Canadian Journal Journal canadien of Chemistry de chimie Publistzed by Publit! par THE NATIONAL RESEARCH COUNCIL OF CANADA LE CONSEIL NATIONAL DE RECHERCHES DU CANADA

Volume 52 Number 8(Part 1) April 15, 1974 Volume 52 numtro 8(partie 1) 15 avril 1974

The High Resolution Photoelectron Spectra of some Iodoalkanes, Iodocycloalkanes, Iodoalkenes, and Fluoroiodohydrocarbons

ROBERTO ANGELO ANTONIO BOSCHI Str~ldo; A G , CH4OO?, Btr.sc.1, Sn~i / :er l t r~~t l

A N D

DENNIS RUSSELL SALAHUB QI IOI I / I I I ) I Tlreory Group. Dcptrr/ri~e~rr nf Applicd Mtr/lrc~r~rtr/ic.\, Utri~~c,r.si~v uJ' Wotcrloo, Wu/t.rloo, O/l/trrio N2L 3G1

Received October 17, 1973

The high resolution photoelectron spectra for the following iodine-containing molecules have been measured: iodomethane, iodoethane, 1-iodopropane, 1-iodobutane, l-iodopen- tane, 1-iodohexane, 2-methyl-1-iodopropane, 3-methyl-1-iodobutane, 2-iodopropane, iodo- cyclopentane, iodocyclohexane, 2-methyl-2-iodopropane, trifluoroiodomethane, 2,2,2- trifluoro-1-iodoethane, pentafluoroiodoethane, 3,3,3,2,2,1,1-heptafluoro-1-iodopropane, iodobenzene, pentafluoroiodobenzene, vinyl iodide, ally1 iodide. The spectra are discussed in terms of extended Hiickel ( E H ) molecular orbital calculations and of the qualitative changes upon variation of the alkyl group.

On a mesurt les spectres de photo6lectrons i haute rtsolution des iodures suivantes: iodomithane, iodotthane, iodo-1 propane, iodo-l butane, iodo-1 pentane, iodo-1 hexane, methyl-2 iodo-1 propane, mtthyl-3 iodo-1 butane, iodo-2 propane, iodocyclopentane, iodocyclohexane, mtthyl-2 iodo-2 propane, trifluoroiodomtthane, trifluoro-2,2,2 iodo-1 tthane, pentafluoroiodotthane, heptafluoro-l,l,2,2,3,3,3 iodo-1 propane, iodobenzsne, pentafluoroiodobenzGme, iodure de vinyle et iodure d'allyle. On discute les spectres a I'aide des calculs d'orbitales molCculaires dans l'approximation de Hiickel ttendue e t aussi en fonction des changements qualitatifs lors de la variation du groupe alkyle.

[Traduit par le journal]

Can. J. Chem., 52, 1217 (1974)

Introduction We have recently presented the far U.V. spectra

of a large number of simple molecules containing a single iodine atom (1, 2). In these papers we found it useful to make comparisons with the photoelectron (p.e.) spectra and discussed some of the principle features of the latter. In the present paper we present the high resolution p.e. spectra in their own right. These are in almost all respects simpler than the U.V. spectra and give directly the ionization potentials (i.p.'s) of the molecule and (in the approximation of Koop-

mans' theorem (3)) the orbital energies, that is, the experimental quantities corresponding to the eigen values of the Hartree-Fock equations.

The molecules studied are (iodomethane), iodoethane, (1-iodopropane), (1-iodobutane), (1-iodopentane), 1-iodohexane, 2-methyl-1-iodo- propane, (3-methyl-1-iodobutane), 2-iodopro- pane, iodocyclopentane, iodocyclohexane, (2- methyl-2-iodopropane), trifluoroiodomethane, 2,2,2-trifluoro-1-iodoethane, pentafluoroiodo- ethane, 3,3,3,2,2,1,1-heptafluoro-1-iodopropane, iodobenzene, pentafluoroiodobenzene, vinyl io-

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1218 C A N J . C H E M . VOL. 5 2 . 1974

dide, ally1 iodide. Because of space restrictions the spectra of the molecules in parentheses are not presented here. '

Some of these molecules have been studied in part by previous workers. The p.e, spectrum of iodomethane is well known (4-8). It is included here for com~leteness and since correlations will be drawn with it in the discussion of the remain- ing spectra. The spectra of the I-iodoalkanes (C,,H,,,+ , I ; n = 1-5) and a few branched mole- cules have been measured (9-1 1) and the first two i.p.'s discussed in terms of the coinpetition between spin-orbit coupling and conjugation; however, the discussion so far has been limited to the two lowest bands. Baker et al. (10) pre- sented the spectra of iodoethane, I-iodopropane, 2-iodopropane, and I-iodopentane and discussed them very briefly. Very recently Kimura et al. (11) studied the spectra of RX (R = ethyl, n-propyl, n-butyl; X = Cl, Br, 1) and attempted to correlate the spectra with the Pauling electro- negativity of the halogen atom and gave an inter- pretation in terms of equivalent orbitals. A cor- relation diagram for the vinyl halides including vinyl iodide has been given by Turner et al. (12) without presenting the spectra themselves. The iodobenzene spectrum is known (12). Below we give an interpretation of some of the higher bands which differs from that of ref. 12. The spectrum of 2,2,2-trifluoro- I -iodoethane has been- presented by Turner (4) with, however, only a very brief discussion of the spin-orbit splitting of the first two bands.

The unifying feature of the present work is of course the presence of the iodine atom and the behavior of the various bands upon changing of the alkyl group. Below we attempt to give a con- sistent assignment for most of the bands out to about 18 eV. The assignment of the methyl iodide spectrum is quite straightforward. For the other molecules, with the exception of the work by Kimura et al. (1 1) on ethyl, propyl, and butyl iodide, virtually no attempt has been made to assign the bands at higher .energy. Also since many of our spectra are better resolved than those available in the literature we feel that any repetition implied in the last paragraph is justified.

'The spectra are available at a nominal charge from the Depository of Unpublished Data, National Science Library, National Research Council of Canada, Ottawa, Canada K I A OS2. The spectra are numbered D l to D6 in the order in which they appear above.

To help in our interpretation of the spectra we have carried out molecular orbital (m.0.) calculations of the extended Huckel (EH) type (13) for all of the molecules.

Experimental and Computational Methods The p.e. spectra were measured with a Perkin-Elmer

PS 16 instrument using a He1 source and a n electrostatic analyzer with a sector field of 127". The resolution was approximately 20 mV. All chemicals were from Koch- Light or Fluka A.G. and were purified by gas-phase chronlatography before use.

The EH program was based on a standard program (14) modified to calculate overlap integrals involving 5s and 5p orbitals (15, 16). The orbital exponents used were those proposed by Burns (17) which we have found usual- ly give better overall agreement with observed p.e. spectra than the usual Slater values. A value of 1.5 for the con- stant k which enters the off-diagonal matrix elements was found to give the best results.

Calculations using a modified C N D O method (18) were also carried out; however, the agreement between these and the observed spectra was not nearly as good as that of the EH calculations. A re-evaluation of the param- eters which enter the C N D O method or the inclusion of one-center exchange integrals to yield an iNDO method could conceivably improve the agreement. Below we report only the EH results. For the fluorinated molecules the EH results failed to correlate with the observed spectra. This is a usual situation for EH calculations o n molecules containing highly electronegative atoms. The EH results are not given in these cases but rather qualita- tive considerations are used to assign the bands.

Results and Discussion

Those spectra which we present are in Fig. 1 and the observed maxima (vertical i.p.'s) for all of the molecules, including those whose spectra are in the Depository of Unpublished Data, are gathered in Table 1 along with the results of the calculations. In all cases where the i.p.'s were also determined from Rydberg series in the far- u.v. spectrum (1) the two values agree within the experimental uncertainty. The molecules may be conveniently divided into four categories, viz., normal iodoalkanes, branched and cyclic iodoalkanes, fluoroiodoalkanes, and iodoalkenes.

Normal lodoalkanes The prototype of this series is methyl iodide,

the p.e. spectrum of which is well documented (4-8). We will give a brief description of this spectrum since the succeeding spectra may then be discussed with reference to it. The valence electronic configuration of ground-state methyl iodide (C,, symmetry) is

(lal)2(2al)2(le)4(3a1)2(2e)4

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BOSCHl A N D SALAHUB: PHOTOELECTRON SPECTRA 1219

The lowest lying m.o.'s l a , and 2a, are essen- tially the in-phase and out-of-phase combina- tions, respectively, of carbon 2s and iodine 5s atomic orbitals. The l e m.0. may be regarded as a degenerate C H bonding orbital; the 3a, m.o. corresponds to the CI bond and the 2e level to the 5p lone pairs of iodine which have n sym- metry with respect to the CI bond.

The presence of the iodine atom leads to large interactions between electron spin and orbital angular momenta so that configurations involv- ing unpaired electrons in degenerate m.o.'s with large coefficients on the iodine must be treated using double groups (19). Thus removal of an electron from 2e gives rise to E3/, and El states which are observed at 9.54 and 10.14 eV, respec- tively. Ionization from 3a, occurs at 12.5 while the two Jahn-Teller split components of l e are observed at 14.7 and 15.5 (shoulder). A peak at 19.5 eV is assigned to 2a, . Ionization from la , has not yet been observed. Price2 predicts a value of about 21 to 21.5 eV for HI on the basis of an extrapolation in the series XH,, (17 = 0, 1, 2, 3, 4) across a row in the periodic table. Since a carbon 2s electron is more tightly bo~ lnd than a hydrogen Is we would expect l a , to be a bit deeper than this in CH31, perhaps at around 22 or 23 eV. The He11 spectrum would locate this level. For CH3Br, ,&brink3 in fact found the lowest band in the Hell spectrum just a b o ~ ~ t 1 eV lower than the value which Price predicted for HBr.

Thus the whole of the p.e. spectrum of methyl iodide is qualitatively understood. The EH calcu- lations mirror the observed spectrum very well, except of course for the spIitting of the 2e and I e levels due to spin-orbit coupling and Jahn- Teller distortion, respectively.

What happens when the symmetry of the moIecule is lowered by substituting a longer alkyl group for the methyl group? lnasfar as the first two bands (corresponding to E,;, and El l , of CH31+) are concerned the question has been studied by Brogli and Heilbronner (9). If we as- sume C, symmetry for the I-iodoalkanes, then the 2e level is split into an a' and an a" level, both yielding E,,, states of the molecular cation, the a' orbital belng less tightly bound. For the case of zero spin-orbit coupling this is the only effect. The a'-a" energy difference found in the E H

ZProfessor W. C. Price. Personal communication. 3Dr. L. Asbrink. Personal communication.

results indicates that this split is rather small. The observed split is nearly the same as that of CH,I (between 0.58 and 0.56 eV for all the I-iodoalkanes) so it seems that the orbital is very much localized on the iodine atom a n d "sees" only the local cylindrical symmetry about the CI bond and that the orbital angular momentum is only slightly quenched by the alkyl group.

The observed and calculated i.p.'s for the five lowest states are shown in Fig. 2 as a function of the number of carbon atoms in the alkyl group. The gradual shift to lower i.p. and smaller spin- orbit splitting for the first two bands with in- creasing length of the alkyl chain can both be explained by a slight delocalization o f the lone pair onto the alkyl group. This shift to lower i.p. is present but less accent~~ated in the E H results. The pop~~la t ion on the iodine atom in the a' orbital is 92, 91, 89, 87, 87, and 8 6 x a s the series is ascended. This slight delocalization is also con- sistent with the short vibrational progressions observed. Each of the bands shows a spacing near 1100 cm-' which is assigned t o a CH, wagging vibration ( I , p. 289).

The third band in all of these spectra is in analogy with CH31 and as supported by the EH r e s ~ ~ l t s assigned to ionization of a C I 2po, f 5pal bonding electron. It too ~ ~ n d e r g o e s a shift to lower i.p. as the length of the alkyl group in- creases. This behavior is shown in Fig. 2, along with the EH results which reflect the shift satis- factorily. The shift is due to a gradual increase in the delocalization of the orbital over the moIecule.

The following bands out to about 18 eV are due to ionizations from the CH and C C bonds. With the delocalized m.o.'s obtained from the EH calculations it is often not possible t o identify orbitals corresponding t o localized bonds so that when we speak of C C and CH bonds below it should be understood that we are referring to m.o.'s which are mainly bonding con~binations of atomic orbitals between carbons o r carbon and hydrogen.

For iodoethane and iodopropane there is a one-to-one correspondence between the calcu- lated levels and the observed maxima while for the larger n~olecules fewer maxima are observed than calculated indicating an overlapping of close-lying bands. It would be interesting to know which of these peaks corresponds t o mainly C C and which to CH orbitals. Calculations using the equivalent orbital method which is based on

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1220 C A N . J . C H E M . VOL. 5 2 , 1973

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BOSCHI AND SALAHUB: PHOTOELECTRON SPECTRA 1221

FIG. 1. The photoelectron spectra o r (a) iodoethane, (b) I-iodohexane, (c) 2-methyl-I-iodopropane, (d ) 2-iodopro- pane, ( r ) iodocyclopentane. (f) iodocyclohexane, (g) trifluoroiodomethane, ( / I ) 2,2,2-trifluoro-1-iodoethane, ( i ) penta- fluoroiodoethane, ( 1 ) 3,3,3,2,2,1 , I -heptafluoro-I -iodopropane, (k) iodobenzene, (I) pentafluoroiodobenzene. ( 1 1 1 ) vinyl iodide. ( 1 7 ) ally1 iodide.

the concept of interacting bonds and is "cali- brated" with observed spectra could be interest- ing in this respect. Kimura et al. ( I I) came to the conclusion, on the basis of a type of equivalent orbital calculation, that in ethyl iodide the ion- ization from the CC orbital precedes that from a CH orbital. They made several drastic simplifi- cations, however, and we show below that in the case of ethyl iodide the substitution of fluorine for some or all of the hydrogens allows

us to make this distinction assigning the bands at 12.9, 14.9, and 15.5 eV to CH orbitals, and that at 13.7 eV to the CC bonding electron. Similarly for propyl iodide the band a t 12.9 eV is assigned to ionization from a CC orbital and that a t 12.3 eV to a CH orbital. Throughout the series of I-iodoalkanes the two bands which appear at 12.9 and 13.7 eV in iodoethane appear with nearly the same form only shifted slowly to lower i.p. This shift is well accounted for by the

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1222 C A N . J . CHEM. VOL. 5 2 . 1974

TABLE I. Observed and calculated (extended Hiickel) ionization potentials (eV) and symmetry species for iodine containing molecules

Ionization potential Ionization potential Symmetry - Symmetry

Molecule Obs. Calcd. species Molecule Obs. Calcd. species - -

C6H131 (C,) (confd.) 11 . 5 12.48 13.10 13.34 13.78 13.89 13.91 13.93 14.20 14.71 14.78 15.23 15.58

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BOSCHI AND SALAHUB: PHOTOELECTRON SPECTRA 1223

TABLE I. (Concluded) .- -- . -- -

Ionization potential Ionization potential - Symmetry - Symmetry

Molecule Obs. Calcd. species Molecule Obs. Calcd. species -.

c-C5Hl ,I (C,) (cotztd.) 13.69 a t-C,HJ (C3") 14.15 : 9.87 e a 14.33 a 10.75 11.14 a ,

14.40 14.40 a 15.01 a

16.3 15.40 a c - C S H ~ ~ I (C,) Equatorial 8.91 9 .88 a"

Axial

Iodobenzene (C,,) 8.67 9 .39 9.66

10.46

Vinyl iodide (C,) 9 .32 10.09 11.55 12.2 14.6 15.7

Allyl iodide (C,) 9 .32 9 .72

10.26 12.25 12.75 13.9 14.7 15.8

O B S E R V E D

FIG. 2. Calculated and observed i.p.'s of 1-iodoalkanes as a function of the number of carbon atoms in the alkyl group.

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1224 C A N . J . CHEM. VOL. 5 2 . 1974

EH calculations; however the order of the CH one correspondence between the observed and and CC levels is reversed from what we expect calculated bands, only one calculated level being on the basis of the spectra of the fluorinated unaccounted for in the spectrum. This is pre- molecules, the uppermost calculated m.0. being dicted to occur in the 14-15 eV region where the primarily in the CC bonds. They are, however, spectrum consists of broad overlapping bands reasonably close in energy and in view of the and is surely present although unresolved. simple nature of the calculations it is not sur- prising to find inversion of some close-lying Fluoroiodoalkanes levels. We believe the qualitative arguments Brundle and co-workers (20, 21) have ex- given below are in this instance more reliable amined the effect on p.e. spectra of substituting than the calculations. so that we assign the first fluorine for all of the hydrogens of a molecule. of these bands to ionization from a CH orbital Their main conclusion was that in planar mole- and the second to a CC orbital. For the larger cules perfluorination causes the i.p.'s to shift n~olecules this area of the spectr~~m degenerates upwards by 0-1 eV for n electrons and by 2-3 eV into a single broad band because of the presence for (3 electrons. In nonplanar molecules the of a large number ofclose-lying levels (see Fig. 1 b). above distinction cannot be made; however, it

As was the case for methyl iodide there is then has been observed that all the levels are shifted a large gap in the spectrum until the onset of to higher i.p. by varying amounts. ionization from levels involving carbon 2s and The spectra of CF31 (I), CF3CH,I (2), C,F,I iodine 5s orbitals. (3), and C3F,I (4) are shown in Fig. 1g;j. Below

we discuss the effect of fluorination on the posi- Branclled and Cvclic Alkanes tion of the lone-pair bands and their spin--orbit

As was the case for the U.V. spectra (I, p. 735) splitting and the effect on the CC and CH levels. the p.e. spectra of the slightly branched mole- The region above about 15.5 eV is complex owing cules, 2-methyl-I-iodopropane and 3-methyl-1- to the presence of bands due to the ionization of iodobutane, are very similar to those of their fluorine lone pairs and electrons in CF bonds straight-chain isomers so that the discussion (see Fig. lg). No attempt is made t o analyze this above holds. region. As stated above, rhe EH calculations are

When the iodine is attached to a secondary or not much help in this respect so that we prefer tertiary carbon (Fig. Id-f) the mixing of the to use qualitative arguments. lone-pair orbitals with the alkyl group is pre- The presence of two relatively sharp bands sumably greater and results in a slight lowering separated by nearly the same amount as the two of the first i.p. and a small decrease in the spin- lone-pair bands in the perhydrogeno compounds orbit splitting. The appearance of the first com- leaves the assignment of the first peaks of 1-4 ponent is essentially unaltered whereas the to lone-pair ionizations beyond reasonable second has lost its discrete fine structure. A doubt. The center of gravity of these two bands similar diffuseness was noted for the Rydberg has ~~ndergone a shift of 1.25-1.35 eV for the bands leading to this i.p. (1, p. 735) and may be perfluoro compounds and of 0.65 eV for the attributed to predissociation. The lowering of partially fluorinated molecule 2. the i.p. for the first two bands is reflected in the The most striking difference in these peaks is EH results in a reasonable manner. The calcu- that they have become much broader and exhibit lated first i.p. for cyclopentyl iodide is a bit too some poorly resolved fine structure. This broad- low compared with the other values. ening is of course least for 2 where the bands still

The next band is once again assigned to ion- resemble those of iodoethane, except that now ization of a CI bonding electron. It too has the 1-0 transition has gained significantly in undergone a shift to lower i.p. with respect to intensity. This fine structure indicates that the the 1-iodoalkanes and once again these shifts are "lone pairs" are in fact somewhat delocalized closely paralleled by the EH results. The remain- over the molecule. ing peaks out to 18 eV are due to ionization of The splitting between the bands has, however, CC and CH bonding electrons although it is not remained practically constant being 0.56, 0.62, easy to make definite assignments. 0.62, and 0.60 eV for 1-4 (taken between the most

For isopropyl iodide there is nearly a one-to- intense vibrational peak of each band) and 0.62,

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BOSCHI AND SALAHUB: PHOTOELECTRON SPECTRA

0.58, and 0.57 eV for iodomethane, iodoethane, and iodopropane, respectively. A mechanism similar to that proposed by Brogli and Heil- bronner (9) may be applicable. That is, we know that the orbital is delocalized because of the broadness of the band and the vibrational struc- ture. We would therefore expect the spin-orbit splitting to be smaller. The inclusion of spatial effects, however, (by coincidence) nearly cancels this out. As the interaction with the rest of the molecule increases we would expect to see larger splittings dominated by the conjugative effects (see vinyl iodide below).

The following band in all the spectra is well separated .both from the lone-pair bands and from the bands at higher i.p. and is assigned to ionization of a CI bonding electron. The shifts undergone with respect the perhydrogen' FIG. 3. Con~parison of the spectra of iodoethane (c), molecules are 0.85, 1.5, 1.15, and 1.2 eV for 1-4. 2,2,2-trifluoro-1-iodoethgne (b), and pentafluoroiodo- I t is not immediately apparent why the largest ethane (a) shifted so that the bands due to ionization of shift occurs for the partially fluorinated mole- the CI bonding electrons coincide.

cule. A plausible explanation in terms of equiva- propyl iodide is absent i n the (shifted) spectrum lent orbitals proceeds as follows. The self-energy of and is thus assigned to a CH orbi tal . of the CI bond becomes more negative upon inclusion of fluorine. The presence of fluorines in the cr position then leads to large interaction matrix elements with the fluorine lone pairs and the CF bonds causing a shift to lower i.p. for the perfluoro compounds but not for 2 since it only has fluorines in the p position. A study of further partially fluorinated iodides might throw some light on this.

The position of bands due to the CC and CH electrons is not obvious as these fall in the same region as the fluorine ionizations. However, a comparison of the spectra of C2H,I, 2, and 3 is illuminating. Let us assume that in each molecule the CI and the CC bond undergo nearly the same shift upon fluorination. This is not unreasonable since they have the same symmetry and not too different energies. I f we then shift the spectra so that the CI bands coincide (Fig. 3) we see that the 12.9 eV band of ethyl iodide becomes weaker for 2 and disappears altogether for 3 indicating that this band is due to ionization of a CH elec- tron. This band is followed by a band which very nearly coincides in the three (shifted) spectra and is reasonably assigned to a CC electron. In a similar manner the band at 14.3 eV of 4, which is now clearly separated from the fluorine bands is assigned to a CC orbital as is the 12.9 eV band of n-propyl iodide. The band at 12.3 eV in n-

Unsaturated Molecules For most of the iodoalkanes discussed above

it was rather easy to identify the bands due to the iodine lone-pair electrons. The lone pairs are reasonably well localized on the iodine atom and are influenced by a potential due to the alkyl group which has nearly C,, symmetry so that a discussion in terms of spin-orbit coupling is meaningful and in fact the lone-pair bands are very similar to those in methyl iodide.

The situation in unsaturated iodides is some- what more complicated since large conjugative effects may come into play, yielding rather de- localized lone pairs which give bands in the p.e. spectrum which have lost some of their character- istic sharpness.

We start our discussion of the unsaturated molecules with iodobenzene (Fig. Ik) since our discussion of vinyl iodide (Fig. 1\77) will depend to some extent on this.

The p.e. spectrum of benzene is well known (22) and has bands at 9.6, 1 1.4, and 12.1 eV which are assigned to e,,(n), e,,(o) and a2,(x) orbitals, respectively. I f a halogen atom is now substituted for one of the hydrogens one of the lone pairs (out-of-plane) can conjugate with the n system of the ring. The other lone pair (in-plane) has o symmetry and would interact with the ring CJ

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1226 C A N . J . C H E M . VOL. 5 2 , 1974

electrons. We would expect this interaction to be eV A E ( b 2 ( n ) - b , ( n ) ) less and might expect an energy level diagram, such as that of Scheme 1, to result. I ,

FIG. 4. Splitting of halogen lone-pair orbitals in halobenzenes us. the separation of the unperturbed levels (ev).

SCHEME I

t rum and exhibits some vibrational structure so he e, , level of benzene is split through mixing we must assign the 9.66 eV band of iodobenzene with the iodine x lone pair giving b~ and a2 levels. t o the b2(n) orbital. In summary then we agree The a2 orbital has a node running through the with the assignments of Turner et al. (12) for the iodine atom so that this level should be quite first four bands as b,(n), a,(n), b,(n), b,(n). close to the degenerate e, , level of benzene. The T~~~~~ then assigned the fo l lowing band a t b, orbitals are mixtures of ring and halogen 11.4 eV to the third n level (a,, in benzene). orbitals, the relative weights depending on which H~~~~~~ it is now thought that the highest halogen is considered. orbital precedes the lowest n (22) a n d the bands

It is instructive to consider the behavior of in benzene at 11.4 and 12.1 e~ a re assigned to these levels throughout the series of halobenzenes ionization from the 3e2,(o) and 1 levels, (C6H5X; X = C1, Br, 1). In Table 2 we have respectively. We make the analogous assignment gathered the i.p.'s for the b ~ ( n ) , a2(n), b2(n), for the bands at 11.4 and 12.2 in iodobenzene. and b,(n) levels for all of the molecules. These (I t should be mentioned that agreement is not are taken from Turner's book (12) or the present unanimous on this ordering (23).) The 3e2, work. The difference between the a2(n) and b2(n) level should of course be split under the lower levels may be taken as the difference between the symmetry; however the magnitude of the split- unperturbed orbitals. In Fig. 4 we have plotted ting is liable to be small for o electrons and is the energy shift of the n lone-pair orbital us. the not resolved. Further confirmation of this assign- distance between the unperturbed levels for the ment comes from the spectrum of pentafluoro- three halobenzenes. The variation is monotonic iodobenzene (see below), Here the deepest x level and we will use this figure later on in the discus- occurs at 13.20 eV and the upperlnost o orbital sion of vinyl iodide. a t 13.9. The above assignment then gives shifts

I t might at first sight seem strange to have on perfluorination of 1.0 and 2.5 eV which are assigned the band at 9.39 eV to the a2(n) level typical values for x and o electrons, respectively since this level was much closer to 9.7 eV in the (20,21). The three other possible orderings of the chloro and bromo compounds, and in fact there lowest n and the highest o orbitals in iodoben- is a band at 9.66 in the iodobenzene spectrum. zene and pentafluoroiodobenzene would lead to However in both chloro- and bromobenzene the shifts of the n level by 2.5, 1.8, and 1.7 eV and of lone-pair band is the strongest band in the spec- the o level by 1.0, 1.7, and 1.8 eV, respectively,

which are not in keeping with previous ex- TABLE 2. Ionization potentials of halobenzenes (eV) perience.

We would expect ionization of a CI bonding Ionization potential (ev) electron to give rise to a band somewhere near

Molecule 12.5 eV. There is a well pronounced shoulder on b1(7t) a2(d bz(n) bl(n) - the high-energy side of the peak a t 12.2 eV and

CsH6 9.6 9 .6 we tentatively assign this to expulsion of an C6H,C1 9.06 9.69 11.32 11.69 electron from the C I bond. C6H5Br 9.05 9.67 10.61 11.18 We have seen above the effect o f fluorination C ~ H S I 8.67 9.39 9.66 10.46 - on the spectra of ethyl and propyl iodide. As

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BOSCHI AND SALAHUB: PHOTOELECTRON SPECTRA

TABLE 3. Predicted ionization potentials of vinyl halides (eV)

Ionization potential (eV)

Molecule a* b c d e f

'a, the unperturbed rr level; b. the unperturbed lone-pair level (values taken from cor- respon~ling halobenzene); c, b - a ; d, the predicted shift of the a" lone pair (taken from Fig. 4); e, predicted value of a" lone pair; f, observed values.

Brundle et al. (21) pointed out, in aromatic molecules perfluorination usually affects the n levels only slightly while shifting the o levels to higher energy by about 2-4 eV. Thus in per- fluorobenzene (12, 21) we find that the el, level occurs at 10.1 eV (a shift of 0.5 eV). This is followed by a gap of about 2.5 eV until a band at 12.8 eV which is due to the e2,(n) orbital (21). In iodobenzene the corresponding "gap" was about 3 eV and was filled with the two lone-pair bands separated by about 0.8 eV. We would expect to find these somewhat shifted in the fluorinated molecule.

In fact the first two n bands in pentafluoroio- dobenzene occur at 9.84 and 10.25 eV and show some vibrational structure. These are shifted with

This region is complicated; however, there is a small peak just at 13.7 eV.

We now turn to vinyl iodide (Fig. Im). This is quite similar to the case of iodobenzene, one of the iodine lone pairs (a" symmetry) mixing with the double bond and the other (a' symmetry) remaining relatively unaffected.

Let us also refer to the spectra of vinyl chloride (24) and vinyl bromide (25). The relevant data are gathered in Table 3. If we take the value for the unperturbed lone pair from the corresponding halobenzene and the unperturbed n level as the ethylene value 10.51 eV (12) and use Fig. 4 we can predict the location of the lone-pair bands. We see that the agreement (Table 3) is quite reasonable. Note that in vinyl iodide the un-

respect to the corresponding bands in iodo- perturbed lone pair has a lower i.p. than ethylene benzene by 1.1 and 0.9 eV, respectively, which so we would expect the uppermost orbital to be are reasonable values for n electrons. We then mostly on the iodine, in vinyl bromide the two look for the third n band about 3 eV higher and assign the band at 13.20 accordingly.

This leaves two bands in the gap at 10.96 and 11.72 eV which are assigned to the in-plane and out-of-plane lone pairs, respectively. These are both shifted by about 1.3 eV. The band at 10.96 eV is sharp and the strongest in the spectra and must be assigned to the lone pair in the molecular plane. The shift of 1.3 eV is rather small for a o electron so we presume this lone pair to be

are very close and highly mixed i~ a n d lone-pair orbitals would result, while in vinyl chloride the uppermost level will be mostly the C=C n orbital. The mixing can be seen in the shape of the bands in the vinyl iodide spectrum, the second band being very sharp and intense, a typical lone-pair band, while the first and especially the third are somewhat broader indi- cating a larger mixing. The first band is still, however, reasonably sharp and is surely pre-

reasonably well localized and in fact the band is dominately a lone-pair band. sharp and shows only a limited fine structure. The next band at 12.2 eV is assigned to ex- The band at 11.72 on the other hand is consider- pulsion of a CI bonding electron (1 2.5 eV in ably wider and shows three or four vibrational CH,I, 11.6 eV in C2H51). The remaining bands components indicating a larger degree of de- at 14.6 and 15.7 eV are due to ionization from o localization for this n lone pair. The following levels in the CC and C H bonds. Both the EH band at 13.9 is then assigned to the uppermost o and CNDO calculations indicate that the pre- ring orbital as explained above. dominately CC orbital has a lower i.p. than the

As for the CI ionization, in the C2H,I-C2H51 CH; however, this should be treated cautiously. case this band was shifted by 1.2 eV so we might A study of CF,CFI might be illuminating in this look for it around 13.7 eV in the present case. respect.

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1228 C A N . J . CHEM. VOL. 5 2 , 1974

In ally1 iodide (C, symmetry) direct conjuga- The award of post-doctorate fellowships from the U.S.

tion is not possible and because of the low sym- Air Force (to R.A.B.) and the National Research Council of Canada (to D.R.S.) are gratefully acknowledged. We metry we expect a large amount of mixing thank Messrs. G. King and B. Wilkins for measuring

the The structure of ally1 iodide is some of the spectra and Professor W. C. Price and Dr. (26) L. Asbrink for useful discussions

so that rather large "through space" interaction of the iodine with the double bond might be expected.

In fact the spectrum shows three reasonably sharp structureless close-lying bands at 9.32, 9.72, and 10.26 eV which should be due to mixtures of iodine lone pairs and the ethylenic link. The second band is somewhat broader and less intense than the first or third. Since lone-pair bands are normally the sharpest and most intense bands of a spectrum we assign the bands at 9.32 and 10.26 eV to ionizations from m.o.'s with appreciable populations on the iodine. The difference between these, 0.94 eV, when com- pared with the spin-orbit split in CH,I (0.62 eV) indicates that the degree of delocalization is large.

There then follows a gap in the spectrum until a band at 12.25 eV which is likely due to expul- sion of a CI bonding electron. This band is followed by at least four bands before 18 eV which are due to C C o and CH electrons al- though definite assignments cannot be made.

T o summarize our work, the high resolution photoelectron spectra of a large number of iodine containing molecules were presented. Extended Hiickel molecular orbital calculations were used to help in the interpretation of these spectra and with some exceptions gave remarkably good agreement with the observed spectra in light of the severe approximations made in the method. These results co~lpled with some qualitative arguments led to reasonable interpretations of a large number of bands out to 18 eV. The utility of studying molecules in which some or all of the hydrogens are replaced by fluorines was further demonstrated.

This work was carried out in part at the University of Sussex and we would like to thank Professors M. F. Lappert and J. N. Murrell for their hospitality. Most of the spectra were measured on the PS-16 at the Perkin- Elmer Laboratories, Beaconsfield, and we would like to thank Dr. N. Ridyard for experimental assistance.

I. R. A. B ~ S C H I and D. R. SALAHUB. MoI. Phys. 24,289, 735 (1972).

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A268,7 ( 1970). 5. W. C. PRICE. Summer school in photoelectron spec-

tl-oscopy. The Chemical Society, Lecture 9, 1972. 6. J . L. RAGLE. I . A. STENHOUSE, D. C. FROST, and C.

A. MCDOWELL. J. Chem. Phys. 53, 178 (1970). 7. T. BAER, W. B. PEATMAN, and E. W. SCHLAG. Chem.

Phys. Lett. 4,243 (1969). 8. T . BAER and B. P. TSAI. J . Electron Spectros. Relat.

Phenom. 2.25 (1972). 9. F. B R ~ G L I and E. HEILBRONNER. Helvet. Chim. Acta,

54. 1423 (1971). 10. A. D. BAKER, D. BET-TERIDGE. N. R. K E M P , ~ ~ ~ R. E.

KIRBY. Anal. Chem. 43.375 (1971). I I. K. K I M U R A , S. KATSUMATA. Y. AGHIBA, H. MAT-

SUMOTO, and S. NAGAKURA. Bull. Chem. Soc. Japan, 46,373 ( 1973).

12. D. W. TURNER. C. BAKER, A. D. BAKER, and C. R . BRUNDLE. Molecul~~r photoelectron spectroscopy. Wiley-Interscience, New York. 1970.

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Program Exchange, Indiana University, Blooming- ton, Indiana 47401.

15. K. RUEDENBERG. C . C. J . ROOTHAAN, and W. J-\uN- ZEMIS. J. Chem. Phys. 24,201 (1956). C. C. J. ROOTHAAN. J. Chem. Phys. 24,947(1956). G. BURNS. J. Chenl. Phys.41. I521 (1964). R. GLEITER and T . KOBAYASHI. Helvet. Chim. Acta, 54. 1081 (1971). G. HERZBERG. M ~ l e ~ ~ l i i r s t r~~cture and molecular hpectra Ill. Electronic spectra and electronic S~I.LIC- ture of polyatomic molecules. Van Nostrand. Prince- ton. 1966. C. R. BRUNDLE. M. B. ROBIN. N. A. K U E B L E R , ~ ~ ~ H . BASCH. J . Am. Chem. Soc. 94, 145 1 (1972). C. R. BRUNDLE, M. B. ROBIN.^^^ N. A. KUEBLER. J. Am. Chem. Soc. 94, 1466(1972). B. 0. JOHNSON and E. L INDHOLM. Arkiv for Fysik, 39,65 (1969) and refs. therein. A. W. PoTTs. W. C . PRICE, D. G. S T R E E T S , ~ I ~ ~ T. A. WILLIAMS. Disc. Faraday Soc. 54. 168 ( 1972). R. F. LAKE and S I R H. THOMPSON. Proc. Roy. Soc. (London). A258.459 ( 1970). D. CHADWICK, D. C. FROST. A. KATRIB, C. A. MCDOWELL, and R . A. N. MCLEAN. Can. J. Chem. 50,2647 ( 1972). Tables of interatomic distances and configurations in molec~lles and ions. Etlitc,tl hy L. E. Sutton. Publica- tion No. 1 I, The Chemical Society. 1958.

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