J. Phys.
1488
Chem. 1981, 85, 1486-1489
Photoelectron Spectra of and Ab Initio Calculations on Chlorobenzenes. 1. Chlorobenzene and Dichlorobenzenes Branko RugEiC, * Leo Klaslnc, Ruder BoSkovl6 Institute, B!ienCka 54, Zagreb, Croatia, Yugoslavia
Andreas Wolf, Institut fur Theoretische Chernie, DiisseMorf, Federal Republic of Germany
and Jan V. Knop Rechenzentrurn der Unlversitat DlisseMorf, DusseMorf, Federal Republlc of Germany (Received: August 8, 1980)
Photoelectron (PE) spectra of chlorobenzene and dichlorobenzenes were recorded by using He I and He I1 excitation and are discussed by assuming the validity of Koopmans’ theorem. The assignment given down to ionization energies of 24 eV is based both on the results of ab initio calculations and on empirical arguments considering the shapes and intensities of PE band systems and vibrational fine structure. This assignment is in agreement with the one reached by simple interaction schemes within the symmetry rules based on the assignment of benzene and not taking into account spin-orbit interaction. Special attention is paid to ionization of chlorine lone pairs. Ten years ago, Turner and co-workers1 pioneered the investigation of substituted aromatic compounds with the aid of photoelectron (PE) spectroscopy. Today, this field is still generating a great deal of interest, to the extent that a systematic and detailed study of substituted benzenes has been proposed.2 Meanwhile, the P E spectra of monoand disubstituted bromo- and iodobenzenes have been successfully analyzed by a simple treatment,2” and the two or three lowest energy systems in the PE spectra of chlorobenzenes have been interpreted within a framework of mesomeric and inductive Recently, an investigation of emission spectra of radical cations and P E spectra of chlorobenzenesEwas reported. The interpretation of the PE spectrum of chlorobenzene for ionization energies up to 24 eV, reported as part of a semiempirical investigation of substituted pyridine^,^ nearly coincides with the nonempirical assignment presented here. Experimental Section Low- and high-resolution (30 and 10 meV, respectively) P E spectra of mono- and disubstituted chlorobenzenes were recorded at room temperature on a Vacuum Generators UVG-3 instrument,1°using He I and He I1 excitation. The energy scale was controlled and calibrated by simultaneous addition of argon, xenon, nitrogen, and methyl iodide to the sample flow. The compounds were of commercial origin and highest purity. Ab Initio Calculations Single determinant ab initio MO wave functions were derived for the investigated compounds by using minimum basis sets of contracted Gaussian orbitals. The benzene ring was assumed to be a regular hexagon, and the C-C, (1)A. D. Baker, D. P.May, and D. W. Turner, J. Chern. SOC.B, 22 (1968). (2)T.CviM, H.Glisten, and L. Klasinc, J. Chern. Soc., Perkin Trans. 2, 962 (1977). (3)T.CviM and L. Klasinc, Croat. Chern. Acta 60, 291 (1977). (4)J. N. Murre11 and R. J. Suffolk, J. Electron Spectrosc. Relat. Phenorn., 1,471 (1972-73). (5)D. G. Streets and G. P. Ceasar, Mol. Phys., 26, 1037 (1973). (6)F.Marschner, Tetrahedron, 31,2303 (1975). (7)C. N. R.Rao, Tetrahedron, 32,1561 (1976). (8)J. P.Maier and 0. Marthaler, Chern. Phys., 32,419 (1978). (9)L.Klasinc. I. Novak, M. Scholz. and G. Kluge. - . Croat. Chern. Acta, 51, 43 (1978). (10)L.Klasinc, B. KovaE, and B. RulEiE, Kern. Ind. (Zagreb),23,569 (1974). 0022-3654/81/2085-1486$01.25/0
C-H, and C-C1 bond lengths were taken as 139.5, 108.5, and 174 pm, respective1y.l’ All calculations were performed with two systems of programs, PHANTOM^^^ and MANY ATOM,^^^ using full symmetries of the molecular frameworks. For carbon the (7,3) primitive set of Whitman and Hornback13was used, while the 1s orbital of hydrogen was represented by Huzinaga’s four-term expan~ion.’~Successful use of both basis sets has already been made.15 The computations on mono- and dichlorobenzenes were thought of as test calculations for the higher substituted derivatives. Therefore, two primitive sets were used for chlorine: the (12,9) basis of Veillard16 and the (6,4) basis of Claxton and Smith.17 From one basis set to the other, the calculated energies for the core orbitals differed by as much as 3 eV. However, in the valence orbital region, these differences were smaller (maximum of 0.5 eV). An inspection of the resulting total energies and virial ratios gave preference to calculations performed with the Veillard basis set. (The complete results, including the Mulliken population analysis, are available from the authors upon request.) Results and Discussion Low-resolution He I and He I1 PE spectra of chlorobenzene and 1,2-, 1,3-, and 1,6dichlorobenzene are presented in Figures 1-4. The vertical ionization energies, defined at the highest peak in the system, are given above the spectra (in eV, with an absolute error of f0.03 eV for the He I spectra and f O . l eV for the He I1 spectra). The (11)“Tables of Interatomic Distances and Configuration of Molecules and Ions”, Special Publication No. 11,Chemical Society, London, 1958. (12)(a) PHANTOM, system of ab initio programs by D. Goutier, R. Macaulay, and A. J. Duke, QCPE, 10,241(1974);(b) MANYATOM, modified closed-shell CDC 7600 version of POLYATOM II by D. B. Neumann, H. Basch, R. L. Kornegay, L. C. Synder, J. W. Moskowitz, C. Hornback, and S. P. Liebmann, QCPE, 10,199(1974);see A. I. Duke and R. F. W. Bader, Chern. Phys. Lett., 10,631 (1971). (13)D. R. Whitman and C. J. Hornback, J. Chern. Phys., 61, 398 (1969). (14)S. Huzinaga, J. Chern. Phys., 42, 1293 (1965). Schmidtke, and J. V. Knotx (15)A. Wolf, H.-H. _ . Theor. Chirn.Acta. 48,‘37’(1978). (16)A. Veillard, Theor. Chim.Acta, 12,405 (1968). (17)T.A. Claxton, and N. A. Smith, Theor. Chirn. Acta, 22, 378 (1971). ’
0 1981 Amerlcan Chemical Society
The Journal of Physical Chemistty, Vol. 85,No. 11, 198 1 1407
Photoelectron Spectra of Chlorobenzenes 1553
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Flgure 3. Low-resolution He I (upper right) and He I1 (lower) PE spectra of 1,3-dichl~roben~ene, and high-resolution He I PE spectrum (upper left) of the X and A systems.
assignment of the resolved vibrational progressions is marked above the appropriate high-resolution spectra in Figures 2-5 together with the wavenumbers (in cm-', with an absolute error of 140 cm-l). Unless otherwise stated,
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Figure 5. High-resolution He I PE spectra of B, 6, and b systems of 1,3-dichlorobenzene (left), 1,Cdichlorobenzene (middle), and 1,241chlorobenzene (rlght).
all mentioned vibrations are totally symmetric. The assignment is presented in a diagram (Figure 6) correlating the experimental ionization energies for chlorobenzenes, chlorine,18and benzenelSz1 and showing approximate shape of the corresponding orbitals, as obtained from the MO coefficients. The validity of Koopmans' theorem is assumed throughout the discussion. The final assignment is based both on the results of ab initio calculations and on empirical arguments. These arguments resemble those used in the elucidation of PE spectra of bromo- and iodobenzenes3f2and use simple schemes of interaction between the orbitals, intercomparison of the PE spectra of chlorobenzenes, analysis of the resolved vibrational structure, as well as comparison of relative intensities of band systems in He I and He I1 PE spectra. Such an assignment necessarily relies on the assignment for the molecular frame, which is benzene in this case.22 In the present (18) T. A. Carlson, "Photoelectron and Auger Spectroscopy", Plenum, New York, 1976, p 337. (19) J. Almlof, B. Roos, U. Wahlgren, and H.Johansen, J. Electron Spectrosc. Relat. Phenom., 2 , 61 (1973). (20) D. M. W. van der Ham,M. Beerlage, D. van der Meer, and D. Feil, J . Electron Spectrosc. Relat. Phenom., 7, 33 (1975). (21) E. Lindholm, Faraday Discuss. Chem. SOC.,54, 200 (1972). (22) W. von Niessen, L. S. Cederbaum, and W. P. Kraemer, J. Chem. Phys., 65, 1378 (1976).
1408
The Journal of Physical Chemistty, Vol. 85,No. 7 1, 198 1
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Figure 6. Orbital correlation diagram for chlorobenzene and dichlorobenzenes, based on experimental ionization energies (left, together wlth and and on calculated orbital energies (right). The approximate shape of the orbltals is obtalned from the calculated MO coefficients.
discussion, however, spin-orbit interaction has not been taken into account. First, there is no place for it in the context of empirical arguments, and, second, it is believed that the main interaction between the ring and the substituent is conjugative, as was shown in the case of bromoand iodoben~enes.~,~ The inclusion of spin-orbit interaction would not affect the assignment. In all of the discussed chlorobenzenes, systems and A (Figures 1-4) correspond to ionizations from r orbitals produced by lifting the degeneracy of the lelg benzene 0rbital.l In chlorobenzene (Figure l),the splitting between these orbitals (4bl and la2)amounts 0.57 eV, which nicely compares to the related values for other halobenzenes: 0.44 eV in fluorobenzene,5 0.64 eV in bromobenzene,3 and 0.71 eV in iodobenzene.2 All of these values can be qualitatively explained by the energy difference of the interacting orbitals, i.e., benzen_e lel orbital and halogen p orbital. Systems B and C in tke P E spectrum of chlorobenzene correspond to ionization from the in-plane (9b2) and out-of-plane (3b1) lone pairs on chlorine, respectively. They are split by 0.36 eV as a result of different interactions with the benzene orbitals (Figure 6). The out-ofplane chlorine lone pair is destabilized by an antibonding interaction with the la2,, benzene orbital (12.35 eVm),although there is an additional, but weaker, bonding interaction with the bl component of the degenerate lelg benzene orbital. The in-plane chlorine lone pair is destabilized by a whole series of antibonding interactions with benzene u orbitals. The strongest interaction is with the b2 component of the benzene 3e2, orbital, somewhat weaker is the interaction with the b2 component of the benzene 3elu orbital, while the interaction with the lbzu benzene orbital is the weakest one. This explains why the in-plane chlorine lone pair is more destabilized then the out-of-plane lone pair. Such an assignment of the B and systems is suggested by the ab initio results, too, and is supported by the emisEion spectrum of chlorobenzene radical cation8where the B X transition is not observed.
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The P E spectrum of chlorobenzene provides additional evidence for this assignment. Namely, a measurement of the system areas in He I and He I1 spectra shows that the relative intensity of the B system in going from He I to He I1 excitation is reduced to less than half, while the system retains approximately the same relative intensity in both He I and He I1 spectra. Such a reduction in the ionization cross section for chlorine orbitals from He I to He I1 excitation was observed earlier for some other corn pound^.^^ However, the different behavior of the two lone-pair systems in chlorobenzene (and in many other molecules) implies, in agreement with Murre11 and Suffolk: that the in-plane orbital possesses more “lone-pair” character than the out-of-plane one. The system at 13.21 eV in the PE spectrum of chlorobenzene (Figure 1) corresponds to the ionization of the lowest ?r orbital (2b1). This assignment is supported by comparison with the P E spectra of higher chlorobenzenes, as well as the ab initio results. the earlier investigations,9~~~ The stabilization of the 2bl orbital by 0.86 eV, as compared to the parent benzene orbital, is due to the bonding interaction with the energetically near chlorine 3p orbital. The degeneracy of the 3ezgand 3elu benzene orbitals is lifted in chlorobenzene, yielding two pairs or orbitals: 15al and 8b2, and 14al and 7b2, respectively (Figure 6). The stabilization of the al components can be explained by the fact that the u 3p chlorine orbital takes the role of 1s hydrogen orbital, presumably increasing the overlap with the carbon 2p orbital. However, this type of interaction is not very strong, in the sense that it rapidly decreases with the energy difference between the interacting orbitals. This is evident in the case of the 12a1orbital, which has nearly the same energy as the parent 3alg benzene orbital. Where it is symmetry allowed, the ring u orbitals enter a much stronger interaction with the tangential chlorine 3p orbital (defined as the in-plane lone pair). This interaction accounts for the strong stabilization of the 8b2 orbital (as compared with the 15al orbital; both orbitals originate from the same parent 3e2, benzene orbital), accounting at the same time, for the destabilization of the 9b2orbital (i.e., in-plane lone pair). A similar explanation can be applied to the 7b2and 6b2 orbitals, although their stabilization is somewhat weaker because of a greater energy difference between the chlorine 3p orbital and the parent benzene orbitals. The remaining valence orbitals of chlorobenzene can be considered as linear combinations of 2s carbon orbitals and the 3s chlorine orbital. They, as can be follow an orbital interaction pattern very similar to the one for i~ orbitals, as can be v_isualiz_edfrom Figure 6. The assignment of X and A systems in the PE spectra of 1,4- and 1,3-dichlorobenzenes is in agreement with the earlier results for halobenzenes,2-6while the same cannot be said for 1,2-dichlorobenzene. Previous investigations>6 similar to those for 1,2-dibromo- and 1,2-diodoben~ene,~~~ assign the top two systems in 1,2-dichlorobenzene to ionizations from 3a2 and 4bl orbitals, respectively, while the ab initio results suggest an inverse assignment. The comparison of the Franck-Condon profiles of X and A systems in all three dichlorobenzenes, as well as the comparison with the P E spectrum of 1,2,3-trichlorobenzene, gives
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(23)(a) A. Katrib, T. P. Debies, R. J. Colton, T. H. Lee, and J. W. Rabalais, Chem. Phys. Lett., 22, 196 (1973); (b) C. R. Brundle, M. B. Robin, and H. Basch, J.Chem. Phys., 53, 2196 (1970);(c) T.CvitaB, H. Gusten, and L. Klasinc, ibid., 67, 2687 (1977). (24)J. A. Sell and A. Kuppermann, Chem. Phys., 33, 367 (1978). Faraday Trans. 2, (25)A. W.Potts and D. G. Streets, J.Chem. SOC., 875 (1974).
Photoelectron Spectra of Chlorobenzenes
preference to the assignment suggested by the a b initio results. InJhe PE spectrum of 1,4-dichlorobenzene (Figure 2), the X system is located at 8.97 eV and under high resolution shows vibrational progressions of 1100, 1580, and 330 cm-'. These coincide with the ring-breathing u4 (1106 cm-l), the C-C-stretching u2 (1573 cm-l), and the C-Clstretching vg (330 cm-l) in the ground state of the neutral molecule.26 The hot band corresponds to the wavenumber of the C-C1-stretching vg, The A system, located at 9.84 eV, has a poorly resolved fine structure. The two visibl_e progressions, 1100 and 330 cm-', similarly as in the X system, correspond to the ring-breathing u4 and C-CIstretching vg. The top system a t 9.14 eV in the PE spectrum of 1,3dichlorobenzene (Figure 3) shows the excitation of vibrations of 1130, 1420, and 390 cm-l. These very probably correspond to the ring-breathing vg (1124 cm-l), the C-Cstretching u5 (1412 cm-'1, and the C-C1-stretching ul0 (398 cm-l) in the molecular ground state.27 The hot band (400 cm-l) corresponds to the wavenumber of the C-C1stretching vlO. The next system, at 9.70 eV, does not show such a prominent fine structure. The vibrational progression of 390 cm-l corresponds to the C-C1-stretching ul0, while the progression of 810 cm-l corresponds either to the ring-breathing vg or to the ring-deformation us which has a frequency of 997 cm-l in the ground state.27 The P E spectrum of 1,2-dichlorobenzene (Figure 4) shows under high resolution that in the ground state of the radical cation the excited vibrations have wavenumbers of 1140, 660, and 480 cm-l. They most probably correspond to the ring-breathing u7 (1129 cm-l), the ring-deformation ug (660 cm-'), and the C-C1-stretching ul0 (480 cm-l) in the molecular ground statesz8 The first excited state of the radical cation shows a vibrational progression of 940 cm-l which probably can be attributed to the ring-breathing u7. In t h e P E spsctrum of 1,4-dichlorobenzene (Figure 2) systems C and D arise from ionizations of the positive and negative combination of the in-plane chlorine lone pairs (6bzuand 5b,,). Both systems are located at the same energy (11.50 eV). The high-resolution spectrum (Figure 5) shows that they are split only by 50 meV. Therefore, it is hard to pick out clear vibrational progressions. It seems that progressions of 680 and 1120 cm-l are excited in both systems and can be assigned as ring-deformation u5 and C-H-bending us which have in the ground state of the neutral molecule wavenumbers of 747 and 1169 cm-l, respectively.26 In 1,3-dichlorobenzene the splitting of the lone-pair orbitals amounts to 150 meV. This shows, as one can expect, that the interaction of the in-plane lone pairs is greater than in 1,6dichlorobenzene. In the PE spectrum, these systems are located at 11.58 and 11.73 eV (Figure 3) and do not show fine structure under high resolution (Figure 5). In 1,2-dichlorobenzene the splitting between the in-plane lone pairs reaches 1.12 eV due to the favorable condition for their through-space interaction. The related systems, (26)A. StojiljkoviE and D. H. Whiffen, Spectrochim. Acta, 12, 47 (1958). (27)J. H.S.Green, Spectrochim. Acta, Part A , 26, 1523 (1970). (28)J. R.Scherer and J. C. Evans, Spectrochim. Acta, 19,1739(1963).
The Journal of Physical Chemistry, Vol. 85, No. 11, 198 1
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B and E, in the PE spectrum are located-at 11.25 and 12.37 eV, respectively (Figure 4). System B shows a poorly resolved vibrational progression of 200 cm-l (Figure 5) and probably corresponds to the C-C1 stretching ulO. A similar progression can be seen in the system E, too. Measurement of the system areas in the P E spectra of dichlorobenzenes reveals that the relative intensities of the systems herein interpreted as ionizations from the in-plane chlorine lone pairs drop in the He I1 spectra to approximately half their value in the He I spectra. This effect can be clearly seen in the case of 1,2-dichlorobenzene, where the related systems are well resolved in both the He I and He I1 spectra. Among the investigated compounds, the splitting of the out-of-plane chlorine lone pairs reaches the maximum value of 1.41 eV in 1,4-dichlorobenzene, At the same time, the positive combination of these pairs (3b3,) is destabilized by an antibonding interaction with the la2, benzene orbital, while the negative combination (2b2,) is stabilized by a bonding interaction with the appropriate component of the degenerate lel, benzene orbital. A similar statement can be made for the case of 1,3-dichlorobenzene, although the splitting between the related orbitals (3bl and 2aJ is somewhat smaller (1.33 eV). However, in 1,2-dichlorobenzene the splitting amounts to only 50 meV. This shows that in 1,2-dichlorobenzene mutual through-space interaction of the out-of-plane lone pairs is much stronger than their interaction with the fng T orbitals. The assignment of the B systems in 1,4- and 1,3-dichlorobenzene as ionizations from the out-of-plane orbitals of chlorine can be supported by the eqissiog spectra of appropriate radical cations, where the B X transition has been nicely observed.8 The wavenumbers of the 0 0 transitions, amounting to 19620 and 18770 cm-', respectively, correspond within the experimental error to the differerye of the adiabatic ionization energies for systems B and X, namely, 2.40 and 2.33 eV, respectively. In addition, the emission spectrum of 1,4-dichlorobenzene reveals vibrational progressions of 1590-and 330 cm-l, which are the same as those found in the X system (Figure 2), and the hot band in the emission spectrum (330 cm-!) coincides with the C-C1-stretching Vg excited in the B system (Figure 5). The interactions concerning the remaining orbitals are analogous to the interactions discussed for chlorobenzene, and the corresponding assignment is given in Figure 6.
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Conclusion If one neglects the discrepancy between the absolute values, a close correlation of theoretical results and the experimental values shows that, in a qualitative sense, the computed results give a fairly good prediction of orbital interaction trends. However, in a quantitative sense the strength of the interaction is sometime exaggerated, thus giving, e.g., a wrong prediction of the sequence of orbitals described as combinations of chlorine lone pairs or overestimating the stabilization of the lowest a orbital. A similar statement should be made for the orbitals connected with systems appearing in the 15-17-eV region of the PE spectra. Nevertheless, nonempirical calculations appear to be, in the cases where Koopmans' theorem is valid, a useful aid to the interpretation of PE spectra, if combined with empirical arguments.
