The ultraviolet photoelectron spectra of the chlorobenzenes and chloropyridines

Journal of Electron Spectroscopy and Related Phenomena Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

THE ULTRAVIOLET CHLOROBENZENES

J. N. MURRELL

PHOTOELECTRON SPECTRA AND CHLOROPYRIDINES

OF THE

and R. J. SUFFOLK

School of Molecular Sciences, University of Sussex, Brighton BNI 9QJ (England) (Received 7 December 1972)

ABSTRACT

The photoelectron spectra of the chlorobenzenes and chloropyridines have been measured, and a classification of the bands has been made on the basis of perturbations to the states of a six-membered ring by Cl and N. An atom-additive model is found to correlate the shifts of the x and n states of pyridine by Cl, but the correlation is not as good as for the shifts by F studied in an earlier paper. INTRODUCTION

An examination of substituent shifts has been valuable for the assignment of photoelectron bands of aromatic molecules. Although there have been suggestions that the bands arising from n, r~or YE(non-bonding) electrons might be distinguished by the shape of their band envelopel, this is not now considered a reliable criterion’. On the other hand, it is known that the shifts in rc and n bands arising from fluorine substitution may be quantitatively interpreted by a rather simple theory and such bands may be distinguished readily from Q bands 3 - 5 . Perturbations arising from alkyl and silyl groups have also been used to distinguish n and y1levels in the azaaromatics”. In this paper we examine the influence of chlorine as a substituent on aromatic molecules and also the replacement of a ring C-H by N. The project was originally conceived to study the effect of Cl on the 7~,TVand n bands of pyridine, but examination of the spectra showed that in assigning the bands it was equally helpful to consider aza-substitution as a perturbation of the chlorobenzene spectra. The reason for this is that chlorine, unlike fluorine, introduces new low-energy bands in the spectrum because of the relatively low energy of the chlorine 3p electrons. Spectra have been published previously of the three mono-chloropyridines and of the mono and dichlorobenzenes. Only a tentative assignment was made, and at that time the assignment of the pyridine spectrum was uncertain. J. Electron Spectrosc., 1 (1972/73)

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EXPERIMENTAL

Most of the chloropyridines were produced by I.C.I. Mond Division and had a purity of approximately 95 %. The exceptions were 2-chloropyridine (Koch-Light, purity > 98 %) and 4-chloropyridine which was prepared by hydrolysis of its hydrochloride salt (Koch-Light pure) with NaOH. After purification the product was obtained as a yellow oil with a vapour pressure of 10 mm at 48°C. All liquid samples were purified on a vacuum system attached to the inlet of the spectrometer (Perkin-Elmer P.S. 16) and run at room temperature. A heated inlet probe (N 80 “C) was used to produce sufficient vapour pressure with the solid samples of pentachloro-benzene and pyridine. The spectra were recorded with a He I source, and typical resolution obtained was 24 meV measured as the width at half height of the argon 2P3,2 line. The spectra were calibrated with rare gas and O2 lines. RESULTS

The spectra are shown in Figures 2-6, and we will discuss the range 9-14 eV. Above this range there is the usual multiplicity of features arising from a-electron ionisation which we do not wish to comment upon. Before discussing the spectra in detail it is relevant to describe the spectrum of benzene (Figure 1) as all spectra can be considered as derivatives of this. The lowest energy band is due to ionisation from the le, B rc orbitals. It has an onset at approximately 9.3 eV and shows some evidence of the expected Jahn-Teller splitting in its band envelope. The next region of absorption starts at 11.4 eV, and continues to 13 eV. There are two maxima, at 11.4 and 12.1 eV, and the most generally accepted (a)

(b)

IO

12

14

16cV

Figure 1. Photoelectron spectrum of 472

(a)

pyridine and (b)

benzene.

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Figure 2. Photoelectron spectrum of (a) 2-chloropyridine, (b) 3_chloropyridine, (c) 4-chloropyridine and (d) chlorobenzene.

assignment is to attribute the first to a G molecular orbital (probably 3ezg), and the second to the lowest energy z orbital lazU 39 ’. The identification of the luzU level at 12.1 eV appears conclusive when one examines the spectra of the fluorobenzenes3. Further cr ionisation starts at approximately 14 eV. The introduction of a nitrogen atom into the six membered ring has two large effects on the spectrum (Figure I). Firstly it removes any degeneracy in the molecular orbitals, so that there are now two low energy z bands. Secondly it introduces a new J. Electron Spectrosc.,

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(d)

IO

12

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16eV

Figure 3. Photoelectron spectrum of (a) 3,5-dichloropyridine, (b} 2,6-dichloroppidiue, (c) 2,4dichloropyridine and (d) m-dichlorobenzene.

band in the spectrum associated with the nitrogen lone pair electrons. The n band has been assigned by us4 to a peak in the photoelectron spectrum of pyridine at 9.8 eV with the z3 Level giving the peak at 9.6 eV. However, the difference between these two bands is so small that this assignment is far from conclusive and the reverse order of this pair has been adopted by others’. The second 7~ionisation band is identified with the band at 10.5 eV. The introduction of a chlorine substituent has a somewhat similar effect to 474

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-J,

.,

I

I

(b)

10

12

Figure 4. Photoelectron

14

16eV

spectrum of (a) 2,4,6_trichloropyridine

and (b) 1,3,%richlorobenz.

(b)

IO

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Figure 5. Photoekctron benzene. J. Electron

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spectrum of (a) 2,3,4,5-tetrachloropyridine

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and (b) 1,2,3,4-tetrachloro-

475

(al

(b)

IO

12

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Figure 6. Photoelectron spectrum of (a) pentachloropyridine and (b) 132 I 3,4 ,5-pentachlorobenzene.

nitrogen on the benzene spectrum. The degeneracy of the first x band is removed and a new band is introduced that arises from the non-bonding chlorine 3p orbital of c symmetry. This is typically a sharp band and has been identified in the 11-12 eV region in the spectra of the chlorobenzenes. The chlorine 3p orbitals of 7~ symmetry will also give new bands in the spectrum but the molecular orbitals involved will also contain contributions from the carbon rc orbitals. The separation of the first two n bands in the spectrum of pyridine is 1 eV and in that of chlorobenzene is 0.68 eV. The similarity however is deceptive because the symmetries of the 7~levels are not the same in the two cases. The coefficients of the le, g rc molecular orbitals of benzene, may be taken in real form as follows:

A nitrogen atom being more electronegative than carbon lowers the energy of a molecular orbital, roughly by an amount proportional to the orbital density at the position of substitution. Thus a nitrogen at position 1 will lower elg relative to 476

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elg’ and the lowest energy band of pyridine wiI1 correspond to ionisation from an orbital which is approximately e,,‘, with some admixture of other 7c orbitals of the same symmetry*. A chlorine substituent raises the energy of any z molecular orbital which has a higher energy than that of the chlorine 3p n: orbital through a mesomeric (conjugative) interaction. The inductive effect of a chlorine on ;n levels is also probably such as to raise their energy. Once again the shifts (a mesomeric interaction in second order or an inductive interaction in first order) will be proportional to the square of the molecular orbital coefficients so that a chlorine introduced at position 1 will raise the energy of elg with respect to e 1g’. The first n: band of chlorobenzene is therefore due to ionisation from an orbital which is approximately e 1B. A critical test of this interpretation is in the spectrum of 4-chloropyridine for which the N and Cl shifts should be in opposition, and indeed there is only one broad low energy band in the spectrum of this compound (Figure 2). The first three bands are well resolved in the spectrum of 2-chloropyridine and the assignment here is critical for an understanding of the other spectra. In 2-fluoropyridine the order was 4 7c3 < ?l < 712. It can be seen from the benzene orbitals that a nitrogen at position 1 and a chlorine at position 2 will give maximum stabilization to e, and destabilization to eg’. There will also be a cross term of interaction which will increase the splitting. In short we expect a large splitting of 7c3 and nn, in 2-chloropyridine and we therefore adopt the same order of levels as for fluoropyridine. Tee situation for the x bands of 3-chloropyridine is rather similar to that for 2-chloropyridine. Fluorine was found to have a smaller perturbation on the n band when in the 3-position than when in the 2-position and we therefore assign the n band in 3-chloropyridine to the shoulder on the first band in the spectrum. In our study of the fluoropyridines4 we found that the shift in the n2, 7t3 and 12bands was closely additive for each substituent.

E = E, + n,E,+ n,& + nyEy

(1)

where n,, nS, and nY are the number of tl, p and y substituents respectively, and E,-, is the energy of the band for the unsubstituted molecule. We expect this pattern to be followed less accurately for chlorine because chlorine itself perturbs the n bands to as large an extent as the nitrogen**. Nevertheless if we assign bands in the spectrum on the assumption that formula (1) holds then the observed shifts do correlate reasonably well with this model, particularly for the n bands as can be seen from Figure 7. The regression coefficients and the constants E,,Ej,E,, for 712, Z~ and n are given in Table 1. We recognise that there is no independent evidence to support our assignment of the first three bands in some of the compounds. It rests solely on the fact that -* If the nitrogen is introduced at any other position then elg and elg’ are mixed together to give rzzulting orbitals which are equivalent to elg and el g’ but rotated about the six-fold axis. For example, in fluorobenzene the first 7z band is split by only 0.4 eV. J. Electron Spectrosc., 1 (1972173)

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Figure 7. Graph of observed versus calculated ionisation potentials for azs,zs and n.

TABLE 1 REGRESSION COEFFICIENTS FOR THE II AND ICBANDS OF THE CHLOROPYRIDINES ASSUMING AN ADDITIVE MODEL FOR THE PERTURBATION BY CHLORINE (equation 1)

7-a

Ea Efi EY root mean square deviation

(14

-0.07 -0.07 0.25 0.10

n

x2

0.35 0.09 -0.03 0.06

0.33 0.02 -0.47 eV 0.10eV

(261)

expression (1) gives a satisfactory correlation of the band positions. We can conclude from this approximate additivity that the chlorine is not behaving as a strong inductive substituent. This is because the inductive effect leads to non-additive terms, arising from the mixing of different z states of the ring, if taken beyond first-order of perturbation. In principle an analysis of the shifts in the photoelectron bands on substitution could lead to a separation of inductive and mesomeric effects in the same way as has been possible with electronic spectroscopys9 g. However, this could only be done if the resolution of the photoelectron spectrum was considerably improved. The shifts on which Figure 7 are based are at best 0.03 eV but due to overlapping and broad bands are in general no better than 0.1 eV and in some cases far more uncertain. The chlorine lone pair bands can be identified by a comparison of the chlorobenzene and pyridine spectra. In other words we consider the nitrogen as a perturbing 478

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influence on the chlorobenzene spectra. It is seen that in most cases the bands between 11 and 12.5 eV show a strong similarity in the two sets of spectra. The number of lone-pair bands should be twice the number of chlorine substituents so that for pentachloro-pyridine there should be ten bands. The region between 1 I and 12.5 eV has an intensity consistent with this but the incomplete resolution makes a detailed assignment impossible. A surprising feature is that the positions of the sharp bands in chlorobenzene and the monochloropyridines differ quite considerably. It has in past work been assumed that the lower energy band can be attributed to ionization of the chlorine 3p CJ electron. If this is so then the nitrogen in 4-chloropyridine, for example, increases the ionization energy by 0.4 eV. Either the 3p orbitals are considerably delocalized to the ring, which is somewhat inconsistent with the sharpness of the band, or the ionization is appreciably affected by the detailed charge distribution of the ring. The latter appears to be the most likely. A net charge of 0.05 units at the carbon atom attached to the chlorine would, by the electrostatic interaction, change the lone pair ionisation potential by about 0.5 eV. We note however that the order of the ionisation potentials from this first band in the three monochloropyridines is 3 > 4 > 2 and this is the reverse to that expected from the a electron densities at the carbon atoms (2 > 4 > 3) and also not the same as the total electron density calculated by Clementi” (4 > 3 > 2). The second sharp band (at 11.68 eV in chlorobenzene) has been assumed to arise from the chlorine 3p 7t orbital mixed with the carbon rc orbitals. This energy is close to that of the first benzene rc orbital (la,,) which we assume to be 12.1 eV. In the mono-chloropyridines the corresponding chlorine 3p n band occurs at 12.1-12.5 eV in all three compounds which is close in energy to that of the band in pyridine. On this interpretation of the bands there appears to be little interaction between the chlorine 3p rr and the lowest carbon 7~molecular orbital. The alternative assignment of a 3p c ionisation at higher energies than the 3p z is unlikely to be correct because the most striking feature of the spectra (Figure 2) is that the separation of the two chlorine peaks under consideration is much smaller for chlorobenzene than for any of the monochloropyridines. In other words the higher energy band in chlorobenzene appears to undergo a greater shift than the lower energy band. This greater shift is to be expected if the appropriate molecular orbital has a greater contribution from nitrogen orbitals which is likely to be the case for that of 7t symmetry. There are strong similarities between the spectra of some of the chlorobenzenes and corresponding pyridines in the 11.5-12.5 eV regions. The most striking differences appear to be only due to superposition of bands as in the 2, 4, 6 compounds (Figure 4). We do not believe that a more detailed assignment of the spectra can be made until polarization data are available on these compounds. There is some evidence that n: and (T levels have different polarization ratios (B coefficient)“.

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REFERENCES 1 D. W. Turner, C. Baker, A. D. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy, Wiley-Interscience, London, 1970. 2 R. GIeiter, E. Heilbronner and V. Hornung, Nelv. Chim. Acta, 55 (1972) 255. 3 B. Narayan and J. N. Murrell, Mol. Phus., 19 (1970) 169. 4 G. H. King, J. N. Murrell and R. J. Suffolk, J.C.S. Dalton, 1 (1972) 564. 5 M. B. Robin, N. A. Kuebler and C. R. Brundle, J. Amer. Chern. Sot., 94 (1972) 1451. 6 E. Heilbronner, V. Hornung, H. Bock and H. Alt, Angew. Chem. bzt. Ed. Engl., 8 (1969) 524. 7 L. &brink, 0. Edquist, E. Lindholm and L. E. Selin, Cfzem. Phys. Lett., 5 (1970) 192, 609. 8 J. Petruska, J. Chem. Phys., 34 (1961) 1120. 9 J. N. Murrell and K. L. M&we& J. Chem. Phys., 25 (1956) 1143. 10 E. Clementi, J. Chem. Phys., 46 (1967) 4731. 11 T. A. Carlson and R. M. White, Faraday Discuss., 54 (1972) in press.

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