Negatlve Ion Photoelectron Spectroscopy of HCF-,, HCCr ... · Spectroscopy of HCF, HCCl-, HCBr-,...

1130

nuclei X,(g) with aging. The aging process could consist of the formation of agglomerations of dust particles, a single dust particle possibly not having a crevice to stabilize a gas pocket. The experiments of Figure 1 show once more that the inception of cavitation by ultrasound is decisively determined by the pre-

J. Phys. Chem. 1992,96, 1130-1141

treatment of the solution.

Acknowledgment. We thank Dr. J. Lilie and Dr. E. Janata for technical advice. This work was supported by Deutsche Forschungsgemeinschaft und Fonds der Chemischen Industrie.

Negatlve Ion Photoelectron Spectroscopy of HCF-,, HCCr, HCBr-, and HCI‘: Photoelectron Angular Distrlbutlons and Neutral Triplet Excltation Energies

Mary K. Gill- Kent M. Ervin,+ Joe Ho, and W. C. Lheberger* Joint Institute for Laboratory Astrophysics, University of Colorado and National Institute of Standards and Technology, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440 (Received: August 2, 1991)

Photoelectron spectra and angular distributions are reported for the HCX(’A’) + e- - HCX-(k 2A’’) and Hex(””) + e- + HCX-(k A”) transitions of the halocarbenes (X = F, Cl, Br, and I). Taking photoelectron spectra at parallel and perpendicular laser polarizations with respect to the direction of the photoelectron detection allows us to distinguish the triplet transition from the overlapping singlet transition. Ab initio calculations used to simulate the Franck-Condon envelope for the triplet states combined with the experimental data predict that HCl has a triplet ground state. Best estimates for the triplet excitation energy based on these simulations are 14.9 f 0.4 kcal/mol (HCF), 4.2 f 2.5 kcal/mol (HCCl), 2.6 f 2.2 kcal/mol (HCBr), and -2 to -10 kcal/mol (HCI). Vibrational intervals of 850 f 60 cm-I (HCCI), 725 f 70 cm-I (HCBr), and 637 f 80 cm-I (HCI) in the HCX(’A’’) + e- + HCX-(R 2A”) transitions are attributed to the C-X stretch of the neutral. Adiabatic electron affinities for the singlet states are found to be 0.542 f 0.005 (HCF), 0.535 f 0.005 (DCF), 1.210 f 0.005 (HCCl), 1.454 f 0.005 (HCBr), and 1.680 f 0.005 eV (HCI). The electron affinity of ’HCI is expected to lie between 1.25 and 1.59 eV. Asymmetry parameters are also reported for photoelectrons from F, Br-, and I- (hv = 351.1 nm).

I. Introduction Carbenes have intrigued chemists for a number of decades, with

their fascinating chemistry,’” two low-lying electronic states, and their challenge to ab initio theory. Interesting problems include the determination of carbene geometries, ground-state multiplic- ities, and the energy difference between the low-lying lA’ and 3Atr states. The chemistry is quite different for these two states. For example, singlet methylene undergoes stereospecific cis addition to olefins while triplet-state methylene undergoes nonstereapecifk addition to olefins. In the early studies of carbene chemistry the degree of stereospecificity in reactions with olefins was used to infer the ground spin state of the carbene.Zc6 Later these types of reactions were used to determine that HCF,’ HCC1,8 and HCBr9 possessed ‘A’ ground states.

The halocarbenes (HCX) have been the subject of many ab initio calculations, both on the k ‘A’ state geometries for HCF,’&I8 HCCl,Ie17 and HCBr1e16 and their H 3A’r state geometries.’*15 Computations by Bauschlicher et al.14 including configuration interaction obtained geometries and vibrational frequencies for the ground-state singlet and low-lying triplet spin states of HCF, HCCl, and HCBr. Scuseria et a1.I6 determined molecular geometries and frequencies for the lowest singlet and triplet states of HCF, HCC1, and HCBr using triplet plus double polarization basis sets. Geometries, frequencies and force fields for the k IA’ and the H 3AN states of HCF were computed by Weis et al.IO using highly correlated electron wave functions. Tomonari et- al.” obtained geometries, frequencies, and force fields for the X ‘A’ and the g ’A” neutral states of HCF and for the k zA’t state of HCF. Predictions for the HCF tri let excitation energy have

Weis et al.Io and Shin et alSz predict the energy difference between the lowest singlet and triplet states to be 13.9 and 14.5 kcal/mol,

varied from 0 to 26.7 kcal /m~l .~*~~J E 25 Recent calculations by

To whom correspondence should be addressed. Present address: Department of Chemistry, University of Nevada, Reno,

NV 89557-0020.

0022-3654/92/2096-1130$03.00/0

respectively. Additional computations on HCC11e16*23*24,26 and HCBr14J6 predict that the triplet excitation energy decreases in

(1) Gaspar, P. P.; Hammond, G. S . In Carbene Chemistry; Kirmse, W., Ed.; Academic: New York, 1964; Chapter 12.

(2) Gasper, P. P.; Hammond, G. S. In Carbenes; Moss, P. A,, Jones, Jr. M., Eds.; Wiley-Interscience: New York, 1975; Chapter 6.

(3) Gilchrist, T. L.; Rees, C. W. Carbenes, Nitrenes, and Arynes; Apple- ton-Century-Crofts: New York, 1969; Chapter 6.

(4) Skell, P. S.; Klebe, J. J . Am. Chem. Soc. 1960, 82, 247. (5) Skell, P. S.; Woodworth, R. C. J . Am. Chem. Soc. 1956, 78, 4496.

Skell, P. S.; Woodworth, R. C. J . Am. Chem. Soc. 1956, 78,6427. Wood- worth, R. C.; Skell, P. s. J . Am. Chem. Soc. 1959,81, 3381.

(6) Closs, G. L. In Topics in Stereochemistry; Eliel, E. L., Allinger. N. L., Us.; Wiley: New York, 1968; p 193.

(7) Tang, Y.; Rowland, F. S. J. Am. Chem. Soc. 1%7,89,6420. (8) Hine, J. Diualenf Carbon; Ronald Press: New York, 1964; p 75. (9) Closs, G. L.; Coyle, J. J. J . Am. Chem. Soc. 1965, 87, 4270. (10) Weis, B.; Rosmus, P.; Yamashita. K.; Morokuma, K. J. Chem. Phys.

(1 1) Tomonari, M.; AlmlBf, J.; Taylor, P., private communication. (12) Baird, N. C.; Taylor, K. F. J. Am. Chem. Soc. 1978, 100, 1333. (13) Staemmler, V. Theor. Chim. Acta 1974, 35, 309. (14) Bauschlicher, Jr., C. W.; Schaefer 111, H. F.; Bagus, P. S. J . Am.

(15) Hoffmann, R.; Zeiss, G. D.; Van Dine, G. W. J . Am. Chem. Soc.

(16) Scuscria. G. E.; DurBn, M.; Maclagan, R. G. A. R.; Schaefer 111, H.

(17) Mueller, P. H.; Rondan, N. G.; Houk, K. N.; Harrison, J. F.; Hooper,

(18) Carter, E. A.; Goddard 111, W. A. J . Chem. Phys. 1988,88, 1752. (19) Goldfield, D.; Simons, J. J. Phys. Chem. 1981, 85, 659. (20) Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M.-B.; Apeloig, Y.;

Karnie, M.; Chandrasekhar, J.; Schleyer, P. v. R. J . Am. Chem. Soc. 1986, 108, 270.

(21) Dixon, D. A. J . Phys. Chem. 1986, 90, 54. (22) Harrison, J. F. J . Am. Chem. SOC. 1971, 93, 4112. (23) Carter, E. A.; Goddard 111, W. A. J . Phys. Chem. 1987, 91,4651. (24) Carter, E. A.; Goddard 111, W. A. J . Phys. Chem. 1986, 90, 998. (25) Shin, S. K.; Goddard 111, W. A,; Beauchamp, J. L. J . Chem. Phys.

1990, 92,6635.

Chem. Soc. 1977, 99, 7106.

1968, 90, 1485.

E. J. Am. Chem. SOC. 1986, 108, 3248.

D.; Willen, B. H.; Liebman, J. F. J . Am. Chem. Soc. 1981, 103, 5049.

1990, 93,4986.

0 1992 American Chemical Society

Spectroscopy of HCF, HCCl-, HCBr-, and HCI-

the order HCF > HCCl > HCBr. To our knowledge, no ab initio results on HCI have been published.

Additional information has been obtained about the geometries and electronic structures of the halocarbenes through optical spectroscopy. Thtse-studies have focused on the transition between the R ‘A’ and the A lA” electronic states of HCF>7-39 DCF,@ and HCC1.28~4143 Merer and Travis’) observed absorption bands of HCF between 4300 and 6000 A in the flash photolysis of HCFBr2. They assigned a 1403.2-m-’ feature to the ground-state bending mode, and determined that HCF is nonlinear in both the upper and lower states, with bond angles of -127O and -102O, respectively. They did not observe any bands corresponding to excitation of the stretching modes. H a k ~ t a ) ~ obtained a value of 1406.87 (27) cm-’ for u2 (bending mode) by laser-induced fluorescence, in accord with the earlier observation of Merer and Travis.’)

In infrared matrix isolation studies of HCF, Jacox and Mil- ligan“ observed absorptions at 1405 and 1181.5 cm-’, corre- s nding to u2 (bend) and v3 (C-F stretch), respectively, for the Ip” ‘A’ state, produced by vacuum-ultraviolet photolysis of CH3F, CD3F, and W H 3 F in argon and nitrogen matrices. Since they were unable to observe any absorption due to the C-H stretching mode, ul, they calculated force constants for the 2 ‘A’ state using assumed values for v l . Recently Suzuki and H i r ~ t a ) ~ observed the C-H stretching mode of HCF (2643.0393 (26) cm-’) in a stimulated emission pumping experiment and improved the force field of Jacox and Milligan“ for HCF.

Merer and Travis42 analyzed the band system between 5500 and 8200 A arising from the photolysis of HCC1Br2. They attributed this band system to the A ‘A” - % ‘A’ transition of HCCl and determined the ground-state geometry of HCCl. Kakimoto et al.41 obtained a rotational spectrum of HCCI. They carried out a normal-coordinate analysis and calculated centrifugal dis- tortion constants and the inertial defect. Their HCCl ground-state force field is in reasonable agreement with that obtained by Jacox and Milligan.4s Results are not available from optical spectroscopy on HCBr and HCI.

A previous photoelectron spectroscopy study on the halocarbene and CC12 and CF2 anions from this laborat09 yielded vibrational frequencies for the C-X stretch of the anions and singlet neutrals. Adiabatic electron affinities were found for HCF, DCF, and HCC1. Tentative origin assignments were also made for HCBr and HCI. Upper bounds of 14.7 f 0.2 kcal/mol (HCF, DCF), 11.4 f 0.3 kcal/mol (HCCl), and 9 f 2 kcal/mol (HCBr) were given for the triplet excitation energies. More precise values could not be obtained, because the portion of the spectrum corresponding to the triplet origin was obscured by much stronger transitions to excited singlet vibrational levels. The upper limits were based on the values that produced significant deviations of the Franck-Condon singlet simulation from the data. Only an ab-

(26) Shin, S. K.; Goddard 111, W. A,; Beauchamp, J. L. J . Phys. Chem. 1990. 94. 6963.

(27) AshGld, M. N. R.; CastaAo, F.; Hancock, G.; Ketley, G. W. Chem.

(28) Qiu, Y.; Zhou, S.; Shi, J. Chem. Phys. Lerr. 1987, 136, 93. (29) Dixon, R. N.; Wright, N. G. Chem. Phys. Lerr. 1983, 100, 311. (30) Patel. R. I.: Stewart. G. W.: Casleton. K.: Gole. J. L.: Lombardi. J.

Phys. Len. 1980, 73, 421.

. . . . R. Chem. Phys. 1980, 52,461.

’

(31) Suzuki, T.; Saito, S.; Hirota, E. Can. J. Phys. 1984, 62, 1328. (32) Kakimoto, M.; Saito, S.; Hirota, E. J. Mol. Spcrrosc. 1981,88, 300. (33) Merer, A. J.; Travis, D. N. Can. J. Phys. 1966, 44, 1541. (34) Hakuta, K. J . Mol. Specrrosc. 1984, 106, 56. (35) Dixon, R. N.; Wright, N . G. Chem. Phys. Lerr . 1983, 100, 311. (36) Butcher, R. J.; Saito, S.; Hirota, E. J . Chem. Phys. 1984,80, 4000. (37) Suzuki, T.; Hirota, E. J . Chem. Phys. 1986, 85, 5541. (38) Ibuki, T.; Hiraya, A.; Shobatake, K.; Matsumi, Y.; Kawasaki, M. J.

(39) Suzuki, T.; Hirota, E. J . Chem. Phys. 1988, 88, 6778. (40) Suzuki, T.; Saito, S.; Hirota, E. J. Mol. Specrrosc. 1981, 90, 447. (41) Kakimoto, M.; Saito, S.; Hirota, E. J. Mol. Specrrosc. 1983,97, 194. (42) Merer, A. J.; Travis, D. N . Can. J . Phys. 1966, 44, 525. (43) Hirota, E. Faraday Discuss. Chem. Soc. 1981, 71, 87. (44) Jacox, M. E.; Milligan, D. E. J. Chem. Phys. 1969, 50, 3252. (45) Jacox, M. E.; Milligan, D. E. J . Chem. Phys. 1967, 47, 1626. (46) Murray, K. K.; Lcopold, D. G.; Miller, T. M.; Lineberger, W. C. J .

Chem. Phys. 1990, 92,4277.

Chem. Phys. 1988.89, 5442.

The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1131

solute value of the upper bound for the first excited state of HCI (9 f 2 kd/mol) was given, since the ground-state spin multiplicity of HCI was not known.

In this study we use the same flowing afterglow negative ion photoelectron spectrometefl6 to examine the HCX(’A’) - HCX-(f( 2A“) and HCX()A”) - HCX-(X 2A”) transitions (X = F, C1, Br, and I) in more detail. There are two major differences between this study and the previous one. First, we use 351.1 nm (hv = 3.531 eV) radiation rather than 488-nm (hv = 2.540 eV) laser light, which allows us to study higher vibrational levels of the triplet states. Second, with the new laser system we can rotate the laser polarization using a X/2 plate, permitting us to study the angular distributions of the photoelectrons. Polarization studies aid in identifying the symmetries of transitions in the photoelectron spectrum of HCI and can provide a means to separate the overlapping singlet and triplet states.

The outline of the paper is as follows: In section I1 we describe the experimental techniques, the current laser system, and the method of ion production, and include a brief explanation of the angular distributions of photoelectrons. In section I11 we discuss our experimental results which include spectra taken at several laser polarizations and values for the asymmetry parameters. Rotationally corrected adiabatic electron affinities are presented in section IV. A discussion of the Franck-Condon analyses and the determination of anion geometries is also in this section, followed by a detailed analysis of the individual halocarbenes and a comparison of our results with previously reported experimental and theoretical results. We also include a qualitative discussion on triplet excitation energies and on the angular distributions of photoelectrons for the singlet and triplet states of the halocarbenes. We summarize our results in section V.

IT. Experimental Section A Wotoelectron -. The photoelectron spectrometer

has been described in detail re~ently;~’.~~ therefore, a short explanation suffices here. Briefly, negative ions are produced in a flowing afterglow s0urce,4~ accelerated, focused into an ion beam, mass selected with a Wien velocity filter, and crossed by a con- tinuous laser beam of fixed photon energy. Photodetached electrons are collected perpendicularly to the ion and laser beams. The kinetic energies of the photodetached electrons are measured with a hemispherical electrostatic energy analyzer with a resolution of 9 meV. The spectra are obtained by measuring the kinetic energy of photodetached electrons from the process

HCX-(u’? + hv - HCX(u9 + e-

where u“ and u‘ denote the vibrational states of the ionic and neutral species. The spectra are taken as a function of electron kinetic energy ( em) , and are converted to binding energy (eBE), which is the difference between the photon energy and the electron kinetic energy (eBE = hv - e m ) . Therefore peaks at low binding energies correspond to high-energy photoelectrons. The absolute energy scale of the spectrometer is calibrated with 0- (EA = 1.461 121 5 (10) eV).So In addition a small linear correction is determined,s1 which accounts for the relative energy scale com- pression of the hemispherical electron energy analyzer (typically about 0.5%) by photodetaching W- and comparing the W fine structure splittings observed in the W- photoelectron spectrum to known values.s2 The experimental uncertainty is f0.005 eV for the absolute electron kinetic energy of well-resolved peaks in the photoelectron spectra.

(47) L,eopold, D. G.; Murray, K. K.; Stevens Miller, A. E.; Lineberger, W.

(48) Ervin, K. M.; Ho, J.; Lineberger, W. C. J. Chem. Phys. 1989, 91,

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5974.

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Natl. Bur. Stand. No. 467; US. GPO Washington, D.C., 1958.

1132 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992

9. Ultraviolet Laser System. The 351-nm output of a single-mode argon-ion laser is amplified in an optical buildup cavity, where the mirrors of the cavity also form the windows of the chamber!* The buildup cavity is a high-finesse Fabry-Perot interferometers3 that is electronically stabilized and locked to resonance. A servoamplifier system matches the resonant frequencies of the laser and the buildup cavity by adjusting the lengths of the two cavities (laser and buildup cavities) with pie- zoelectric translators on the cavity mirrors. When starting with 150-200 mW (single frequency, single mode) of incident laser power, we routinely achieve buildup factors of 150-250, producing 30-50 W inside the buildup cavity. Rotation of a X/2 plate external to the buildup cavity rotates the electric vector of the laser radiation relative to the direction in which the photoelectrons are detected.

C. Angular Distribution of Photoelectrons. The differential photodetachment cross section using linearly polarized light in the electric dipole approximation is generally written ass4

du/dSl = (u0/4?r)[l + j3P2(cos e)] (1)

where uo is the total photodetachment cross section, P2(cos e) = (3 ax2 0 - 1)/2, e is the angle between the direction of the ejected electron and the polarization of the incident light, and @ is the asymmetry parameter (-1 I @ I +2). At the *magic" angle of e = 54.7*, P2(cos 0) is equal to zero and the photodetachment signal is proportional to the total photodetachment cross section.

We have taken full photoelectron spectra for H C F , HCCl-, HCBr-, and HCI- at three different laser polarizations, 8 = 90°, 8 = 54.7O, and e = Oo, giving approximate asymmetry parameters. More precise values were obtained for the asymmetry parameter, 8, at a fixed photoelectron energy by changing e in 5-10° in- crements through several periods and monitoring the photoelectron intensity normalized by laser power and ion current. A least- squares fit of the data to eq 1 was used to obtain the best value for 8. D. Ion Roductioa Halocarbene anions were produced in the

flowing afterglow source by reacting 0- with a singly halogenated methane (CH3X). The 0- ions (300-1000 PA) were produced in a 2.45-GHz microwave dmharge of helium seeded with either 025 or N20?9 Typical flow rates were between (3-10) X IO3 std cm3 min-' of the helium buffer gas and 5 std cm3 min-' (N20) or 10 std cm3 min-' (02). After optimizing the 0-ion signal, 1-5 std cm3 min-' of the singly halogenated methane, CH3X, was introduced downstream of the microwave discharge. The 0- reacted with the halogenated methane by H2+ abstraction:

Gilles et al.

0- + CH3X -. H20 + HCX-

to produce the halocarbene anion.s658 Ion currents following mass selection ranged between 60 and 165 PA for the haloearbenes. By varying the inlet position for the singly halogenated methane, inmasing or decreasing the helium buffer gas pressure, and adding 100-400 std cm3 min-' of Ar, we were able to either cool or heat the anions vibrationally. Spectra taken at different vibrational temperatures were used as an aid in the confirmation of origin assignments.

In addition to 0-, a considerable quantity of the 0,- ion was also produced in the flowing afterglow source when O2 was used to generate 0-. Since HCF and 02- have the same mass and overlapping photoelectron spe~tra,~5~@ the interpretation of minor

(53) Siegman, A. E. In Losers; University Science Book. Mill Valley, CA,

(54) Cooper, J.; Zare, R. N. J . Chem. Phys. 1%8,48,942; erratum, 1968,

(55) Bohme, D. K.; Fehsenfeld, F. C. Cun. J. Chem. 1%9, 47, 2717. (56) Tanaka, K.; Mackay, G. I.; Payzant, J. D.; Bohme, D. K. Cun. J.

(57) Dawson, J. H. H.; Jennings, K. R. J . Chem. Soc., Furuduy Trum. 2

1986; pp 413ff.

49, 4252.

Chem. 1976,54, 1643.

1976. 72. 7 M . - - . - , . -. . - -. (58) Grabowski, J . J.; Melly, S. J. Inr. J . Muss Spectrom. Ion Proc. 1987,

(59) Celotta, R. J.; Bennett, R. A.; Hall, J. L.; Siegel, M. W.; Levine, J. 81, 147.

Phys. Rev. A 1972, 6, 631.

1 .I

111 2 0.1

3 0

0.1

2 p 0.4 t 2 0. i

U

I - a

0 .(

111 ;; 0.8

e 0 . f

2 p O . r

2 0.2

a 0 V

U - W 0 CI

a 0 .c

111 CI # 0 .E a V # 0 .E

0

0

u W 0.4

CI

I

t 2 0.2 U

a

0 .o

111 g 0.8 3 0 V

0.6

2 U

p 0.4

W 0

- c)

2 0 .2 &

0 .o

Electron Kinetic Energy (eV) 1 .o 2 .o 3 .O

HCF -

HCCl' 1

ucr- L

3 :O 2:o 1:o E l e c t r o n B i n d i n g E n e r g y ( e V )

Figure 1. Photoelcctron spectra of the halocarbenes, H C F , HCCI-, HCBr-, and HCI- taken at the "magic angle" with hv = 3.531 eV. Photoelectron counts are plotted as a function of the electron binding energy, the difference between the photon energy and the measured electron kinetic energy. The HCX('A') + e- - HCX-(k 2At') transitions are seen at lower electron binding energies and the vibrational origins are indicated by an arrow. The vertical HCX()At') + e- - HCX-(% 2A'') transitions are seen to the left, at higher electron binding energies.

features in the H C F spectrum can easily be compromised by an 02- contaminant. Several steps were taken to minimize the contaminant 02- present in the ion beam. First, because N20

(60) Travers, M. J.; Cowles, D. C.; Ellison, G. B. Chem. Phys. Lett. 1989, 164, 449.

Spectroscopy of H C F , HCCl-, HCBr-, and HCI-

produces less O;, NzO was used as the source of 0-. Second, after taking a spectrum of H C F (with contaminant Or) , the CH3F was shut off and a spectrum of only 0; was taken. This spectrum was normalized to the peak at 0.644 eBE and subtracted from the contaminated H C F spectrum. For all of the HCF spectra we discuss below the contaminant Of spectrum has been subtracted. Typically the 0; contamination was less than 20%.

The halogen atomic anions were normally present in the ion beam when the halogenated compounds were used in the ion source. The F, Br, and I- currents present in the ion beam, when the singly halogenated methanes were used to make the halocarbenes, were sufficient for measurement of their asymmetry parameters.

III. Results A. Spectra Taken at the Magic Angle. The 351.1-nm photo-

electron spectra of the halocarbenes displayed in Figure 1 were obtained with the laser polarization at the ymagic angle”. Transitions from the anion ground state to two neutral electronic states are seen in all of the spectra in F w e 1. The photoelectron spectra of H C F , HCCl-, and HCBr- exhibit a vibrational progression in the C-X stretchin mode (v3) of the neutral, arising

the ground-state neutral, % ‘A’. The second vibrational progression, at higher binding energies, arises from the photodetachment to the first excited state of the neutral, H 3A”. The transition to the H 3A” electronic state appears to consist of a single active vibrational mode for HCF; however, it will be seen from the isotopically substituted DCF that there are two nearly de- generate modes active in this progression. The photoelectron spectra of the H 3A” state for HCCl and HCBr display congested vibrational progressions arising from the two active vibrational modes.

The HCI- photoelectron spectrum exhibits structure similar to that of HCCl- and HCBr-, that is the appearance of a single vibrational progression in the state at lower bmdw energies while the state at higher binding energies is severely congested. This spectrum additionally exhibits a sharp peak at 3.059 eV, the binding enerd’ of I-. This peak arises from a two photon process, consisting of the photodissociation of HCI- into HC and I-, followed by the subsequent photodetachment of I- to I + e-. This results in the appearance of a photoelectron at the correct energy for detachment of free I-. Similar twephoton processes have been observed previously for 03- and Au<.62.63

B. Polarization Dependence. Spectra taken at two different laser polarizations, B = Oo and = 90°, are shown in Figure 2. The HCX(’A’; u’ = 0) - HCX-(k zA”; u” = 0) transitions are marked with an arrow and correspond to the vibrational origin. The asymmetry parameter can be approximated from the ratio of the spectral intensities taken at B = Oo and B = 90°. Cooper and Zares4sM have shown that /3 is a function of the angular momentum of the orbital from which the photoelectron originates and can be a function of photoelectron kinetic energy. Because the singlet and triplet transitions arise from electron detachment from different orbitals in the anion, we anticipated that the photoelectron arising from the lA’ - k zA” transition would have a different angula: distribution than the photoelectron resulting from the 3A” - X ZA” transition. If the angular distributions were sufficiently different, separation of the overlapping portions of the two electronic transitions would be possible.

The photoelectron spectra of HCX- at 0 = 0’ and B = 90° reveal ieveral definite trends. For HCF, the relative intensities of the X ‘A‘ and H 3A“ transitions are about 2:l for both Oo and

from the transition from the # zA” ground state of the anion to

(61) Hotop, H.; Lineberger, W. C. J. Phys. Chem. Re/. Duru 1985, 14, 731. (62) Novick, S. E.; Engelking, P. C.; Jones, P. L.; Futrell, J. H.; Line-

berger, W. C. J . Chem. Phys. 1979, 70,2652. (63) Ho, J.; Ervin, K. M.; Lineberger, W. C. J . Chem. Phys. 1990, 93,

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C d i s b n Processes; Geltman, S., Mahanthappa, K. T., Brittin, W. E., Eds.; Gordon and Breach: New York, 1969; Vol. llc, p 317.

The Journal of Physical Chemistry, Vol. 96, No. 3, I992 1133

TABLE I: Asymmetry Parameters for Photodetachment of HCX- and X- (X = F, CL Br, and I)

neutral species ‘A‘ 3All

anion species peak” B peak” B H C F 2.830 0.14 f 0.1 2.030 0.15 f 0.1 HCCI- 2.015 -0.30 f 0.1 1.185 0.13 f 0.1 HCBr- 1.899 -0.50 f 0.1 1.050 0.21 f 0.1 HCI- 1.776 -0.67 f 0.1 0.900 0.28 f 0.1

*p3,*

F 0.132 -0.13 f 0.2 Br- 0.166 -0.22 f 0.2 1- 0.47 1 -0.65 f 0.1

Electron kinetic energy, electronvolts.

90°, although the absolute intensities are lower at 90° (as seen from the signal-to-noise levels of the spectra). In other words, /3 is nearly the same for both electronic transitions from H C F . For the heavier halocarbenes, noticeable differences are observed between the spectra at Oo and 90°. For B = Oo, the intensity of the ‘A’ state (lower binding energies) decreases relative to the 3A” state (higher binding energies) from HCCl to HCBr to HCI. For the B = 90° spectra, the opposite trend is observed, the intensity of the lA’ transition increases relative to the 3A” transition from HCCl to HCBr to HCI.

In the absence of autoionization, one expects a nearly constant value of /3 over an entire vibronic progression. This expectation is consistent with the data shown in Figure 2. Consequently, for each electronic state a single intense peak was chosen and the normalized intensity of this peak was monitored as a function of B to obtain accurate angular distributions. The asymmetry parameters for F, Br-, and I- were also measured to compare the halocarbene and halogen values for j3 as a function of electron kinetic energy. Table I summarizes the asymmetry parameters of each of the halocarbene neutral electronic states and for the lowest spin-orbit states of the halogens. Trends for the values of the asymmetry parameters are discussed in section IV.

The difference in the asymmetry parameters of the 3A” and ‘A’ states of HCCl, HCBr, and HCI was exploited to separate the two transitions. First, in the spectrum taken at B = 90°, the intensity of a single peak in the ‘A’ state (well removed from observed triplet transitions) was normalized to match its intensity in the spectrum taken at B = Oo. This normalized B = 90’ spectrum was subtracted from the B = 0’ spectrum. After the singlet state was subtracted out the resulting spectrum, consisting primarily of the triplet state, was smoothed. The subtracted data for HCCl, HCBr, and HCI are shown in Figure 3. The binding energy at which the triplet peak progression becomes nonzero provides a direct experimental upper bound on the triplet excitation energy. For HCF, the minimal difference in the asymmetry parameters for the two states made an analogous subtraction impossible.

IV. Discussion We first discuss general trends in the photoelectron spectra and

use these to assign the state symmetries of HCI. We then describe the Franck-Condon simulations. Detailed descriptions are given of the singlet-state analysis for each of the halocarbenes, including anion and neutral vibrational frequencies, normal-mode displacements, and calculated anion geometries. Adiabatic electron a f f ~ t i e s for the singlet states are then presented. In general, our analysis of the triplet states was limited due to the lack of information available. Detailed Franck-Condon simulations were done for two possible origin assignments for the triplet states of HCF and DCF. Franck-Condon envelopes were simulated for HCC1, HCBr, and HCI. F d y , the electron angular distributions were used to verify neutral-state symmetry assignments for HCI.

A. State Symmetry Assignments for HCI. The halocarbenes HCF, HCC1, and HCBr are known to possess singlet (‘A’) ground states and low-lying triplet states (3A”). The ground-state spin

1134 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 Gilles et al.

e = 0” 8 = 90” 1.0 I

a 0

e t - a t 0.5 q u

0 .o

a 0

e t qx 0.5 q u

0 .o

e 0

P t 22 0.5 I

2; U

0 .o

a a-

I o d 0.5 .“ *’,

u

0 0 3 0 2 0 1 0 3 0 2 0 1 0

Figure 2. Photoelectron spectra of the halocarbenes taken at 0 = Oo (left) and 0 = 90’ (right) where 0 is the angle between the polarization of the incident light and the elcctron dctcctor. The origins of the HCX(’A’) + e- + HCX-(f( *A”) transitions are indicated by arrows. The vertical transitions to the triplet state are seen at higher electron binding energies. The sharp peak in the HCI- spectra at 3.06 eV electron binding energy arises from a two photon process.

multiplicity of HCI has not been firmly established in previous for the lower binding energy state and ’A’’ symmetry for the higher experiments. In the earlier photoelectron study it was not possible energy state. to determine which of the spin states was the ground state of HCI. The halocarbenes belong to the C, point group and the three A singlet ground state has been suggeated from the stereospacific vibrational modes have A’ symmetry; all three vibrational modes addition of iodomethylene to 2 - b u t e n ~ ~ ~ and from a study on are symmetry allowed for both electronic transitions. For a given reactions of HCI with ethylene in the presence of OZnSa electronic transition, one expects to see vibrational progressions

Several trends appear in the photoelectron spectra of the in the modes with Franck-Condon overlap and those which have halocarbenes in Figure 1. First, HCF, HCC1, and HCBr display a substantial geometry change. Intuitively, we predict that the a single active vibrational mode, the neutral C-X stretch (v3) , in geometry changes for the ‘A’ - ft ZA’f and 3A’r - 2 2Afr the HCX(ft ‘A’) - HCX-(% zA”) transition. second, the HCX(H transitions of HCI- follow the same trends as the other halo- 3A’’) - HCX-(R 2A”) transitions appear to be quite congested carbenes. due to two active modes, the bending mode (v2) and the C-X The electron removed in the HCX(’A’) - HCX-(ft ZAff) stretch (+). For HCI, the state with a maximum intensity at 1.9 transition originates from an antibonding orbital consisting pri- eV eBE has a progression in a single active mode while the state marily of a contribution from the out-of-phase, out-of-plane, pz with a maximum at 2.7 eV eBE is quite congested. This similarity orbitals of the carbon and halogen. One anticipates that the C-X between HCI and the other halocarbenes suggests ‘Af symmetry bond length should decrease when an electron is removed from

this orbital and that the C-X stretching mode (v3) should be active. (65) Yang, N. C.; Marolewski, T. A. J . Am. Chem. Soc. 1968,90,5644. Similar to the other halocarbena, one swdses that the c-I (66) Kikuchi, M.; Church, L. B. Radiochim. Acta 1973, 20, 81. stretch should be active in the ‘A‘ - ft zA” transition, since this

Electron Binding Energy (eV)

Spectroscopy of HCF, HCCl-, HCBr-, and HCI-

3 .o 2.5 2 0 1 5 c

3 .o 2.5 2 .o 1 .5 ELECTRON BINDING ENERGY (eV1

Enspc 3. Subtracted photoelectron spectra of HCCl-, HCBr-, and HCI-. Spectra from Figurc 2, at B = 90°, were normalized and subtracted from their respective spectra taken at B = Oo. The resulting spectra consist primarily of the HCX(3A”) + e- + HCX-(R ZA”) transitions. The arrows correspond to rigorous experimental upper limits for the triplet excitation energy.

is the vibrational mode that should reflect the geometry change. Elaborate calculations on HCF,’*I72mJ0J3 HCCl,’e1a.41,42 and

HCBr1c’6 indicate that the halocarbenes have bond angles at lea$ 20° larger in the H 3A’’ state than in either the ft ‘A’ or the X 2A’’ state. The C-X bond length is much shorter in the triplet state than in the anion. Results of our Hartree-Fock geometry computations on HCI,67 executed using an STO-3G* basis set, suggest a comparable trend for HCI. The anion has a C-I bond length 20.3 A longer than the singlet or triplet neutrals. The bond angle is expected to be = 3 3 O larger in the 3A” neutral state than

(67) Frisch, M. J.; Head-Gordon, M.; Schlegel, H. B.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A.; Steger, R.; Melius, C. F.; Baker, J.; Martin, R.; Kahn, L. R.; Stewart, J. J. P:; Nuder, E. M.; Topiol, S.; Pople, J. A. GAUSSIANBB; Gaussian, Inc.: Rttsburgh, PA, 1988.


TABLE II: Geometries of k aA“ HCX- a d ‘A’ HCX (X = F, Br, Cl, I)g

HCFb HCCI” HCBr’ HCI’

HCFd HCCI‘ HCBrf HCIE

1.12 1.14 1.10 1.13

1.130 1.130 1.091 1.128

1.46 1.91 2.10 2.30

‘A‘ 1.305 1.687 1.884 2.104

99.9 99.5 99.0 98.1

103.0 105.1 102.0 100.6

‘Anion geometries were obtained using the singlet geometries given in this table, the experimental normal-coordinate displacement from our Franck-Condon simulation, and the force fields in Table IV. For both HCF and HCCl- experimental singlet-state geometries and force constants are known. The uncertainties in the anion geometries of h0.02 A in the bond lengths and *3O for the bond anglts arise from a 20% uncertainty in the normal-coordinate displacement from the Franck-Condon simulation. ‘For HCBr- and HCI-, since both the singlet state geometries and the force fields are calculated, significant errors may be introduced by the adopted neutral geometries and force fields. E~perimental.’~ e Experimental!’ f Ab initio.I6 8 Hartrct Fock calculation using an STO-3G* basis set!’ The ’A” state geometry calculated was 1.087 A (rC-H), 2.036 A (rc-x), and 125.7O (fHCX).

in the anion *A” state. Therefore, both vibrational modes u2 and v3 should be active in the HCI(3A”) - HCI-(ft *A,’) transition.

Assigning the electronic state with maximum intensity at 1.9-eV eBE to the ‘A’ state of HCI accounts for the solitary active mode in the singlet state. Selection of the electronic state with an intensity maximum at 2.7 eV eBE as the 3A” state is consistent with the vibrational congestion in this part of the spectrum. Additional, conclusive support for our state symmetry assignments is given in section F, where angular distributions of photoelectrons are examined. B. Franck<ondon Analysis of the HCX Singlet States. To

confirm origin assignments and obtain more quantitative information about the halocarbenes, a Franck-Condon simulation of the spectra taken at the “magic angle” was performed. Only a very brief description is given here, as a more detailed explanation of the Franck-Condon analysis is available in a recent paper by Ervin et a1.68

The vibrational potentials for the C-X stretches were modeled as independent Morse oscillators for the anion and neutral electronic states. Franck-Condon factors were calculated by nu- merically integrating the overlap between trial Morse wave functions of the neutral with those of the anion. Variable parameters in the Franck-Condon simulations are w, and w , ~ , for both states, anion vibrational temperatures, origin positions, and normal-coordinate displacements. Unknown neutral frequencies were obtained by examining the well-resolved vibrations within a vibrational progression. The anion frequencies were found from the hot bands, HCX(’A’; u’ = 0,l) - HCX-(ft 2A”; u” = 1,2). The hot band progrmions are not long enough to allow accurate determination of anion anharmonicities. Anion vibrational temperatures were varied to fit the hot-band intensity in the vibrational progression of the ‘A’ transition and ranged from 300-500 K.

The vibrational peaks in the ‘A’ states were tit with rotational contours. The rotational contours69 of the vibrational transitions were calculated by generating a rotational spectrum using dipole selection rules and convolving the result with a Gaussian line shape at the experimental resolution (9-meV fwhm). Experimental rotational constant^^^*^^-^' were employed where available; oth- erwise values calculated from the experimental or theoretical geometries in Table I1 were used. Singlet-state rotational transitions were calculated according to asymmetric rotor selection

(68) Ervin, K. M.; Ho, J.; Lineberger, W. C. J . Phys. Chem. 1988, 92,

(69) Engelking, P. C. J . Phys. Chem. 1986,90,4544. 5405.


TABLE IIk (HCX) 'A' .nd (HCX-) % *A" C-X Stntebing ( V S )

Frequencies and Anl18dcIties~ (cui') Determined from This Experiment

NCD (~'3): EA,' k 2A'' 1 A' Jf3 wff3 x f f3 ) I / ) d3 xb3 (g/mo1)1/2 A eV

HCF 720 748 -14 1189 1205 3 -10 -0.41 0.542 DCF 709 735 -13 1193 1213 -10 -0.43 0.535 HCCl 445 810 819 -5 -0.68 1.210 HCBr 393 683 694 -6 -0.74 1.454 HCI 370 578 581 -2 -0.69 1.680

"Harmonic C-X stretching frequencies, d3 = J3 - 2d3, - 1/2(x'13 + xf2)) , where ~ ' 1 3 and x i 3 are assumed to be zero. bNCD, normal-coordinate displacement obtained from our Franck-Condon simulation (+20%). CUncertainties in the electron affinities are k0.005 eV.

TABLE W. Force FkMa for 'A' HCX (X = F, Cl, Br, I; Frequencies in P", cm-1)

kHCXi ~ C H , kcxg md@l ~ C H . C X , ~ C H . H C X ~ ~ C X H C X ,

mdm1A mdvnlb rad2 mdyn/A mdynlrad mdynlrad H C P 4.303 6.743 1.332 0.351 0.082 0.647

(2799) (1214) (1441)

(2800) (815) (1201) HCCl 4.29' 3.8W 0.978' 0.W 0.W 0.523c

HCBP 6.38 4.99 1.27 -0.199 0.071 0.57 (3413) (866) (1355)

(3427) (722) (1283)

OAUSSIANB~~' with an STO-3G' basis.

HCId 6.43 3.68 1.15 -0.245 0.023 0.47

Ab initio.1° bAssumed.'l CExperimental.4s dCalculated using

rules (perpendicular transition, B type).7O Because R-T energy relaxation is generally faster than V-T, the rotational temperature is expected to be lower than the vibrational temperature in a flowing afterglow source, as long as low extraction voltages are used to prevent collisional reheating. Since the extraction voltages used were normally less than 2 V, rotational temperatures were constrained to be less than the vibrational temperatures (300-500 K). These calculated rotational contours were used to fit the experimental peaks in our Franck4hndon simulations. The origin position of the contour, when fit to the experimental data, cor- responded to the rotationally corrected electron affinity.

If the origin assignment was not clearly indicated from the spacing between vibrational peaks, a nonlinear least-squares op timization procedure was used for each possible origin for the 'A' state to find the best fit to the observed spectra. The origin choice both minimized the residuals and accounted for all of the vibrational peaks seen in the spectra. In every case, an unequivocal origin choice emerged for the singlet state. Values obtained for frequencies and anharmonicities of the singlet neutral and anion states from the Franck4hndon simulation are given in Table 111.

In the following sections, we discuss each carbene separately. For each of the halocarbenes, the neutral ground state singlet geometry (Table 11) was used with a force constant matrix (Table IV) and our experimental normal-coordinate displacement from the Franck4hndon simulation (Table 111). to generate an anion geometry (Table II).71 The Franck-Condon analysis provided information on the magnitude of the geometry change in the electronic transition but not on the direction of the geometry change. Here, the sign of the normal-coordinate displacement was chosen so that the C-X bond length decreased in the HCX- (IA') - HCX-(J( 2A") transition, reflecting the antibonding character of the orbital from which the electron was detached.

1. HCF ad DCF. The C-F stretching frequency for the anion is 720 k 25 cm-'. The IHCF C-F stretching frequency of 1189 * 25 cm-' is in excellent agreement with the 1182-cm-l matrix

(70) Herzberg, G. Molecular Spectra and Molecular Structure III Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1966; p 248.

(71) Wilson, Jr., E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1955.

4 o o j 3:

L 3 0 0 1

HCBr-

1.60 1.50 i .40 1.30 ELECTRON BINDING ENERGY (eV)

Figure 4. Photoelectron spectrum of HCBr- taken at the "magic angle" with hv = 3.531 eV. The Franck-Condon simulation, resulting in an adiabatic electron affinity of 1.454 * 0.005 eV (assignment a), is compared to the simulation for the previous origin assignment (assignment b) which differed by one C-Br stretch quanta.

value of Jacox and Milligan4 Frequencies of 709 f 25 and 1193 f 25 m-l are observed for the C-F stretching frequencies of the neutral J( 'A' and anion zA'' states of DCF, respectively; the frequencies and anharmonicities from the Franck-Condon simulations appear in Table 111. All of the HCF and DCF frequencies found in this study are in agreement with previously reported

Combining our normal-coordinate displacement of -0.41 (g/ mol)'/2.A for vj of HCF with literature values for the neutral geometry (Table 11) and force field (Table IV), we obtain the anion geometry given in Table 11. Our H C F bond angle of 99.9 f 3O is slightly larger than the bond angle obtained from the previous photoelectron experiment.& The difference lies in the force fields used to determine the anion geometry. Only recently the C-H stretch was identified by Suzuki and H i r ~ t a . ~ ~ They found the C-H stretch to be unusually low (2643.04 an-'). Using the calculated force field of Weis et al.I0 increased the anion bond angle and brought it into closer agreement with the ab initio value (98.9O) of Tomonari et al." Due largely to an improved force field, the anion geometry reported here should be more accurate than the previous experimental estimate.&

2. HCCI. The singlet C-Cl stretching frequency found for HCCl, 810 f 25 cm-' (Table 111), is identical to that of Murray et a1.* and is in agreement with the 815-cm-' value found in a matrix isolation study by Jacox and Milliga11.4~ The anion C-Cl stretching frequency (445 f 25 cm-l) is also in agreement with earlier results.46 Although our Franck-Condon displacement (-0.68 (g/mol)l/Z-A) is consistent with the previously used value (-0.66 (g/mol)'/*.A),& the calculated C-H bond length (1.14 A), in Table 11, does not agree with the earlier results even though the same force field45 was used. The C-H bond length of 1.21 A given by Murray et ala& is a typographical error and should read 1.12 A.

3. HCBr. To our knowledge, spectroscopic studies of HCBf and its corresponding neutral singlet and triplet states are limited

Spectroscopy of HCF, HCCI-, HCBr-, and HCI-

to those performed earlier in this laboratory.& In the previous work only a tentative fi: ‘A’ origin assignment was possible for HCBr. h e to slightly better resolution, higher photon energy, and larger ion currents, the quality of the spectrum has been improved in this experiment. Figure 4 displays the experimental data with the best fits of the earlie@ tentative origin (b) and the current origin assignment (a). The earlier tentative origin assignment was in error by a single quantum of the C-Br stretch. The C-Br stretching frequency is 393 f 25 cm-’ in the anion and 683 f 25 cm-I in the ground-state singlet.

for HCBr have been published, neither force constants nor the HCBr- geometry were available. We computed the force field of the ff ‘A’ state of HCBr (given in Table IV) from HartreeFock level ab initio calculations, using G ~ u s s I A N 8 8 ~ ~ with an STO-3GZ basis set. The anion geometry in Table I1 was calculated using the experimental normal coordinate displacement (-0.74 (g/ mol)’/z.A), the calculated force field (Table IV), and the singlet geometry of Scuseria et (Table 11). Since our estimate of the anion geometry was made using a force field from a relatively low level calculation, there could be considerable error in the result.

4. HCI. Vibrational frequencies were found for u3 of the ‘A’ state (578 i 25 cm-’) and the fi: 2A” (370 25 cm-l) states of HCI, both of which are in agreement with previously reported values? As with HCBr, there is a lack of experimental and theoretical information on HCI, and it was necessary to calculate both the singlet-state geometry (Table 11) and the force field (Table IV) using an STO-3G’ basis set.67 The HCI- geometry was generated using the STO-3G* singlet HCI geometry and force field with a normal coordinate displacement of 4 .69 (g/mol)’/z.A. This geometry has a C-I bond length of 0.10 A larger, and a bond angle 6’ larger, than the anion geometry from the Hartree-Fock ab initio calculations. When improved force constants or singlet geometries h m e available, appropriate corrections can be made in the anion geometries. Minor contributions from the 3A” state were neglected in this portion of the analysis.

Although there are no other experimentally derived values with which we can compare the geometries of HCBr- and HCI-, they follow the same trend as HCCI- and HCF. The C-X bond lengths of the anions are all about 10% longer than the corresponding ‘A’ neutrals. The neutral bond lengths are expected to be shorter because the photodetached electron is removed from an antibonding orbital. There is very little change in the C-H bond length for any of the halocarbenes, which is consistent with the removal of an electron localized on the carbon and halogen antibonding orbital. The small change in bond angle between the anions and the singlet state neutrals arises from the C-X stretch normal coordinate, which has some bending character. The C-X frequency is largest for fluorine and d e ” with incmsing mass for the larger halogens. The anion C-X frequencies are 5 5 4 % of the neutral values, reflecting the longer and weaker bond because of the presence of an electron in an antibonding orbital.

C. Adiabatic Electron Affioitk On the basis of the prccediig determinations of single-state vibrational origins, the rotationally corrected adiabatic electron affinities are 0.542 0.005 (HCF), 0.535 f 0.005 (DCF), 1.210 f 0.005 (HCCI), 1.454 f 0.005 (HCBr), and 1.680 f 0.005 eV (HCI). The number reported here for EA (HCI) is the origin of the state with a maximum intensity at 1.9 eV eBE. As discussed in section IV.D, this state is the ‘A’ state of HCI. The Franck-Condon analyses described in section 1V.E indicate that the origin of the triplet state may lie even lower.

Electron affinities for HCF, DCF, HCCI, HCBr, and HCI have been previously reported from the 488-nm photoelectron spectra.& All of the origin assignments made in the present study are in agreement with the earlier results, except for HCBr. Although the origin assignments remain the same, our electron affinities for HCF and DCF lie just outside the error limits previously reported. In the previous study the rotational correction was inadvertently applied with the incorrect sign.& The electron affinities reported previously for CF, and CCl, also had a sign error in their rotational correction. Approximate treatment of

Although calculations on the singlet and triplet


the rotational correction indicates that the previous values& should be corrected to 0.165 f 0.010 (CFA and 1.591 f 0.010 eV (CCl,). D. Anrlysis of the HCX Triplet States. The triplet states of

the halocarbenes are poorly characterized experimentally and theoretically, particularly for the heavier halogens. Because of the lower photon energy (2.540 eV) used in the previous photoelectron experiment,& it was not possible to see the entire HCX(3A”) 4- HCX-(I< 2A”) progressions of HCCI, HCBr, and HCI. These are seen quite clearly with the present photon energy (3.531 eV) and are displayed in Figures 1 and 2. The photoelectron spectra of the triplet transitions are more congested than for the singlets due to two active vibrational modes, the C-X stretch (vj) and the bending mode (u2). The individual systems are discussed in the following sections.

1. HCF and DCF. Examining portions of the singlet Franck-Condon fits for HCF and DCF shown in Figure 5, we see that the first portion of the spectra which shows significant residuals after subtraction of the singlet-state Franck-Condon simulation is at 1.19 f 0.02 eV eBE. The peak at this position must belong to the triplet vibrational progression. Although examination of the residuals from the HCF singlet fit shows no significant residual at one C-F stretch quantum lower (1.03 f 0.02 eV eBE), more detailed analysis is required before 1.03 or 1.19 eV eBE can be assigned to the triplet origin. In our earlier study,& the 1.19 eV eBE peak was given as an upper bound on the triplet origin, and the (then more prominent) peak at 1.03 eV eBE was considered to be the more likely triplet origin. The earlier H C F spectrum was contaminated by an 02- impurity which produced a noticeable peak at 1.03 eV eBE, most of which was eliminated with the subtraction process and our present angular distribution capability. In spite of the improvement, a weak “feature” was present at 1.03 eV eBE, and HCF and DCF simulations were needed to establish the triplet origin at 1.19 eV eBE.

Complete Franck-Condon analyses were performed for these two possible triplet origin assignments for both HCF and DCF. The two active vibrations in the triplet state, the stretching and bending modes, were modeled as uncoupled Morse and harmonic oscillators, respectively. The bending and C-X stretching progressions overlap in the photoelectron spectrum of HCF, giving the appearance of a single vibrational progression, but decompose into two progressions in DCF (Figure 5). Since the frequencies of these two modes in HCF were not distinguishable in our experiment, the calculated frequencies and anharmonicities of Weis et al.’O were used as a starting point for the triplet-state fit of HCF. In the DCF@ 3A”) - D C F ( k ,A”) transition the C-F stretch and the bending mode are seen as two clearly separate modes, allowing us to use these frequencies for the Franck-Condon analysis. For the Franck-Condon calculation, the anion C-F stretching frequency, geometry, and vibrational temperature were fured to the values obtained in the simulation of the singlet state progression. The vibrational peaks were fit with a Gaussian of 22-meV fwhm for DCF and 32-meV fwhm for HCF.’,

Results of the Franck-Condon simulations are seen in Figure 5. Figure Sa shows part of the triplet vibrational progressions and portions of the singlet Franck-Condon simulations. The triplet simulation using origin assignments of 1.03 f 0.02 eV eBE and 1.19 f 0.02 eV eBE are added to the singlet simulation (a) and displayed as (b) and (c) respectively. For HCF, the quality of the Franck-Condon simulation is essentially the same for both origin choices. For DCF the Franck-Condon simulation using the triplet origin assignment of 1.03 eV eBE (Figure 5b) fails to reproduce the data in the region 1.2-1.5 eV eBE. The Franck- Condon simulation using the 1.19 eV eBE triplet origin (Figure 5c) produces an improved fit in this region. The optimized vibrational frequencies and normal-coordinate displacements for each origin assignment are given in Table V. The choice of signs for the normal-coordinate displacements shown in Table V was

(72) A rotational contour b a d on the actual molecular geometry and transition selection rules was not generated bccausc the triplet states are not as well Characterized as the singlets and bccausc the rotations that arc allowed in the transition to the triplet electronic state from the anion are hybrid A and B-type transitions.70


DCF- (a )

I1 I - Singlet Simulation

DCF- (b)

v) c)

fi g u 200

3 f i 0

U al c( 8 100 Y 0 5: p1

0 L 6 1.5 1.4 1.3 1.2 1.1 1 0 L 6 1.5 1 . 4 1.3 1 , 2 1.1 1 0 0.9

Electron Binding Energy (eV) Electron Binding Energy (eV) Figure 5. Photoelectron spectra of H C F (left) and D C F (right) taken at the magic angle. The data are plotted as points, and the Franck-Condon simulations are plotted as a line. Spectra in (a) display parts of the singlet state simulation: (b) and (c) show Franck-Condon simulations of the triplet state using triplet origins of 1.03 and 1.19 eV eBE, respectively, added to the simulation shown in part (a). The triplet origins used for each fit in (b) and (c) are indicated by arrows and the resulting triplet excitation is given (kcal/mol).

TABLE V Results of the Frpnck-Condon Simulations for tbe P jA" States of HCF and DCF Using Origin Aesignments of 1.19 and 1.03 eV eBE (Frequcncios in c d , and Normal-Cmrdinate Displacements (NCD) in (n/mol)'/* A)

J2 ~ ' 2 ~ ' 2 2 J3 0'9 ~ ' 3 3 NCD(J2) NCD(J3) 1.19 eV eBE

HCF 1047 1054 -3.5 1232 1248 -8 0.19 -0.44 DCF 834 856 -11 1216 1231 -7.5 0.39 -0.47

1.03 eV eBE HCF 948 966 -9 1283 1305 -9.6 0.13 -0.57 DCF 868 886 -8.9 1197 1208 -5.6 0.60 4 . 3 6

'The bending mode is designated as J2 and the C-F stretch as J3. bThe signs of the normal coordinate displacements were chosen to give the anion geometry closest to that in Table 11, when used with the triplet fora field of Weis et a1.I0

made after examining every permutation and choosing the combination that resulted both in a triplet C-X bond length shorter than the anion (fixed in the singlet simulation) and a bond angle consistent with theoretical predictions.

Comparison of the optimized frequencies for the triplet state of HCF with calculated values of Weis et a1.I0 supports the choice of the 1.19 f 0.02 eV eBE origin assignment. Weis et a1.I0 have calculated the harmonic frequency of the bending mode (a2) of HCF to be 1138.9 cm-' and the harmonic stretching frequency

DCF- ( c ) it

( w 3 ) to be 1269.3 cm-'. Using the 1.03 eV eBE triplet origin assignment for HCF, we obtained optimized frequencies of 966 f 25 and 1305 f 25 cm-', respectively. The optimized frequencies determined for w2 and w3 using the triplet origin of 1.19 f 0.02 eV eBE were 1054 f 25 and 1248 f 25 cm-', respectively. Both of the optimized frequencies found using the 1.19 eV eBE origin are in much better agreement with the calculated values than the corresponding frequencies for the 1.03 eV eBE origin. For DCF, these two modes were clearly separated and the optimized bending and stretching frequencies for both origin choices were quite similar. Although an absolute unequivocal origin assignment can not be made for 3HCF and 3DCF, the experimental evidence and Franck-Condon fits strongly support our triplet origin choice of 1.19 eV eBE.

2. HCCL, HCBr, and HCI. Since little information is available on the triplet states and the photoelectron spectra are severely congested, detailed Franck-Condon analyses similar to those carried out for the singlet states were not attempted. Photoelectron spectra showing all of the HCX(3A") - HCX-(% *A") transitions are shown in Figures 1 and 2. The transitions to the excited state show vibrational features at 850 f 60 (HCCl), 725 f 70 (HCBr), and 637 f 80 cm-l (HCI). Calculations of Scuseria et a1.16 predict C-X stretching frequencies of 869 (HCCl) and 753 (HCBr). Because of the larger mass of the iodine, the stretching frequency of HCI should be lower than that of HCBr. The calculated bending frequencies, HCCl(lO89 cm-l) and HCBr (1031 cm-l),

Spectroscopy of HCF, HCCI-, HCBr-, and HCI-

TABLE VI: Singlet-Triplet Energy Splittings of the Haloarbems (kd/mol)

exptl determined from upper bound” FCF simulationb

HCF <14.9 14.9 4 0.4 HCCl <14 4.2 f 2.5 HCBr <12.8 2.6 f 2.2 HCI <8.5 -2 to -10

” Rigorous experimental upper limit based on observed triplet transitions. bPossible values obtained by comparison of calculated and observed Franck-Condon envelopes (see text).

are similar because the bending normal coordinate involves motion primarily of the lighter hydrogen rather than the halogen. Since the observed frequencies follow the decreasing trend HCCl > HCBr > HCI and similar structure was observed in the deuterated analogues, we assign the observed vibrational progression in the HCX(3A”) - HCX-(ft 2A”) transition to the C-X stretch. The assignment of a vibrational peak as the triplet origin, however, is much more difficult. In fact the next section shows that the triplet origin is most likely not observed directly for the heavier halocarbenes because of weak Franck-Condon intensities.

E. Triplet-State Excitation Energies. Because of optical spin selection rules, a direct determination of the energy difference between singlet states and triplet states of these transient species is very difficult. Provided that Franck-Condon factors are sufficiently large near the origin in the transition from the stable anionic species to the neutral and that both spin states are en- ergetically accessible, photoelectron spectroscopy offers an un- ambiguous method for probing these optically forbidden states.

Predictions of the energy difference between the lowest singlet and triplet spin states have varied widely. Calculations on the excitation energy for the triplet state of HCF have ranged from 0 to 26.7 kcal/mol; more recent higher level calculations have agreed on a value of about 14 k c a l / m ~ l . ~ ~ J ~ , ~ ~ Calculations on the triplet excitation energies for the other halocarbenes range from 1.6 to 16.7 kcal/mol for HCC114J6*23.24,26 and from -1.1 to 4.1 kcal/mol for HCBr.14J6 Although the values for the triplet excitation are still unknown, there is general agreement that the triplet excitation energies decrease in the order HCF > HCCl > HCBr > HCI. previous work using photoelectron spectroscopy obtained upper

limits for the triplet excitation energy by examining Franck- Condon simulations of the singlet states. The upper limits for the triplet excitation energies were obtained from the electron kinetic energy where significant deviations of the experimental data from the Franck-Condon simulation of the singlet state occurred. In the present study, we had hoped that complete separation of the singlet and triplet vibrational progressions would be possible by exploiting the angular distributions of the photoelectrons. The subtracted data in Figure 3 were used to obtain values for the lowest eBE peaks definitely belonging to the triplet state. These are marked by arrows in Figure 3 and occur at 1.82 eBE (HCCl), 2.01 eBE (HCBr), and 2.05 eBE (HCI). The corresponding rigorous, direct experimental upper bounds on the triplet excitation energy from these peaks are given in Table VI.

Since the change in geometry going from the negative ion to the triplet state neutral is substantial, there exists the possibility that the Franck-Condon transition intensity at the triplet origin will be so low as to be unobservable. This is in fact the case for the heavier halocarbenes, and an alternative approach must be taken to further improve the upper bounds to the triplet excitation energy. This alternative approach involves calculating the Franck-Condon profile from theoretical negative ion and triplet-state geometries. While high-level ab initio calculations would be required to obtain meaningful singlet-triplet energy splittings, the geometries and frequencies from lower level calculations should be reasonable. The Franck-Condon envelopes calculated from these theoretical geometries can then be compared to the experimental profdes, and thus better estimates of the origin positions can be obtained than is possible from either experiment or theory alone.


Franck-Condon envelopes were determined from the results of ab initio Hartree-Fock calculation^^^ employing an STO-3G* basis set. Normal coordinate displacements were calculated for the transition using the triplet force fields. The normalaordinate displacements were combined with scaled (90%) vibrational frequencies to obtain a calculated Franck-Condon profile. These calculations showed that the intensity of the triplet origin transition should be much weaker than the maximum in each case: HCCl

and HCI (10-4). Since the dynamic range of the subtracted spectra described above is only a factor of 6-10, it is immediately apparent that the triplet origin must lie at considerably lower electron binding energy than given by the upper bounds given in Table VI. The calculated Franck-Condon profiles were used to estimate the triplet origin by matching the experimental maximum of the triplet vibrational profile (which is well determined) to the calculated maximum and using the calculated energy difference between the Franck-Condon maximum and origin to establish the position of the triplet origin. In each case, the calculated Franck-Condon profde width agreed reasonably well with the experimental width, giving some additional credence to this fitting procedure. With allowances for uncertainty in the calculated geometry, this analysis indicates that the triplet origin should lie between 1.28 and 1.50 eV eBE for HCCI, between 1.47 and 1.66 eV eBE for HCBr and 1.59-1.24 eV eBE for HCI. Combining this analysis with the observed C-X triplet stretching frequencies and the singlet vibrational origin assignments, we obtain the best estimates of the singlet-triplet energy differences shown in Table VI. In general, the origina lie 10 kcal/mol below the upper bound. In the case of HCI, this analysis predicts a ground state triplet for HCI!

One test of this procedure is to apply this analysis to HCF, where we have determined independently that the triplet origin is at 1.19 eV eBE, with a resulting triplet excitation energy of 14.9 f 0.4 kcal/mol. When this Franck-Condon estimation procedure was applied to HCF, we determined that the triplet origin is =1/4 of the maximum intensity and that the triplet origin must lie at 1.19 eV, lending more confidence in the procedure. The triplet excitation energies determined in this way were 4.2 f 2.5 kcal/mol for HCCl and 2.6 f 2.2 kcal/mol for HCBr. For HCI, the triplet is expected to lie -2 to -10 kcal/mol lower than the singlet state. All of the origin choices are significantly lower than the rigorous upper bound established with the subtraction process.

Although previous calculations of the triplet excitation energies have predicted a wide range of values, several of the more recent ones are in agreement with our current estimates. Both of the recently calculated values for the triplet excitation of HCF, 13.9 f 1.4 kcal/mol by Weis et a1.I0 and 14.5 kcal/mol of Shin et a1.,25 are consistent with our result of 14.9 f 0.4 kcal/mol. The triplet excitation energy of 4.2 f 2.5 kcal/mol for HCCl, established from the calculated Franck-Condon profile, is in agreement with calculated values of 5.4 f 2,16 and 6.0 kcal/mo1.26 Calculations on HCBr have predicted both a triplet14 and a singlet16 ground state. The calculated triplet excitation energy of Scuseria et a1.16 (4.1 f 2 kcal/mol) is consistent with the results of the Franck- Condon simulation (2.6 f 2.2 kcal/mol). There are no reported calculations on the singlet-triplet splitting with which to compare the prediction of a ground state triplet for HCI.

F. Electron Angular Distributions. Angular distributions of photoelectrons are a function of both the symmetry of the orbital from which an electron is ejected and the kinetic energy of the ejected e l e c t r ~ n . ~ ~ * ~ * ’ ~ Cooper and Zare54 predicted that an electron originating from an s orbital has an outgoing p wave and exhibits an angular distribution of photoelectrons that follows a cos2 B behavior that is independent of electron kinetic energy. For photoelectrons originating from a 2p orbital, the angular distribution is a function of electron kinetic energy. At threshold the angular distribution is nearly isotropic with /3 close to zero, and as the electron kinetic energy increases, /3 decreases to -1 (showing sin2 B behavior) about 1 eV above threshold. From this

of the maximum intensity), HCBr

(73) Hall, J. L.; Siegel, M. W. J. Chem. P h p . 1968,48,943.


state halocarbenes reflect the localization of the detached electron. Asymmetry parameters for the singlet states of the halocarbenes are given in Table I. Examination of the singlet state asymmetry parameters shows that 6 becomes increasingly more negative as one proceeds from HCF to HCI. In Figure 6, one sees that the singlet-state asymmetry parameters (open squares designated 'HCX) follow the same general trend as the halogen and the generalized curve of Hanstorp et for photodetachment from a p orbital.

In an earlier study on methylene, angular distributions for photodetachment to the singlet and triplet states were 0btained.7~ Photodetachment to the singlet state resulted in a negative asymmetry parameter, -0.27 f 0.02, at 1.485 eV eKE. De- tachment to the triplet state was nearly isotropic, @ = 0.02 i 0.05 (1.885 eV eKE). Similar to methylene, the halocarbene triplet states have nearly isotropic angular distributions (/3 = 0). Even in the simplest bonding picture, that of an spZ hybridized orbital, the angular distribution would be much more difficult to predict than for a simple p or s orbital. The more complex combination of in-plane p orbitals with the carbon 2s orbital results in a more isotropic distribution. Asymmetry parameters for the triplet states of the halocarbenes are sctn in Figure 6 (open circles designated 3HCX). They are all nearly isotropic, similar to the asymmetry parameter measured for triplet methylene.

Further support for our assignment of the HCI state peaked at 1.9 eV eBE as the singlet state is seen in Table I and in Figure 6. In Figure 6, the asymmetry parameter of -0.67 found for the singlet state of HCI matches the general trend that the asymmetry parameter increases after reaching a minimum and is consistent with photodetachment of an electron localized on a p orbital. If the asymmetry parameters for the two electronic states of HCI were exchanged in Table I, their signs would be completely in- consistent with the trends of the other halocarbenes in each of the spin states, as well as with the expectations for ejection of an electron from a p orbital in this range of electron kinetic energies.

The correlation between the angular distribution for de$achment of molecular ptype electrons (as a function of electron kinetic energy) and the atomic values as shown in Figure 6 is remarkable. If we assume the correlation holds at lower energies, tuning the photon energy such that the singlet transition falls near the minimum in the curve, at eKE = 0.8-1.0 eV, would maximize the difference between the angular distributions of the two transitions. This effect could be exploited to resolve experimentally the two transitions. Tuning the photon energy is not feasible with the present apparatus, but there is no intrinsic obstacle to such an experiment.

v. Canclusions We have measured the photoelectron spectra of HCF, HCC1,

HCBr, and HCI at Oo, 90°, and the magic angle with the 351.1-nm line of an argon ion laser. Rotationally corrected adiabatic electron affinities for the singlet states of HCF (0.542 f 0.005 eV), DCF (0.535 f 0.005 eV), HCCl (1.210 f 0.005 eV), HCBr (1.454 f 0.005 eV), and HCI (1.680 f 0.005 eV) are reported. Anion geometries for each of the halocarbenes have been found using our normal-mode displacements with available neutral singlet geometries and force constants. Best estimates of the triplet excitation energies are 14.9 i 0.4 kcal/mol for HCF, 4.2 f 2.5 kcal/mol for HCCl, and 2.6 f 2.2 kcal/mol for HCBr. HCI is predicted to be a ground-state triplet, lying between 2 and 10 kcal/mol lower than the singlet state with an electron affinity between 1.25 and 1.59 eV. Vibrational peaks in the 3AN states with a spacing of 850 f 60 (HCCl), 725 f 70 (HCBr), and 637 f 80 cm-" (HCI) are assigned to the C-X sttetching mode. Values for the asymmetry parametem have been found for the singlet and triplet states of the halocarbenes and for F, Br-, and I- using a 3.531-eV photon for photodetachment. The spectra taken at 0' and 90° laser polarizations combined with the values of the asymmetry parameters for vibrational peab in the photoelectron

'HCCP

mO. HCBr .I' Br 0 :'

I' I '

,' ,'

,'

, '.,. , - I I I - 1 0 0 0.5 1.0 1.5 2.0 2 5 3 0

E L E C T R O N K I N E T I C E N E R G Y ( e V )

Figure 6. Plot of the asymmetry paramcer, f l , versus electron kinetic energy. Asymmetry parameters for the halogens measured in this laboratory are marked by solid triangles. Asymmetry parameters for both spin orbit states of Br and I measured by Weaver and N e ~ m a r k ' ~ are indicated by solid squares. Open circles (3HCX) denote the asymmetry parameters for the 3Aff states of the halocarbenes and open squares ('HCX) denote the 'A' states of the halocarbenes. The dasheddotted line is a generalized curve for the asymmetry parameter versus electron kinetic energy for photodetachment of an electron in a p orbital and is taken from Hanstorp et al."

point it increases toward 6 - 2, or cosz 0 behavior. This dependence on electron kinetic energy comes from the interference terms of electrons ejected with AI = f l angular momentum quantum numbers and from the energy dependence of photodetachment cross sections for the different partial waves (AI = f l ) .

An elegant study on the angular distribution of electrons originating from a p orbital as a function of electron kinetic energy has been published by Hanstorp et al.74 They employ a few simple assumptions about photodetachment of electrons from p shells of negative ions and obtain a generalized dependence of the asymmetry parameter on electron energy. Their simple model qualitatively reproduces the behavior predicted by the more rigorous Cooper-Zare calculation^.'^

Values for the atomic halogen asymmetry parameters found in this laboratory, using 351.1-nm laser light, are given in Table I. Figure 6 shows the asymmetry parameters versus kinetic energy of the ejected electron for the halogens (filled points) and the halocarbenes (open points). The filled triangles represent data obtained in this laboratory and the solid squares were obtained by Weaver and N e ~ m a r k . ~ ~ The dottedaashed line in Figure 6 is the generaliztd curve for photodetachment of pshell electrons from Hanstorp et al.'4 Comparing the data points for the halogens to the curve of Hanstorp et al.,'4 the general appearance is the same. These results are also in qualitative agreement with the calculations (near threshold) of asymmetry parameters for the photodetachment of halogen negative ions by RadojeviC et al.76

Although the halocarbenes are molecular ions the angular distributions follow a form similar to that of atoms.64 A primary difference is that the molecular angular distribution is an auerage over all molecular orientations, while in the atomic case the laser electric vector defines the same orientation for all of the ions. The body-fixed molecular transition moment in the ions, however, provides some selectivity in the set of photodetached molecular ions. To obtain quantitative results for the angular distributions of photoelectrons, these averaging effects must be properly as- sessed. This type of detailed analysis is beyond the scope of this paper; therefore only a qualitative explanation of the angular distributions is given here.

To form the singlet, an electron is removed from a molecular orbital that consists of the out-of-plane pz orbitals on the carbon and halogen. The electron density is largely localized on the carbon pz orbital, and the asymmetry parameters for the singlet

(74) Hanstorp, D.; Bengtsson, C.; Larson, D. J. Phys. Rev. A 1989, 40,

(75) Weaver, A.; Neumark, D. M., private communication. (76) RadojeviE, V.; Kelly, H. P.; Johnson, W. R. Phys. Rea A 1987, 35,

670.

21 17. (77) Engelking, P. C.; Corderman, R. R.; Wendolceki, J. J.; EUison, G. B.;

ONeil, S. V.; Lineberger, W. C. J. Chem. Phys. 1981, 74, 5460.

J. Pkys. Ckem. 1992,96, 1141-1143 1141

K. Murray and Professors Robert W. Field and G. Barney Ellison. We also thank Professor Daniel M. Neumark and the students in his group for providing measurements on the asymmetry parameters of the halogens. Dr. Mark L. Polak provided insightful criticism on the manuscript and Lorraine Volsky provided expert editorial assistance.

spectrum of HCI have ken used to determine the state symmetries of HCI.

Acknowledgment. This work was supported by National Science Foundation Grants Nos. CHE88-19444 and PHY 90- 12244. We acknowledge stimulating discussions with Dr. Kermit

EPR Spectrum of CuPF$

M. Histed, J. A. Howard,* R. Jones, and M. Tomietto Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario, Canada KIA OR9 (Received: August 5, 1991)

CuPF, is formed from Cu atoms and PF3 on solid adamantane at 77 K in a rotating cryostat. ' It has the magnetic parameters a6, = 4205 f 10 MHz, a31 = 1100 f 10 MHz, alg = 90 f 10 MHz, and g = 1.999 f 0.002, has a 2AI electronic ground state in the point group C,, and can be considered to be a copper-centered radical and not a phosphoranyl formed by addition of a Cu atom to PF,. About 70% of the unpaired spin is located in the metal 4s orbital, 8% in the P 3s orbital, and 0.5% in the F 2s orbitals for a total unpaired spin s population of 78.5%. The remaining 21.5% of unpaired spin is probably located in the Cu 4p orbital. Cu3 is also formed in adamantane and does not appear to react with PF3. Cu, rather than CuPF3 is formed in solid cyclohexane and is also unreactive toward PF3.

Introduction In previous publications'" from this laboratory we have shown

that the group 11 atoms Ag and Cu react with CO on inert hydrocarbon matrices at 77 K on a rotating cryostat to give the Paramagnetic EPR visible mononuclear carbonyls M(CO), where x = 1-3 in the case of Ag5 and x = 1 and 3 for C U , ~ Cu(CO), not W i g visible because it is linear with a ,lI ground state. One form of Ag(CO), is, however, detected because it has a 2A1 ground state with most of the unpaired spin located in the metal 5s orbital? Interestingly, two forms of Ag(CO)3 have been observed, one with a 2A1' ground state with the unpaired electron located in the 5s orbital of AS2 and a second with a Z A P ground state and the unpaired electron located principally in the 5pz orbital on AgaS These results have complemented and extended the pioneering spectroscopic studies of matrix-isolated group 11 carbonyls in rare gasca by Ogden,' 0zin,"lo Moskovits,ll K a ~ a i , ~ Z ' ~ and co-workers.

Trifluorophosphine (PF,) is a ligand that behaves l i e CO and readily stabilizts zero-valent metals because the udonor properties of the P lone pair and the r-acceptor properties of the P 3d orbital are modified by the electronegative fluorine ligands.14 Thus, stable complexes such as Ni(PF3)4,1SJ6 CT(PF~)~," and Fe(PF3)518 are readily prepared. There has been less success in the identification of paramagnetic metal-PF, complexes. Thus, Timms19 reported that no Cu-PF3 compound of appreciable stability is formed at 143 K from reaction of Cu atoms with PF3 at 77 K. BowmakeP did, however, find IR bands from Cu atoms and PH3 in the neat phosphine and in a krypton matrix that he tentatively suggested were from CU(PH,)~ with x = 1-3.

More recently we have shown that Ag atoms react with PF3 to give the mononuclear monoligand complex AgPF3, but spectra could not be assigned to Ag(PF3), and Ag(PFJP2l We have also shown that Cu atoms react with the trivalent phosphorus compounds P(OMe)3 and PMeJ to give the tris(triva1entphosphorus) complexes Cu[P(OMe),], and Cu[PMe,], that are isostkctural with C U ( C O ) ~ . ~ ~

In the present paper we report an EPR study of the paramagnetic products given by reaction of 63Cu atoms with PF3 in solid inert hydrocarbons at 77 K on a rotating cryostat.

Experimental Section was used

to allow 63Cu atoms in their ground electronic state to react with A rotating cryostat, described in detail

Issued as NRCC No. 32935.

PF3 on the surface of an inert hydrocarbon at 77 K. The reaction products were trapped in a fresh layer of matrix. The deposit was scraped into a 3-mm suprasil tube, still under high vacuum and at 77 K, and examined by X-band ( u - 9.3 GHz) and Q-band (v - 35 GHz) EPR spectroscopy in the cavities of a Brukcr ESP 300 spectrometer. Samples were annealed in the cavity of the spectrometer with the aid of a Bruker variable-temperature ac- cessory.

The magnetic field was calibrated with a Varian gaussmeter, and the X-band microwave frequency was measured with a Systron-Donner Model 60 16 frequency counter. Magnetic pa-

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0022-3654/92/2096-1141$03.00/0 Published 1992 by the American Chemical Society

Negatlve Ion Photoelectron Spectroscopy of HCF-,, HCCr ... · Spectroscopy of HCF, HCCl-, HCBr-,...

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