki, Y. Usui and A. Wakisaka, Chem. Letters, 1997,1097-1098.5. "Solvent Effect on Acid-Base Clustering between Acetic Acid and Pyridine," Y. Akiyama, A. Wakisaka, F. Mizukami and K. Sakaguchi, J. Chem. Soc. Perkin Trans. 2,1998, in press.6. "Nonideality of Binary Mixtures: Water-Methanol and Water-Acetonitrile from the Viewpoint of Clustering Structure," A. Wakisaka, H. A-. Carime, Y. Yamamoto and Y. Kiyozumi, J. Chem. Soc. Faraday Trans., 1998, in press.7. "Acid-Base Interaction on the Viewpoint of Molecular Clustering: Effects of Solvent, pKa and Size of Alkyl Group," S. Mochizuki, Y. Usui and A. Wakisaka, J. Chem. Soc. Faraday Trans., 1998, in press.
— 55 —
.2.2.3Process of Cluster Formation in Liquid Droplet Expansion
Hidetoshi Sugahara. Munetaka Nakata, and Harutoshi Takeo*
Graduate School of Bio-Applications and System Engineering (BASE) Tokyo University of Agriculture and Technology Saiwai-cho 3-5-8, Fuchu-shi, Tokyo 183, Japan
and*National Institute for Advanced Interdisciplinary Research (NAIR)
IntroductionAn instrument to form clusters from liquids by an adiabatic expansion of a droplet into
vacuum have been developed by Prof. Nishi [1] and used to investigate the properties of liquid and solution [2]. At NAIR, this method have been improved and applied for various solution to understand the molecular interactions in solution [3]. However, basic question that how clusters are formed in this instrument have not been clearly answered yet. In some cases the results deduced by this experiments contradict those established in solution chemistry. In this study we observed the cluster formation from several pure liquids and solutions of two components system to understand the process of cluster formation occurring in this method.
ExperimentalCluster formations were observed from various liquids. They can be roughly classified into
three categories of interaction as hydrogen bonding, water (H2O), methanol (CHsOH), ethanol (C2H5OH), formic acid (HCOOH), as dipolar interaction, acetonitrile (CHsCN), ethyl cyanide (CH3CH2CN), dimethyl sulfoxide ((CH3)2SO), and as van der Waals interaction, benzene (C6H6) and carbon tetrachloride (CCU). The clusters with relatively larger size were observed only from H2O and CH3CN pure liquids. Up to hexamers and timers are found for CH30H and C2H5OH, respectively, timers for C2H5CN, and no clusters was found for C6H6, CCU and ((CH3)2SO) in our experiment even if the temperatures of nozzle, without heating the nozzle liquid can not be injected into vacuum chamber, were lowered as possible. Since the existence of dimer of formic acid is well known in the gas phase, the result that only a weak signal of dimers and no signal more than timer was observed for HCOOH was a great surprise
for US.
Results and DiscussionAbove results can be explained as follows. A droplet can be taken part into pieces to
molecule by adiabatic expansion into vacuum if no effective cooling process occurs during expansion such as the cases in weakly interacting liquids, C6H6 or CCU. However for strongly interacting cases such as H2O, the evaporation of molecules from droplet can cool down the rest of droplet effectively resulting in the stabilization of clusters. This effect seems larger in case of CH3CN than C2H5CN or ((CH3)2SO), although their dipole moments are very similar.
To confirm above explanation the cluster formation of several molecules from acetonitrile solution have been performed expecting the stabilization of clusters with solute molecule by the use of cooling effect of evaporation of solvent molecule (CH3CN). The formic acid, benzene, phenol (C6H5OH), and imidazole (C3H4N2) in acetonitrile solution were injected into the
—56 —
chamber and the cluster distributions were observed by changing their concentrations. In general, when concentration of solute is small, the clusters of pure acetonitrile, clusters of those with one solute molecule have been observed. For higher concentration, the numbers of solute molecule in the acetonitrile clusters increased as expected. However, when concentration is very large, 85% of HCOOH and 15% of CH3CN, no clusters (even pure clusters of CH3CN) have been observed indicating the dependence of cooling effect of evaporation of molecule on concentration.No clusters without solvent molecules have been observed for any solutes except for imidazole.
In Fig. 1 observed spectrum of the clusters of imidazole molecules from acetonitrile solution is shown. At relatively low nozzle temperature, clusters of mixture of acetonitrile and imidazole were observed. When the temperature of nozzle was increased clusters of imidazole molecules without acetonitrile as shown in Fig. 1 were observed. This reflects the "easier evaporation of solvent molecules at higher temperature and a relatively stronger interaction among imidazole molecules.
mlz
Fig 1. Mass spectrrum of the clusters from imidazole-aetonitrile mixed solutions. Molar ratio of imidazoleraetonitrile = 1:100.
[References][1] N. Nishi&K. Yamamoto, J.Am.Chem.Soc.,10 9,7353-7361 (1987).[2] N. Nishi, S. Takahashi, M. Matsumoto, A. Tanaka, K. Muraya, T. Takamuku &
T. Yamaguchi, J.Phys.Chem., 99,462-468 (1995): A. Wakisaka, Y. Shimizu, N Nishi,K. Tokumaru&H. Sakuragi, J.Chem.Soc.FARADAY trans., 88, 1129-1135 (1992).
[3] A.Wakisaka and Y.Yamamoto, J.Chem. Soc. Chem. Commun., 2105-2106 1994.
-57-
2.3 h ]) —
2.3.1
Structure and Reactivity of Small Metal Clusters (l-10nm) by HRTEMJ. Urban. H. Sack-Kongehl and K. Weiss
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department ofInorganic Chemistry
Faradayweg 4-6, D-14195 Berlin, Germany
Introduction: Lately, small metal particles have become of interest because they
represent a transition from the atomic /molecular to the solid state. They also serve as
model states for the interpretation of heterogeneous catalysis. The present studies were
originally focused on pure Ag, Au, Pd and Ag/Au binary clusters and have now been
extended to more reactive Cu clusters and their oxidation as a function of size and
structure. These investigations will be the subject of the present representation.
Theoretical work on very small clusters has come to describe magic numbers of the
shell model1. Molecular dynamics calculations2 describe structures of larger clusters
with different magic numbers, which have been the focus of interest in the present
experimental study. The latter mentioned structures are mainly influenced by a
geometrical arrangement of the atoms rather than changes in the electronic band
structures as obtained for the smaller clusters consisting of only up to 20 atoms which is
reflected in the appearance of magic numbers from the atomic shell model.
Experimental: The inert gas aggregation technique with argon of partial pressures
between 0.1 and lmbar as aggregation gas was adopted to prepare Cu clusters in the size
range between 1 and 10 nm diameter3. Preparations were also performed by adding pure
O2 with partial pressures between 10"3 and 10"1 mbar to the argon during the evaporation
from the Knudsen cell. After deposition of the clusters on amorphous carbon films of
about 3 nm thickness the samples were transferred to the electron microscope (Philips
CM200 FEG, Cs=1.35 mm, 200 kV, resolution better than .18nm) under argon as
protection gas. The transfer system served also as reaction chamber. The samples were
exposed to air for different time periods after having been inspected in the microscope.
After digitisation of the obtained micrographs in pixel sizes of about 0.03 nm and image
processing, structural characterisations were performed by interpreting the calculated
power spectra (PS, square of the Fourier transform of the images).
—58 —
Results and discussion: The particle size distributions typically showed the shape of
normal distributions. Usually, the maximum of the distributions was centred at 4.5 nm.
However, this could be influenced by variation of the gas pressure and the temperature.
1. Preparation without 0%: Pure Cu clusters of diameters less than 5 nm always showed
the structures of multiply twinned particles (MTP), i.e., particles with 5-fold symmetries
such as icosahedra and decahedra. MTPs have also been observed for other materials
such as Ag, Au and Pd. The particles showed small lattice dilatations compared to the
bulk. The dilatations cause two different bond lengths in the MTPs and are purely
structure-dependent when introducing strain to the deformed tetrahedral subunits. The
introduced strain was often distributed uniformly over the subunits but it was sometimes
concentrated in a small number of subunits. Larger clusters, i.e., larger than 5 nm
diameter, always showed the fee bulk structure with cuboctahedral morphology without
any change in the lattice constant compared to the bulk. After 100 h of air exposure at
room temperature, both cluster types were fully oxidised. They always showed the
cuprite structure of Cu^O. However, for shorter air exposures many intermediate states
were observed exhibiting both the Cu and oxide lattice parameter.
2. Preparation with 0%: Preparation with 0% of different partial pressures resulted in
structures consisting of Cu in coexistence with oxides which repeatedly could not be
identified. They always showed heavy distortions. Step dislocations could be viewed at
the interfaces between different regions of Cu and oxide after image processing. The
particles may be interpreted as sub-oxides. Among well-known oxides as CuzO cuprite
and CuO tenorite, there exist structures as paramelaconite CU4O3 and sub-oxides as
CU4O, CugO, and Cu^O. However, also cubic fee structures with pure Cu lattice
constants were observed even for particle diameters less than 5 nm. We believe that a
small quantity of oxygen not detectable by HRTEM stabilises the fee structure. The
reactivity of these samples was less than for the preparations without oxygen. Even after
100 h of air exposure both structures for pure Cu and oxide were observed.
References
'W. Eckardt, Phys. Rev. B36,4483 (1987)
2S. Valkealahti and M. Manninen, Phys. Rev. B54, 9459 (1992)
3J. Urban, H. Sack-Kongehl and K. Weiss, High Temperature and Materials Science 36,
155 (1997).
-59-
Stable bimetallic icosahedrons formed by annealing
K. Koga. D. K. Saha, and H. Taken
National Institute for Advanced Interdisciplinary Research (NAIR), I -I -4 Higashi,Tsukuba 305, Japan
It has been known that the metallic clusters composed of single elements, such as
Au, Ag, have often shown multiply-twinned (MT) decahedral (Dh) and icosahedral (/c)
structures. Researchers have commonly observed the mixture of different structures in the
deposit of clusters, because the particle formation was achieved under non-equilibrium
condition, i.e. coalescence, high cooling rate, etc. Recently, one sort of stability test for
Au clusters has been done by annealing them in the gas phase, and their structures have
been found to be transformed from MT into the face-centered cubic (Fee) structure,
suggesting that the MT forms of Au clusters were metastable.fi] However,'this sort of
experimental studies to seek the most stable form of metallic clusters have been done in
few cases. We have performed the present study to obtain the stable structure for Au-Cu
bimetallic clusters supported on amorphous carbon film.
The technique used to form, deposit and anneal the clusters involves vaporization
from alloy ingot, formation of clusters with the He gas and deposition on an amorphous
carbon film equipped with an in situ heat treatment mechanism. Bimetallic Au-Cu clusters
with an average concentration of 25 at.% Cu were formed from an Au-10 at.% Cu alloy
ingot, where the variation of concentration between the ingot and the clusters was due to
the different vapor pressures of two elements. The bimetallic clusters were deposited on
an amorphous carbon film at room temperature. Subsequently, the heat treatment was
performed at 723 K for 1 h by a halogen lamp irradiation under vacuum and then slowly
cooled down to room temperature for 5 h. The amorphous carbon film was then
transferred to the high-resolution transmission electron microscope (HRTEM). The same
procedures were also performed for the other samples.
2.3.2
60—
HRTEM observations were performed on both the as-deposited and annealed Au-
25 at.% Cu clusters. In case of the as-deposited clusters, it was difficult to understand
their structures, because most of the clusters were found in peculiar shape without
showing clear structural patterns, although several clusters showed Ic structures along 3-
and 2-fold axes. Contrary to the as-deposited clusters, clear Ic structures along 5-, 3- and
2-fold axes were observed for most of the annealed clusters in the diameter range of 1-6
nm. The observed structures showed the similar images to those obtained by the
multislice calculation HRTEM images for the Au /c.[2] Above the diameter of 6 nm, the
bimetallic clusters were found to be Fee structures, either single-domain, multi-domain,
twinned or distorted structures, indicating bulk nature.
The annealing experiments were done also for single-component clusters of Au
and Cu separately under the same conditions as of the bimetallic one. However, the
formation rate of Ic was found to be low for the both elements even after the annealing.
These facts indicate that neither Au nor Cu individually can form the Ic structure. We also
performed the same experiments for Au-Cu clusters with different average concentrations
as of 13,39,57,75, and 88 at.% Cu. The formation rate of Ic after annealing was found
to be decreased with decreasing or increasing the Cu content from 25 at.%. It is,
therefore, obvious that the special Cu fraction made the Ic arrangement stable.
The origin of the stable Ic configuration in the Au-Cu bimetallic state might not be
easily fixable without any theoretical calculations. However, the geometrical reason for
the present fact is probably found in the Ic arrangement which requires 5 % longer
tangential interatomic distances than radial ones, where the strain energy increases
dominantly with increasing the cluster size. The both factors of the different atomic radius
between Au and Cu (~11%) and the special Cu content might work to relax the local
strains most effectively. Theoretical studies are strongly necessary for full understanding. 1
[1] A. N. Patil, D. Y. Paithankar, N. Otsuka, R. P. Andres, Z. Phys. D 26, 135 (1993).
[2] J. Urban, H. Sack-Kongehl, and K. Weiss, Z. Phys. D 28, 247 (1993).
—61 —
CATALYTIC PROPERTIES OF GOLD CLUSTERS DEPOSITED
ON METAL OXIDES
Masatake Haruta
Osaka National Research institute, AIST, MITI
Midorigaoka 1-8-31, Ikeda 563, Japan
Although gold has been regarded as poorly active as a catalyst, its
chemistry dramatically changes when deposited on select metal oxides as
nanoparticles or clusters. The effective methods for depositing gold with high
dispersion on metal oxides are coprecipitation, deposition-precipitation, co
sputtering, and grafting of organogold complex [1],
We have found that gold nanoparticles and clusters are characterized by
the following four features in their catalytic nature. The first one is that
catalytic properties can be controlled by the appropriate selection of support
metal oxides [1], Metal oxides active for oxidation usually give active gold
catalysts for the complete oxidation of hydrocarbons; Co304 is a typical
example. The oxidation of CO can be catalyzed even at a temperature as low
as -77°C when metal oxides except for strongly acidic ones (Al203-Si02 for
example), and the hydroxides of Be and Mg are used as the support. Only
Ti02 makes gold very selective to the partial oxidation of C3-C4 hydrocarbons in
the copresence of oxygen and hydrogen; propylene oxide from propylene,
acetone from propane, tert-butanol from iso-butane [2], In hydrogenation,
Zr02 provides gold with preference for C=0 bonds over C=C bonds and with
100% selectivity to monoalkenesfrom dienes and alkynes [3], Zinc oxide acts
as an effective support for gold to produce methanol from carbon oxides. For
NO reduction with hydrocarbons in the presence of excess 02 and moisture,
Al203 gives the highest conversion to N2.
The second is that the control of the size of gold particles presents
another measure to create and tune the activity and selectivity. In CO
2.3.3
—62 —
oxidation, turnover frequency (TOP) based on the number of gold atoms
exposed to the surface for Au/Ti02, Au/Fe203., and Au/Co304 markedly increases
with a decrease in the diameter of Au particles. It is interesting that in the
cases of BefOH^ and Mg(OH)2 supports surprisingly high catalyticactivityat
-77°C is obtained only when gold is dispersed as small clusters with a size not
larger than 1.2nm. An increase in TOP with a derease in the size of Au particles
was also observed for Au/ZnO in methanol synthesis. In the reaction of
propylene with oxygen and hydrogen, the switching-over of the main producttakes place from propylene oxide to propane when Au loading is decreased
from 1wt% to 0.05wt%. This phenomenon can be related to the difference in
the diameter of Au particles with the critical size of 2nm and indicates that gold
behaves like platinum in the presence of oxygen when it is minimized down to
clusters composed of less than 200 atoms.
The third is that a strong contact between hemispherical gold particles
and the metal oxide support with long distance around perimeter interface is
indispensable to obtain the above unique catalytic performances. This feature
explain why preparation methods and conditions are crucial to the active and
selective gold catalysts.
The fourth is that moisture often promotes the activity of supported gold
catalysts. This nature is very advantageous in the applications of gold
catalysts to air purification and exhaust gas treatments and has really drove
them to commercial use for oxidation of the odorous compounds (mainly
amines) within the toilette.
REFERENCES[1] M. Haruta.Catal.Today, 36 (1997) 153 and Catal.Surveys Jpn., 1 (1997) 61.
[2] T. Hayashi and M. Haruta.Shokubai (Catalyst),37(1995) 72 and J. Catal.,
accepted.
[3] M. Shibata, N. Kawata, et'al., J. Chem. Soc., Chem. Commun., (1988) 154.
-63-
2.3.4 -Au Multiply-Twinned Particle Micelle
TOMOHTOE TAKAMI AND SHOZO INO*
Research Institute for Scientific Measurements, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-0812, Japan
takami @ rism. tohoku. ac.jp *: Faculty of Engineering, Utsunomiya University,
Ishii-cho 2753, Utsunomiya 321, Japan
In 1966 Ino discovered multiply-twinned particle (MTP) by Au evaporation on NaCl (100) surface. [1] Nanometer-size particle is near borderline between solid and gas, and the property is interested in basic science. However, for the application, the problem is the stability of the particles; they are easy to be sintered when they meet each other.
Recently we developed a method to prevent the Au MTP from sintering by using 3- mercaptopropionicacid (HSCH2CH2COOH) [2]. Figure 1 shows the schematic drawing of the aggregate, and we named it MTP-micelle.
Figure 2 shows the transmission electron microscope (TEM) images of the core Au MTPs of the MTP-micelles. From TEM images we concluded that the average size of the particles in this study was 70 A and most of the core particles were MTPs. The high- resolved TEM images of the MTPs with two types of the structures, icosahedrion (A) and pentagonal decahedron (B), are shown in Fig. 2. From a dark field TEM image, shown in Fig. 4, and high-resolved TEM images we found that some particles had the structure of truncated icosahedron which is a soccerball-like structure.
The proton NMR spectrum of MTP-micelle (Fig. 5), compared with those of 3-mercapto- propionic acid (HSCH2CH2COOH) and 3,3'-dithiodipropionic acid (HOOCCH2CH2S- S CH2CH2COOH), indicates that the thiolates formed disulfide and chemisorbed on the Au MTP (formed a kind of coordinate bonds, presumably). [3] The He I Photoelectron spectra (UPS) and Penning ionization electron spectra (PIES) indicate that the Au MTPs are mostly covered with the thiolates. [4]
Here we have demonstrated an application; forming Au monolayer on Si(lll) surface using the Au MTP-micelle (WITHOUT using vacuum evaporation technique). A drop of the Au MTP-micelle aqueous solution was put on a Si(lll) wafer and dried in air. After introduced into a vacuum chamber the sample was flashed at 900°C for 0.5 sec, then the reflection high-energy electron diffraction (RHEED) pattern shows 6 x6 (Fig. 6) which means that the Si(lll) surface is covered with Au at 1 monolayer. [5] This application method was also comfirmed by UPS, PIES, and X-ray photoelectron spectra (XPS). [4]
References:[1] S.Ino: J. Phys. Soc. Japan, 21 (1966) 346.[2] T. Takami and S. Ino: Surf. Rev.Lett., in press.[3] T. Takami, K. -i. Sugiura, Y. Sakata, T. Takeuchi, and S. Ino: Appl. Surf. Sci., in press.[4] T. Takami, M. Brause, D. Ochs, W. Maus-Friedrichs, V. Kempter, and S. Ino:
Surf. Sci., in press.[5] T. Takami and S. Ino: Jpn. J. Appl. Phys., 36 (1997) L815.
—64—
Multiply-Twinned Particle MicelleAu multiply-twinned particle
disulfide-(S-(CH2)n-X)2,
Q-oh X: tail group
Fig.4: A dark field TEM image of Au MTP-micelles. Two particles having trancated icosahedron structure were observed (circles)
Fig.l: Schematic view of MTP-micelle in this study
solution:D2O
H2O
JUV
MTP-micelle
DSS DSSI I l I j I IT 1 | I I 1 I | I I I l | I I | 1 l'1
.4321 0SH / ppm
Fig.5: *H NMR spectrum of Au MTP-micelle. DSS (2,2-Dimethyl-2-silapentane-5-sulfonate) is a reference material for NMR
Fig.2: TEM image of Au MTP-micelles (Acceleration voltage: 200 kV)
A: icosahedron B: pentagonal decahedron
H50 AHFig.3: Two types of MTP-micelles on TEM image
Fig.6: Si(lll) 6X6-Au RHEED pattern formed by using MTP-micelles
—65 —
CO INTERACTION WITH SIZE-SELECTED, SUPPORTED NICKEL CLUSTERS ON THIN MGO-FILM
U. Heiz F. Vanolli, and W.D. Schneider
Universite de Lausanne, Institut de Physique Experimental, 1015 Lausanne,Switzerland
Particles of nanometer-size (nanoparticles) supported on well-characterized ox
ide surfaces are of particular interest to model the high complexity of real catalysts
in order to answer questions such as the role of intrinsic size effects or the influence
of the support. Model systems so far consisted of size-distributed nanoparticles
deposited on oxide substrates.' Here we demonstrate based on a specific example,
the possibility to observe simple chemical reactions on size-selected and soft-landed
clusters regardless of size and material by using infrared and thermal desorption
spectroscopies.. We investigated the chemical reactivity of monodispersed nickel
clusters, which were deposited with almost thermal energy on thin MgO(.lOO) films.
The thin MgO(lOO) films were epitaxially grown on a Mo(100) single crystal. These
films exhibit properties very similar to bulk magnesia. In particular, we were able
to observe a size dependence of the elementary steps (adsorption, dissociation, and recombination) of the reaction of CO with monodispersed nickel clusters. Monodis
persed Niso clusters show a higher reactivity for CO dissociation than Nin and Nigo-
In particular, each Niso cluster is able to dissociate up to 10 CO molecules at temper
atures below 280 K, whereas the other two sizes dissociate only one of the adsorbed
CO molecules at slightly higher temperatures. Our results demonstrate that such
small clusters are unique for catalytic reactions not only due to their high surface-to-
volume ratio but essentially because of the distinctive properties of different cluster
sizes.
In a second example size-selected Nin clusters were deposited with low en
ergy ( ~ 0.2eV ) on thin MgO(lOO) films. A probe molecule, carbon monoxide,
serves to characterize the supported nickel cluster. From thermal desorption spec
troscopy an average number of five adsorbed CO on each cluster is determined,
where 4 are bonded molecularly. In addition, using an isotopic mixture of 12CO and
13C0, infrared spectroscopy reveals the existence of a vibrational coupling inter
action between the neighboring, molecularly bonded CO’s. A semi-classical model
of interacting dipoles describes well the observed vibrational shifts, when assuming CO-CO distances of 3-5 A. This implies that the four CO molecules are bonded to
neighboring nickel atoms suggesting a three dimensional structure with four nickel
2.3.5
—66 —
atoms exposed at the surface of the cluster.
We have shown that by varying the cluster size from Nin to Niso the average
number of dissociated CO on each active site can be changed deliberately. Using
these activated atoms for further reactions may result in a high selectivity of a
catalyzed reaction at relatively low temperature. The individual character of each
cluster composed of a fixed number of atoms dominates these size effects. Theproposed three dimensional structure of Nin indicates that the varying chemical
reactivity is not due to a transition from a 2- to 3-dimensional cluster structure in
going to larger clusters. But also, as the comparison with free clusters indicates,
the substrate plays a significant role in changing the bonding properties of such
small supported metal clusters. Thus the use of size-selected, supported clusters
on thin oxide films is particularly promising for the investigation of size dependen
cies of simple chemical reactions on small particles, which is an important issue in
heterogeneous catalysis.
—67 —
Mesoporous Silicates Synthesis under Low Temperature and Acidic Medium Condition
and its Chemical Modification
Jun Izumi*l, Akinori Yasutake*!, Nariyuki*! Tomonaga*!,
Yukako M. Setoguchi*2, Isamu Moriguchi*2, Yasutake Teraoka*2, Syuichi Kagawa*2
*1 Mitsubishi Heavy Industries, Ltd. Nagasaki R&D Center ([email protected]. co.jp)
*2Department of Applied Chemistry, Faculty of Engineering, Nagasaki University
Mesoporous silicate is a kind of alumino-silicate compound of which the pore diameter is 10-lOOA. Kuroda and Inagaki, et al. used a layer structured clay compound (kanemite)
as a silica source, they intercalated cationic surfactant to kanemite to rearrange the silica
bridge structure and first prepared the hexagonal structured mesoporous silicate. (FSM-
16) Mobil then also prepared the same structured mesoporous silicate by means of a silicic
acid dehydration-condensation reaction at high temperature in the presence of miscelle
with cationic surfactant of mono-alkyl ammonium compound. (MCM- 41) Stucky
reported that the same structured mesoporous silicate could be prepared by the
dehydration-condensation reaction of mono-silicic acid, which is formed by alcoxy silane
hydration-decomposition, in the presence of miscelle with cationic surfactant at low
temperature near room temperature. Recently, we also found that the mesoporous silicate
could be synthesized with inorganic silica compound, which did not have Q3-Q4 Si-O-Si
bonds, instead of alcoxy silane, in the presence of miscelle with cationic surfactant at low
temperature and low pH value. When a mixture of aluminic acid and silicic acid was used
as a alumino-silicate source and pH-value was adjusted at a higher value(9-ll) ,
mesoporous alumino-silicate, of which the Si02/A1203 ratio was 2-oo could be also
synthesized. As these mesoporous silicates have more than twice the specific surface area
than that of conventional zeolite, it is expected to be used as adsorbents and catalystcarriers. For the evaluation of adsorption behavior of our samples, MEK, C02,H20
adsorption amounts and their adsorption rates were measured by the flow method with
small column. (0.5g loaded column) The high Si02/A1203 ratio sample showed a larger
MEK adsorption amount than that of US Y(conventional VOC adsorbent) at a higher
concentration region and the low S102/A1203 ratio sample showed a larger water vapor
adsorption amount than that of silica-gel(conventional water vapor adsorbent). It is also
confirmed that mesoporous silicates containing some kinds of organo-metal complex
show increases of 02 and NO adsorption amount. For the study of adsorption mechanism
of mesoporous silicates, its simplified model was made and the adsorption behavior was
assumed with the molecular simulation(MD, MC).
2.3.6
—68 —
2.3.7 DYNAMICS OF CLUSTERS STUDIED BY
MATRIX-ISOLATION INFRARED SPECTROSCOPY
COMBINED WITH
PULSED-NOZZLE MOLECULAR-BEAM TECHNIQUE
Munetaka Nakata
Graduate School of Bio-Applications and Systems Engineering (BASE) , Tokyo University of Agriculture and Technology,
Saiwai-cho 3-5-8, Fuchu, Tokyo 183, Japan
The matrix-isolation infrared spectroscopy is one of the ideal techniques for studies of clusters. In low-temperature surroundings made of inert rare-gas atoms, clusters are stabilized by relaxation of the excess energy and monitored by infrared spectroscopy. Vibrational analyses of the infrared spectra of clusters provide important information on their geometrical structures. On the other hand, the pulsed-nozzle molecular-beam technique is an excellent one to produce clusters. Highly sensitive laser spectroscopies remarkably develop with this technique to study dynamics of clusters. In the present study, we combine the matrix- isolation infrared spectroscopy with the pulsed-nozzle molecular-beam technique. Using the combined technique, we compare infrared spectra of HCN or NNO isolated in low-temperature argon matrices deposited through a normal effusive nozzle with those deposited through a pulsed nozzle. It is most interesting that infrared spectra of cyclic (HCN)3 and (NNO)2 were observed when the pulsed nozzle was used.
ExperimentalThe sample chamber was connected to a turbomolecular pump and
a rotary pump. Pressures in the sample chamber were typically less than 1 X 10"7 Torr after baking out at 360 K for 10 h. Traces of residual gas, such as water vapor, were monitored by a gas analyzer with a mass filter. The cryostat used was a closed cycle helium
-69-
refrigeration unit. Infrared spectra were measured by an FTIR spectrophotometer, in which the infrared beam was introduced from the spectrophotometer to the sample chamber using concave and flat mirrors. The liquid nitrogen-cooled MCT detector was also placed outside the spectrophotometer. The diameter of the pulsed nozzle used was 0.3 mm. The pulse width and the pulse interval were 0.2 ms and 0.2 Hz, respectively. The stagnation pressure was. varied between 1 to 3 atm.
ResultsWhen the NNO sample was diluted with argon gas at a ratio of
1/5000, infrared spectra of the matrix sample deposited through a normal effusive nozzle showed, bands of NNO monomer without any polymers. Similar spectra were observed by depositing the same sample through a pulsed nozzle at the stagnation pressure of 1 atom. Figure la shows the NN stretching region, where a peak of NNO'monomer appears at 2219 cm-1. When the stagnation pressure was elevated from 1 to 3 atm, infrared spectra of the same sample showed another peak at 2227 cm-1, which was assignable to NNO dimer (Fig. lb) . Then, we conclude that the situation of molecular beam can be kept in the low^temperature argon matrices to some degree.
(a) Stagnation Pressure 1 abn
lb) Stagnation Pressure 3 atm
2240 2230 2220 2210 2200WAVENUMBER /cm-'
Fig. 1. Infrared spectra of NNO in low-temperature argon matrices deposited through a pulsed nozzle at stagnation pressure of iatm (upper) or 3 atm (lower)
—70 —
THE “DESIGNER” OPTICAL RESONANCES OF METALNANOSHELLS
Naomi J. Halas. Richard D. Averitt, and Steven J. Oldenburg
Department of Electrical and Computer Engineering and the Center for Nauoscalc Science and Technology, Rice University, 6100 South Main, Houston, TX,
77251-1892, USA
2.3.8
Metal nanoshells, consisting of a dielectric nanoparticle core coated with a
metallic shell of nanometer scale thickness, are a new, composite nanoparticlc whose
optical resonances can be “designed in” in a controlled manner. By varying the rel
ative dimensions of the core and shell, the optical resonance of these nanopavlidcs
can be varied over hundreds of nanometers in wavelength, across the visible and
into the infrared region of the spectrum. We will discuss two routes to the synthesis
of metal nanoshells. The first procedure utilizes a two-step reduction, producing
Au-terminated Au%S nanoparticles. As the reaction proceeds, first the nanoshell
core, then the outer metallic layer, grow linearly as a function of time. The growth
kinetics of these nanoparticles can be followed in detail by monitoring the dimension-
dependent shifts in the plasmon resonances of the nanoparticles as growth proceeds
(R. D. Averitt, D. Sarkar, and N. J. Halas, Phys. Rev. Lett., 78, 4217 (1997)).
The second procedure involves a-general approach to the making of metal nanoshell
composite nanoparticles, based on molecular self-assembly and colloid reduction
chemistry (S. J. Oldenburg, R. D. Averitt, S. L. Westcott, and N. J. Halas, Science., submitted). This approach is adaptable to the use of a wide variety of dielectric
core and metallic shell materials. For metal nanoshells of both types, quantitative
agreement between the predicted optical properties obtained using generalized Mie
scattering theory and the experimentally obtained optical properties is observed.
The excitation dynamics of these optical resonances will also be discussed.
-71-
2.4 $nfcOTX-tD—
2.4. l The Effects of Restraints on the Radiation Induced
Chemistry of Cation Clusters and Liquid Clusters in Zeolites
G. Zhang and J. K. Thomas Department of Chemistry and Biochemistry
University of Notre Dame, Notre Dame, IN 46556, USA
Electrons and positive holes are produced in zeolites by short (2 nsec) pulses of
0.4 M electrons. The reactions of the ions with Na+ clusters in the dry zeolites and with
small (nm) liquid pools in the zeolite cavities will be discussed. Typical results are given
below.
High-energy irradiation of sodium zeolites, both short pulses of 0.4-MeV electrons
and steady-state 60Co y-rays, results in the formation of electrons trapped in Na+ clusters,
e.g., Na43+, Nag2*, and Naa"1". ESR spectroscopy is used to identify these species. Only
Na43+ is observed in zeolites X and Y at all temperatures; however, other trapped species
are observed at low temperatures in zeolite A, e.g., Nag2* and Na2+ as well as Na/"1", and in
sodalite, Na/* and Na32+. As the temperature is elevated, the smaller clusters, e.g., Na2+ in
zeolite A and Na32+ in sodalite, become less stable. The spectral absorption peaks are
~550 nm for Na43+ and -680 rim for Na32+, while the Na2+ species absorbs in the near-IR
regime (>750nm). The activation energies for trapped hole-electron (Na/"1) neutralization
are 12.58 ± 1.10 kcal/mol (433-469 K) for zeolite Y arid 7.10 ± 0.71 kcal/mol (308-363
K) for zeolite X. In zeolite A, the activation energy for trapped hole-electron (Na32+)
recombination is 15.32 ± 3.10 kcal/mol (373-421 K), and that for Na2+ is 11.13 ± 0.91
kcal/mol (253-307 K). The temperature range of the measurements is also given. The rate
constant for oxygen quenching ofNa/"1" in NaX is (3.23 ± 0.26) xlO4 Torres'1, and that of
Na32+ in zeolite A is 26.08 ± 6.42 Torres'1. The species Na43+ is located inside the
supercage of NaX and NaY and in the sodalite cage of zeolite A. In zeolite A, the other
two clusters, Na32+ and Na2+, are located in the a-cage and sodalite cages, respectively.
The G value (i.e., radiolytic yield) of Na43+ in NaY is 2.76± 0.30 for 60Co y-ray irradiation.
The study shows the great utility displayed by zeolites is providing unique sites and
environments for the creation of new chemical species.
-72-
The trapping of electrons by water clusters and the reaction of positively charged
holes in pulsed electron radiolysis of hydrated zeolites X and Y were studied using time-
resolved transient absorption spectroscopy. The fully hydrated zeolites, under 12 mbar of
water vapor, exhibit a short-lived structureless absorption band centered at 620 nm. This
is attributed to hydrated electrons confined in the 13 A supercages of the zeolites. The
band is blue-shifted by 0.28 eV relative to that of the hydrated electrons in bulk liquid
water. With the gradual removal of water molecules from the zeolite cavities, a continuous
red shift of the transient absorption spectra is observed in both zeolites X and Y. The
similarity of the spectral features of hydrated electrons in zeolites to those of water cluster
anions in the gas phase suggests that water exists in the form of clusters in the zeolite
supercages. The spectral shift with decreasing size of the water clusters presumably
demonstrates that the confinement of water by the zeolite cages on the nanometer
dimension affects solvation and electronic structures of the excess electrons. It is shown
that water clusters trap electrons more weakly as their sizes become smaller and that
cation cluster trapping sites are gradually formed during dehydration. Electron transfer
from the water cluster trapping sites to the cation cluster trapping sites is clearly observed
when the water content is decreased to ~32 water molecules per pseudocell (a' supercage
plus a sodalite cage) in zeolites X with a Si/Al ratio of 1.0. A high radiolytic yield of Ge =
5.8 is measured for the water cluster solvated electrons in fully hydrated NaY. The unique
transport of hydrated electrons in zeolite cages is understood in terms of an adiabatic
model. The reactivity of positively charged holes generated by the ionizing radiation as
geminate pairs with excess electrons is examined in both hydrated and dry zeolites.
Trapping and reactions of the positive holes with aromatic molecules and water leads to
the formation of organic radical cations and hydroxyl radicals, respectively. Essentially the
same high yield of hydroxyl radicals as that of water cluster solvated electrons is measured
in zeolite Y at the highest water content, Gqh- = 6.0. The addition reactions of OH- with
aromatic molecules included in zeolites is found to be limited by the slow diffusion of OH*
through the zeolite supercages. Time permitting, studies in other liquids such as benzene,
alkanes, and alcohols, will be presented.
73-
2.4.2
Photo excited state of Agl clusters incorporated into cages of zeolites
Tetsuya Kodaira
National Institute for Advanced Interdisciplinary Research,1-1-4 Higashi, Tsukuba, Ibaraki305, Japan
Bulk silver halides are important materials on the application of photograph
technology. They exhibit significant differences on electronic band structures and silver
ion conductivities for various sorts of halogens. In the present study, Agl clusters are
incorporated into cages of zeolite LTA and FAU. Manifestation of Agl clusters in the
cages and their fundamental electronic state are discussed on the basis of their optical data
comparing with micro and macro limit of Agl, such as monomer and bulk, respectively.
Samples were prepared as follows. To avoid the photochromism of Agl bulk,
all process was performed in a dark room. Zeolite LTA or FAU was dehydrated in
vacuum at 420°C. Distilled Agl bulk and dehydrated zeolite were sealed together in a
quartz glass tube without exposing to atmosphere. The sample glass tube was brought
into a electric furnace held at 420°C for a week, and the vapor of Agl is adsorbed to the
zeolite powder. Diffused reflection spectra were measured at room temperature or 77 K,
and the optical absorption spectra were derived from them by using Kubelka-Munk
function.
Figure 1 shows the photo absorption spectra of Agl loaded Na form LTA at
room temperature. Loading density of Agl molecule per a-cage is 4.1. Several
absorption peaks or shoulders are seen in the spectra at 4.1, 4.6 and 5.5 eV. The energy
of lowest absorption band at 4.1 eV is quite large compared with lowest photo excitation
state of bulk Agl which is 2.9 eV. It is well known that if the size of semiconductor is
reduced, absorption edge shifts to the higher energy side compared with the original bulk.
This phenomenon can be explained qualitatively by the model of the quantum confinement
of photo excited electron-hole pair which is called exciton. With increasing the
confinement of exciton by reducing the size of semiconductor fine particle or cluster,
kinetic energy of both electron and hole increases. As a result, energy difference
between electron and hole increases. This is observed as the blue shift of absorption
—74 —
edge. If these Agl clusters in the cages are homogeneous in their size, they will be
(Agl)4 clusters, because the loading density of Agl is ca. four per a-cage. The site of
the clusters seems to be in a-cage as discussed later.
X-ray powder diffraction (XRD) pattern of dehydrated Na-LTA and Agl
included one are also measured. Comparing with dehydrated Na-LTA, several extra
new reflections are observed in the pattern of Agl loaded Na-LTA. The newly appeared
reflections can not be explained by considering the bulk Agl structure. As well as the
optical data, obtained XRD pattern supports that Agl is adsorbed into the cages and form
anew arrangement.The stabilized cluster seems to exist in a-cage of LTA, because Agl is adsorbed
into the cages through the open eight-membered ring. On the contrary, six-membered
ring is capped by an Na+ cation, and this cation seems to interrupt the entrance of Agl
molecule into P-cage.
In this Agl loaded sample, existence of photochromism is checked. White light
of Xe-lamp is irradiated at room temperature and liquid nitrogen temperature. However,
no change in the absorption spectrum is observed in both temperature. The reason is not
well known yet. Photon density may be not enough for inducing the dissociation of Ag
and I, or relaxation process from photo-excitation state may be quite different from Agl
bulk because of the stabilization of Agl cluster in the cage.In the present lecture,
photo excited state of Agl clusters
in FAU is also discussed on the
basis of optical data of samples
where Agl loading density is
systematically changed and alkali-
metal cations in the space of
framework are changed from Na+
to K+ and Cs+. Through these
experiments, it is found that alkali-
metal cations play as an electron
acceptor at photo-excited state.
; AglZNa-LTA(l) RT
Photon Energy (eV)
Figure 1
—75 —
2.4.3METAL AND METAL OXIDE CLUSTERS IN ZEOLITES: GROWTH
AND OPTO ELECTRONIC PROPERTIES
Michael Wark. Gunter Schulz-Ekloff
Institut fur Angewandte und Physikalische Chemie,Universitat Bremen, 28334 Bremen, Germany
Highly dispersed metal or semiconductor nanoparticles stabilized by meso- and
nanoporous solid supports represent a promising class of materials, e.g. as nanoscale
devices in microsystems. Besides this zeolite-supported metal and metal oxide dis
persions find applications as catalysts and represent model systems for the study of
the influence (i) of the particle size and (ii) of the properties of the support on the
electronic structure and on the reactivity of the metal and metal oxide clusters.Stoichiometries and kinetics of the autoreduction and the simultaneous metal
aggregation are thoroughly studied for faujasite hosted ammine complexes of no
ble metals (Pt, Pd, Rh, Ir) using temperature programs and monitoring (i) the
gaseous products by mass spectrometry and (ii) the metal aggregation by X-ray
diffraction and electron microscopy. The autocatalytic reductions start from the
complexes located in super cage sites, which can be unambiguously elucidated by
Rietveld refinement of X-ray diffractograms. The growth of the metal crystals, their
preferential location, the simultaneous fragmentation of the zeolite framework and
the final host-guest orientation-relationship are revealed by transmission electron
microscopy and electron diffraction. The growth of metal particles does not require
nuclei of critical size and can be simulated by an atom capture model. The sinter
ing is dominated by Ostwald ripening depending on the heat of evaporation. The
morphology of the metal particles exhibits a transition from rounded shapes below a
diameter of 2 nm to well-developed crystal shapes with atomically smooth fee {111}
facets above this size. Particles above 2 nm grow in an orientation relationship withthe zeolite lattice.
The dependence of the change of the electronic structure of zeolite-hosted metal
particles on the average particle size, which can be determined with standard devia
tions of 10 - 20%, can be monitored by the shifts of the electron binding energies in
core level and valence-band spectra. The particle-size-dependent shift of the binding
energies indicates a negligible influence of the relaxation effect with reference to the
electrostatic field in the zeolite matrix. Metal clusters of supercage size exhibit a
reduced density of electronic states at the Fermi energy, which is expected theore
tically, i.e., they show non-metallic behaviour in NMR experiments indicating the
spacing of the one-electron energy levels.
The reactivity of the metal clusters with CO depends sensitively on the acid
—76 —
ity/basicity of the zeolite host and can result either in ready reoxidation of the
zerovalent metal (Rh, Ir) clusters in an acidic host or in the formation of stable
multinuclear carbonyls in a basic framework (Ir,Pt). Low-temperature water-gas
shift conversion contributes to the redox processes. Density functional model clus
ter studies confirm the concept of an electron-deficient state of highly dispersed
transition-metal clusters in acidic zeolites and the experimentally found shifts for the CO stretching frequencies. Negatively charged Chini complexes, i.e., [Pt3(CO)6]12~
chains, are observed only in basic faujasite structures, up to now.
A particle size effect in the CO hydrogenation is found revealing an increasing
selectivity for the formation of methanol from synthesis gas with decreasing size of
the noble metal cluster.
The electric and optical properties of semiconductor metal oxide nanoparti
cles, stabilized in the pores of zeolitic hosts, are determined by their sizes, and by
guest/guest and host/guest interactions.
Their characterization succeeded by a combination of UV/Vis-, X-ray photo
electron-, impedance- and MAS-NMR-spectroscopy. Shifts of the absorption edges
in UV/Vis spectra occur due to quantum size effects and due to non-stoichiometries
in the clusters resulting from Coulomb interactions with the host. The latter are
expressed by increasing the binding energies for the core level electrons of the metal
atoms and line broadening of NMR signals.Between nearly stoichiometric neighbouring Sn03 nanoparticles embedded in
zeolites a local charge transport is performed by an electron hopping mechanism,
which is hindered under reductive atmospheres due to the formation of defect sites
acting as hopping barriers.
The redox properties of the metal oxide nanoparticles, which can be followed
by changes in the intensities of absorption bands, depend on their nucleation and
their coordination to the matrix.In general, the reduction rates decrease with increasing nuclearity of the clus
ters. The response times of zeolite supported Sn02 and Ti02 nanoparticles towards
reduction are shortened by factors of 10-50 in comparison to the bulk oxides pro
viding a basis for an application as optical sensor material.
For titanium oxide species the reduction rates increase with the functionality
of the bonding to silanol groups of the matrix leading to an increasing deviation
from the octahedral symmetry of the oxygen coordination sphere indicated also in
an increasing excitation energy of the 0 —> Ti charge transfer.
In the selective catalytic reduction (SCR) of NO by NH3 with vanadium modi
fied ZSM-5 zeolites the activity decreases with increasing nucleation of the clusters;
highest catalytic activities had been found for mononuclear vanadyl cations.
-77-
2.5
2.5.1
Studies on Structure Formation Process by Molecular Dynamics
Tamio IKESHOJI
Cluster Science GroupNational Institute for Advanced Interdisciplinary Research,
1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan ■
Introduction
Structures of either molecules, clusters, or crystals change through energy transfer on a
potential surface of each system. In molecules, it is relatively simple because of smaller degree of
freedom. In crystals, many-body effect is important. Clusters are between them. It may be
quantitatively possible to analyze for clusters how energy is transferred from an atom (or
molecule) to another one to induce the structure change. For this purpose, we have done mainly
two kinds of work: one is molecular dynamics (MD) simulation of formation processes of magic
number clusters to know the structure formation process and another is boundary driven non-
equilibrium molecular dynamics (NEMD) to analyze energy transfer. Along this work, some ab-
initio work was also done, since electronic states make the potential surfaces.
LJ clusters vs. ionic clusters
We could find that evaporation process plays an important role to produce special
structures of clusters (clusters of certain members, which are called magic numbers) of Lennard-
Jones potential (LJ), in which potential energy difference of magic number clusters from others is
small. [I] Aggregation process does not give magic number clusters. [2] In contrast with this,
-78-
water anion clusters of certain numbers which are experimentally observed (or not observed) are
easily formed from small number of clusters with attachment of an electron by path-integral MD
for electron and classical one for water molecules. In case of water anion, there is a big energy
difference between anion clusters of 3, 4, and 5 water molecules and other size clusters. These
water anion clusters (3, 4, 5 members) are not energetically stable. Therefore, aggregation
process can produce predominantly energetically stable clusters but can not selectively produce the
certain size or structure clusters from relatively same energy clusters.
Importance of the evaporation process to produce structures of LJ clusters were also
confirmed by fcc-icosahedron transition. It was experimentally observed to be around 1500 atom
clusters for Ar according to a literature. Molecular dynamics simulation with evaporation also
showed almost the same number, though MD at 0 K gave much larger number.
Non-equilibrium molecular dynamics
NEMD [3] can be used to analyze evaporation process at the liquid/gas interface. It
showed enhanced entropy production and potential energy flux at or near the interface. [3,4]
These observation seems to relate to the evaporation/condensation processes. The detailed
analysis is now going on.
Papers related to this work[1] T. Ikeshoji, B. Hafskjold, Y. Hashi, and Y. Kawazoe, Phys. Rev. Lett., 7 6, 1792-1795 (1996)
[2] T. Ikeshoji, B. Hafskjold, Y. Hashi, and Y. Kawazoe, J. Chem. Phys., 105, 5126-5137 (1996)
[3] T. Ikeshoji and B. Hafskjold, Molec. Phys., 81,251-261 (1994)
[4] B. Hafskjold andT. Ikeshoji, Molec. Simul., 16, 139-150 (1996)
-79-
2.5.2 LIQUID WATER DYNAMICS; HYDROGEN BOND REARRANGEMENT, PHASE SPACE DYNAMICS AND PROTON
TRANSFER
I. Ohmine
Department of Chemistry, Faculty of Science, Nagoya University Furo-cho, Chikusa-ku, Nagoya, Japan 464-01
Water systems (liquid water and water molecular clusters) yield collective mo
tions and extensive energy fluctuations associated with hydrogen bond rearrange
ment (HBNR) dynamics. The collective motions are characterized by the distance
matrix as the intermittent local movements involving few tens water molecules. The
correlation between that these collective motions and HBNR is clearly shown by us
ing the Hamming matrix in a graph theory.
Due to these intermittent collective motions associated with HBNR, the water
systems exhibit so called 1 /f spectra for various time correlation functions. Scalar
quantities, such as the energy fluctuation and a volume change of the Volonois poly
gon of a water molecule, and tensor quantities such as the polarizability change
appears in Raman profile yield spectra of a so called 1/f type. On the other hand
the vector quantities such as the dielectric relaxation is of a Debye type. Direct
measurement of the collective motions has been difficult, because that the funda
mental hydrogen bond network (HBN) structure changes are buried in the noise of
fast molecular vibrational motions. A possible experimental technique which can
measure it is the 5-th order nonlinear photon echo experiment since this experimen
tal method is directly related to the phase space dynamics of liquid water system,
such as Liapunov exponent. We have examine the prospect of this method and its
limit.
We also examine the proton transfer in a water molecular cluster in order to
investigate how the water HBNR dynamics puts effects on chemical reactions. It
is shown that the HBNR creating 3 coordinated water molecules in the system is a
crucial element for the proton transfer.
-80-
2.5.3Theory of Fullerene: A Statistical Model for Growth
Haruo YoshidaNational Institute for Advanced Interdisciplinary Research, Higashi 1 -1 -4, Tsukuba
Ibaraki 305, Japan
The creation mechanism of fullerene is one of the most important theme in cluster science. There have been proposed many models[l]. They have not treated the phenomena statistically. Therefore, it is not clear that the proposed mechanism is major one or not. And the characteristic production ratio of various isomers of fullerenes have not be explained by such models.
It is successfully deduced that the essential information of statistical growth mechanism of fullerenes without treating carbon atoms of which behavior includes miscellaneous information. The creation probability P(Cn) of the network of the fullerene Cn, is obtained as the expression of P and e by the present model. Where, P and e are fundamental parameters of stochastic process: the formation probability of the 5- membered ring that connects with a 5-membered ring in intermediate is given as R=P+ e (Fig.l), and P is the formation probability of the other 5-membered ring. And isolated pentagon rule (DPR) is assumed.(1) Quenched system (production in a buffer gas at temperature not so far from RT)
The expression P(C60) and P(C?o) obtained by the present model are substituted into the equation P(C? o)/P(C6 o)=0.1; the value 0.1 is the typical one reported by many experiments [2], The equation gives a relationship between P and e , e = e 01(P). The R=R+ e 01(P) is ~0.9, for any P in the almost whole region of P, 0.67<P<0.92; in
short, P(C70)/P(C60) has the value of ~0.1, if only the connected pentagon is preferably formed (R~0.9), independently of the formation probability of the other unconnected pentagon. The production of Ceo and C?o are, therefore, in a sense, easily to be in the
ratio 10:1. The higher fullerene C?6 has two isomers satisfying DPR (Fig.2). Under the condition e = e0i(P),P(C76(Td))/P(C76(D2)) <~1%, for e >-0.2 (PC0.7). (1)The Td-isomer is hard to create as compared with the Dz-isomer in the sense of the theory
of probability. This result supports the experimental that Td-isomer has not been observedyet [3]. For C84 fullerene (Fig.3), the present model gives P(C84(D2-(22))) / P(C84(D2d-(23)))~2.3, at P—0.7, R—0.9. (2)The ratio shows a good coincidence with the experimental value, —2 [4], Furthermore,
for C86 fullerene, two C2-symmetry isomers have been observed in 13C NMR spectrum
in the ratio about 4:1 [5]. The two isomers are identified for the first time by the present theory. The obtained expressions of creation probability give P(C86(C2-(2))) /P(C86(C2-(5))) - 4.0, at P-0.7, R-0.9. (3)The structure of C2-(2) and C2-(5) isomer are shown in Fig.4.(2) Annealed system at various temperatures (850-1200°Q
T. Wakabayashi et al. have reported that fractional change in the yield of various isomers of fullerenes strongly depend on temperature of a buffer gas [6]. Their experimental results about D3,C2v, and C'2v-isomer of C78 fullerene are investigated by
—81 —
the present theory. The contribution from the statistical growth patterns started from 6- membered ring-II and IV to the creation probability of Ds isomer is dominant at 850-1050 °C; the statistical growth pattern started from 5-membered ring-1 was dominant at higher temperature than 1100°C(Fig.5). The major statistical growth patterns for Civ and C'lv isomers of C?8 fullerenes do not change at 850-1200°C, however. On the other hand, it is known that the single-step rearrangement, a so-called four-electron pyracylene rearrangement, can sequentially generate the other four isomers of C?8 fullerenes (Fig.6). The analysis of the experiment by the present theory makes it clear that no transformation
between isomers of C?8 has occurred at 850-1050°C, whereas a portion of C?8(C2v)isomer has actually transformed into C78(C'2v) isomer at 1100-1200°C. The transformation C?8(D3h)OC?8(C2v) and C78(C'2v)OC78(D'3h) are, however, negligibly small in quantity, or have not occurred.
The region on the plane of coordinates (R,P), are divided into two parts: they characterize quenched and annealed system respectively. R behaves just like the thermal
average of the variable R, <R> in the potential having a single deep minimum at R= 1. On
the other hand, P behaves just like the thermal average of the variable P, <P> in the
potential haying a shallow true minimum at P~0.3 with many quasi-stable minimums. The formation of fullerene is determined by such R and P with a sort of competing relation due to the restriction that the number of pentagon is 12 in the closed network.
It is concluded that the whole figure of fullerene growth phenomena is originated in file statistics of growth, and the topological structure and the symmetry, .of fullerene network, and the strong restriction of IPR on the formation of 5-membered ring. References[1] for example, H. Schwarz, Angew. Chem. InL Ed. Engl. 32(1993)1412.[2] for example, W. Kraetschmer et al., Nature, 347(1990)354.[3] R. Ettel et al., Nature, 353(1991)149.[4] K. Kikuchi et al., Nature, 357(1992)142.[5] Y. Achibaetal., Proc. Mater. Res. Soc., 359 (Pittsburgh, 1995)3.[6] T. Wakabayashi et al., Z. Phys. D 40(1997)414.
R=P+ e '----- ' 1-RFig.l. Formation probability of 5 and 6-memberedring.
Cs4(Dz-(22)) C84(D2d-(23)) Csi(C2-(2)) C86(C2-(5))
Fig.6. D3h, C2v, C’2v and D’ah-isomer of C?s .
-82-
2.5.4 FULLERENE TO NANOTUBE - THEIR DIFFERENCE AND SIMILARITY IN STRUCTURE AND
GROWTH PROCESS -
Yohji Achiba
Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-03,Japan
Fullerene or carbon nanotube is a good example for exhibiting a nano-sized new material extensively developped in scsience and technology of clusters during
the last decade. Here in the present work, we would like to present our recent works
on the carbon allotropes, placing special emphasis on the differences and similarities
found in their structures as well as growth processes.
Fullerene are all carbon molecules with a closed cage structure consisting of
pentagons and hexagons. Obeying Euler’s rule, the numbers of pentagons arc strictly
limited to be 12 and the numbers of hexagons can be changed by changing the sizes
of the molecules. Up to date, more than 40 different fullerenes have been isolated
and characterized by 13C NMR and other conventional characterization tools. The
sizes of the fullerene molecules examined so far are of the ranges from Coo to Ci2o-
On the other hand, according to the recent research on a single walled carbon
nanotubes (SWNT), the diameter distributions of the SWNT were found to be contorolled ranging from 0.8 nm to 1.4 nm. At least one-side of the tube is known
to be covered by an end-cap. It is very interesting to note that the sizes of the
end-cap of the tube with the smallest diameter are almost the same as those found
in the higher fullerenes with the sizes of Cyg - Cgg, while those with the largest
diameter are close to the sizes of Ci2o»
-83-
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2.6.5
1) BAE-Es 55 SS©#^” (^tUJK-fe>^“x Mzis 1985)
2) G.A. Somorjai, "Introduction to surface chemistry and catalysis” (Wiley, New York,
1994).
3) H. Haberland, "Clusters of atoms and molecules I”, (Springer-Verlag, Berlin
1994).
4) H. Haberland, "Clusters of atoms and molecules II", (Springer-Verlag, Berlin
1994).
5) B. Asamoto, "FT-ICR/MS, Analytical applicaitons of Fourier transform ion cyclotron
resonance mass spectrometry” (VCH Publishers, New York, 1991).
6) I.J. Amster, J. Mass Spectrom. 31, 1325 (1996).
7) S. Becker, G. Dietrich, H.-J. Kluge, S. Kuznetsov, M. Lindinger, K. Lutzenkirchen,
L. Schweikhard, J. Ziegler, Rev. Sci. Instrum. 66, 4902 (1995).
8) J.L. Elkind,F.D. Weiss, J.M. Alfold, R.T. Laaksonen, R.E. Smalley, J. Chem. Phys.
88, 5218 (1988).
9) M.P. Irion, Int. J. Mass Spectrom. Ion Proc. 121, 1 (1992).
10) C. Berg, Th. Schindler, G. Niedner-Schatteburg, V. Bondybey, J. Chem. Phys. 102,
4870 (1995).
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2
2.1 @S£#&iaM1T- (XS*)
2.1.1 Progress towards resolving the mysteries of the CO and NO dimers
A.R.W.NcKellar, M.J.Brookes, and J.KG.Wtson (NRC, Canada)
2.1.2 FTMW spectroscopy of sodium chloride-water Complexes - A model system for
solvation
Y.Endo, Y.Ohshima, and AMizoguchi (Tokyo University)
. 2.1.3 Spectroscopic studies of open-shell clusters: Clues to the nature of intermolecular
interactions
B.J.Howard and CJ.Whitham (Oxford University)
2.1.4 Electronic and vibrational spectra of benzene water ion
K.Ohashi (IMS)
2.1.5 Structure and dissociation energetics of van der Waals and hydrogen bonded clusters
R.M.Helm, Th.L.Grebner, J.Braum, and HJ.Neusser (TU Miinchen)
2.1.6 Infrared depletion spectroscopy of aniline clusters -The hydrogen bond between the
NH2 and an aromatic ring-
-93-
T. Nakanag (NIMC)
2.1.7 Structure and dynamics of size-selected benzonitrile-(H20)n and benzonitrile-
(CH30H)n
clusters investigated by IR-UV and simulated Raman-UV double resonance
spectroscopies
T.Ebata, S.Ishikawa and N.Mikami (Tohoku University)
2.1.8 Solvent effects and chemical reactivity in small molecular clusters
D.Solgadi, CJouvet, S.Martrenchard-Barra, C.Dedonder-Lardeux, G.Gregoire
(Universite Paris-Sud)
2.1.9 Coupling of low frequency vibrations with proton transfer
H.Sekiya, K.Nishi, T.Fukuchi, and N.Nishi (Kyushu University)
2.1.10 Rydberg state of rare gas-NO complexes
K.Shibuya, K.Tsuji, and K.Obi (Tokyo Institute of Technology)
2.1.11 Solvation of single alkali atom in water clusters
K.Fuke, KTakasu, and K.Hashimoto (Kobe university)
2.1.12 Laser spectriscopy of silver-ammonia complexes
J.Miyawaki (NAIR)
. 2.1.13 Competitive coordination in M+(NH3)m(H20)n as studied by laser ablation-
molecular beam method: Experiment and simulation
H.Sato, A. Matsuzaki, S.Nishino, and O.Ito (Mie University)
2.1.14 Reactions of cluster ions by FTICR mass spectrometry
K.Sugawara, E.M.Markin, and AB.Vakhtin (NAIR)
2.1.15 Collision induced dissociation of size selected Aluminum cluster
O.Ingolfsson, H.Takeo, and S.Nonose (NAIR)
2.1.16 Photoelectron spectroscopy of negatively-charged molecular clusters
T.Nagata (IMS, Tokyo University)
2.1.17 High resolution spectroscopy of carbon containg molecules: From 13C to C13
T.F.Giesen and M.Winnewisser (University of Cologne)
2.2 ,
-94-
2.2.1 Clustering in the gas phase and solution: Preferential solvation of neutral species and
hydrohob ic hydration of noble gases
ABango, M.Campulla, M.Pirana, G.Scorrano, and S.Stiz (University of Padova)
2.2.2 Molecular clusters in electrolyte and non-electrolyte solutions
AWakisaka, H.Abdoule-Carime, H.Kobara, and S.Mochizuki (NA1R)
2.2.3 Process of cluster formation in liquid droplet expansion
H.Sugahara, M.Nakata, and H.Takeo
(BASE, Tokyo University of Agriculture and Technology, NA1R)
2.3 h V —
2.3.1 Structure and reactivity of small metal clusters (l-10nm) by HERTEM
J.Urban, H.Sack-Kongehl and K.Weiss (Fritz-Harber-Institut)
2.3.2 Stable bimetallic icosahedrons formed by annealing
K.Koga, D.K.Saha, and H.Takeo (NAIR)
2.3.3 Catalytic properties of gold clusters deposited on metal oxides
M.Haruta (ONRI)
2.3.4 Au mltiply-twinned particle micelle
T.Takami and S.Ino (Tohoku University)
2.3.5 CO interaction with size-selected, supported nickel clusters on thin MGO-film
U.Heiz, F.Vanolli, and W.D.Schneider (Universie de Lausanne)
2.3.6 Mesoporous silicate synthesis under low temperature and acidic medium condition
and its chemical modification
J.Izumi, AYasutake, N.Tomonaga, Y.M.Setoguchi, I.Moriguchi,
Y.Teraoka, S.Kagawa
(Mitsubishi Heavy Industries, Ltd. Nagasaki R&D Center)
2.3.7 Dynamics of clusters studied by matrix-isolation infrared spectroscopy combined with
pulsed-nozzle molecular-beam technique
M.Nakata (BASE, Tokyo University of Agriculture and Technology)
2.3.8 The "designer" optical resonances of metal nanoshells
N.J.Halas, R.D.Aceritt, and S.J.Oldenburg (Rice University)
—95 —
2.4 ^ 7 x * -
2.4.1 The effect of restraints on the radiation induced chemistry of cation clusters and liquid
clusters in zeolites
G.Zhang and J.K.Yhomas (University of Notre Dame)
2.4.2 Photo excited state of Agl clusters incorporated into cage of zeolites
T.Kodaira (NAIR)
2.4.3 Metal and metal oxide clusters in zeolites: Growth and opto-electronic properties
M.Wark, G.Schulz-Ekloff (Universitat Bremen)
2.5
2.5.1 Studies on structure formation process by molecular dynamics
T.Ikeshoji (NAIR)
2.5.2 Liquid water dynamics: Hydrogen bond rearrangement, phase space dynamics and
proton transfer
I.Ohmine (Nagoya University)
2.5.3 Theory of Dullerene: A statistical model for growth
H.Yoshida (NAIR)
2.5.4 Fullerene to Nanotube -Their difference and similarity in structure and growth
process-
Y.Achiba (Tokyo Metropolitan University)
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—96—
