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On the assignment of the He I photoelectron spectrum of silicon tetrafluoride
Journal of Electron Spectroscopy and Related Phenomena EIsevier Publishing Company, Amsterdam - Printed in The Netherlands
Preliminary communication On the assignment of the He I photoelectron
spectrum of silicon tetrafluoride
MB. HALL, M.F. GUEST and I.H. HILLIER Chemistry Department,
University of Manchester, Manchester Ml3 9PL (England)
D.R. LLOYD Chemisrv
Depnrtment,
The University, P.0. Box 363, Birmingham BI5 2TT (Engknd)
A.F. ORCHARD Inorganic Chemistry Laboratory,
University of Oxford. Oxford OXI 3QZ (England)
A.W. POTTS Physics Department,
Kings College, Strand, London
WC2R 2LS (England)
(Received 8 December 1972)
Recently,
high-resolution He I photoelectron (p.e.) spectra have been reported for CF4, SiF4 and GeF4 by Jonas et al.‘, following an earlier report of Bull et al.‘. In these papers assignments for the spectra of SiF4 and GeF4 are proposed which differ markedly from earlier assignments based on low-resolution spectra3, and we wish to comment on the new interpretation. In such tetrahedral AB4 molecules the classical view of the bonding is that it arises mainly from interaction of the valence s and p orbitals of A with “u” orbitals of B, those directed along the bond axes. This description as an sp’ hybrid set may be modified in some cases by m interaction of the empty high lying d orbitals of A with lone pair orbit& of B. In terms of delocaliscd orbitals, the main u bonding occurs through orbitals o1 (s) and ts (p), the n lone pairs giving rise to a set e + tl + t2 ;-any d orbital interaction stabilises e through a pure rr interaction, and to a lesser extent the r2 orbitals through an interaction which is mainly n. It has been shown from He I p.e. studies4 that in tetrachlorides and tetrabromides of Group IVB elements the orbital sequence is tl > fi > e > tz > al. The same sequence has been deduced for tetrahedral oxy-anions on the basis of x-ray p.e. spectra and ab initio molecular orbital calculations; there is evidence from int-ensities that the lowest valence m.o. is the al W. Several ab initi~ calculations upon CF4 indicate this same sequence, and it is generally agreed that the p.e. spectra of CF, are in accord with this’-4,7’ 9 _ In earlier work3 the same sequence was proposed for SiF4 and GeF4, but the new assignment132 for SiF4 brings the al orbital above e, into the lone pair region, while e is strongly stabilised along with the lower t2 orbital.. For GeF4 the stabilisation proposed for e brings it below the lower t2 J. Hectron
Spectrosc.,
1 (1972/1973)
497
while u1 remains in the lone pair region’$ . This means that for both SiF4 and GeF4 there is virtually no (Tbonding interactiou with the valence s orbital, and that in GeF4 the major bonding interaction is with the d orbitah and is of n type. A priori it seems most unlikely that a CJinteraction of fluorine with an s orbital should be very much less than the ff interaction with the much higher lying set of d orbitals. Jonas et al.’ justify the stabilisation of the e orbital set by reference to CNDO/2 calculations withand without d orbitals, though Bull et al.* remark that the d orbital participation is probably over-emphasised in the calculations. The rise in energy of al is justified by Jonas et al. in terms of a reduction in calculated F 2s character, but we note that in this orbital F 2F enters with an mttibonding interaction with the central atom orbital, so that a reduction in F 2s character would be expected to stabilise al rather than destabilize it. Even in transition metal tetrachlorides, where there must be strong d orbital participation, al is &ill the lowest orbita14. By comparison of the p.e. spectra and nb initio calculations we have shown that in the molecules isoelectronic with SiF4 (POF3, SO:! F2, C103 F), the central atom charactei of the lowest valence orbital above -30 eV is predominantly 3s, i.e. the lowest orbital corrlates with a1 of SiF4 3T10.Also, internally consistent correlation of ionisation potentials is possible through the moiecules SiF 4, POF3, PF3 if the SiF4 orbital sequence is assumed tobet >t, >e>fz >a1 ll. The set of ab initio calculations has now been completed . with one upon SiF 4. This calculation was carried out using an extended basis of Slater type orbitab @TO) each expanded in three Gaussian type functiuns” _ The core orbitals (Is, 2s, 2p on Si and 1s on F) were represented by one best atom ST013, while the valence orbitals (3s, 3p on Si and 2s, 2p on F) were represented by two STO’s14. In addition, two 36 polarisation STO’s with the same exponents as the 3p STO’s, and both 4s and 4p STO’s with exponents of 1.0, were added to achieve a very accurate description of the valence -molecular orbitals. In contrast to the CND0/2 calculations, the participation by d orbitals is low; the total Si 3d population is only 0.63 e- , compared with 1.44 e- in the CND0/2 approximation’ . The eigenvalues have been mu1tiplied by -0.92, and are given below, in eV, with experimental values’ ?‘y16 in parentheses: tl 17.5 (16.5); t2 18.4(17.5);e 18.9 (18.1); r2 20.0 (19.5); al 21.7 (21.5). The very good agreement of calculated and experimental values is similar to that observed for the isoelectronic molecules; exactly parallel correlations can be drawn for both theory and experiment through the set SiF4, POFJ , SO2 F2 and ClOsF. We believe that this is very strong evidence for the 21.5 eV band of SiF4 being assigned as al . Further support for this assignment of the 21 S eV band as CI~comes from the low intensity; the ratio of intensities for the last two bands in the He II spectrum is approximately 3: 1 15716,the same ratio as is observed for Sic14 in the He I spectrum3~17. Given the same assignment for CF4 and SiF+ there seems no reason to invoke a different assignment for GeF 4 ; if the lowest orbital in GeF4 is also al , then the similarity of ionisation potentials for this bandI and for that in SiF4 is readily explained in terms of the Si 3s and Ge 4s atomic ionisation potentials3’4. Similar high stabilisations of the e orbital and destabilisations of the a, orbital 498
J. Electron
Spectrosc..
1 (1972/1973)
have been suggested by Jonas et al. for the Group IVB tetramethyls’ , which are isoelectronic with the corresponding fluorides. Since on electronegativity grounds there is much less likelihood of extensive d orbital participation in tetramethyls than in tetrafluorides, or even than in tetrachlorides, it seems to us that this proposed stabilisation of the e orbitals should be treated with reserve. The band in Pb(CH3)4 at ,l 1 eV which Jonas et al. assign as the al orbital is absent in the spectrum published by Evans et al.19, so there is no reason to suppose that a1 rises rapidly on going down Group IVB. The assignments of Evans et al. are analogous to, though not identical with, the assignments proposed above for the tetrafluorides, and the behaviour of the orbital which they assign as 2~~ parallels that of la1 in the tetrahydrides”. The work was supported by the S.R.C. of Great Britain. REFERENCES 1 A_E. Jonas, G.K. Schweitzer, F.A. Grimm and T.A. Carlson, J. Electron Spectrosc., 1 (1972) 29. 2 W.E. Bull, B.P. Pullen, F.A. Grimm, W-E. Moddeman, G.K. Schweitzer and T.A. Carlson, Inorg. Chem.. 9 (1970) 2474.
3 P.J. Bassett and D.R. Lloyd,J. C&em. Sot. (A), (1971) 641. 4 J.C. Green, M.L.H. Green, P.J. Joachim, A.F. Orchard and D.W. Turner, Phil. Trans. Roy. Sot. London, A268 (1970) 111. 5 J.A. Connor, I.H. Hillier, V.R. Saunders and M. Barber, Mol. Phys.. 23 (1972) 81. 6 R. PrinsandT. Novakov, Chem fiys. Lea., 8 (1971) 593. 7 K. Siegbahn et al., ESCA Applied to Free Molecules, North-Holland Publ. CO., Amsterdam, 1969. 8 C.R Brundle, M.B. Robin and H. Basch, J. Chem. Ph.ys.,’53 (1970) 2196. 9 A.W. Potts, H.J. Lempka, D.G. Streets and W.C. Price, Phil. Trans. Roy. Sot. London, A268 (1970) 59. 10 RL. DeKock, D.R. Lloyd, 1-H. Hillier and V.R. Saunders,fioc. Roy. Sot. London, A328 (1972) 401. 11 P.J. Bassett and D-R Lloyd, J. Chem Sot. (A), (1972) 24%. 12 RF. Stewart,J. Chem Phys., 52 (1970) 431. 13 E. Qementi and D.L. Raimondi,J. Chem. Phys., 38 (1963) 2686. 14 E C!le~enti,IBMJ. Res. Dev., 9 (1965) 2. 15 W.C. Price, kW. Potts and D.G. Streets, in D.A. Smley (editor), Electron Spectroscopy, North-Holland Publ. Co., Amsterdam, 1972, p.187. 16 AW. Potts, unpublished observations. 17 The intensities given in ref. 4 need to be corrected for the effect of analyser transmission with electron energy. 18 S. Cradock, Chem Phys. Left., 10 (1971) 291. 19 S. Evans, J.C. Green, P.J. Joachim, A.F. Orchard, D.W. Turner and J.P. Maier, KS Frrra&y II, 6% (1972) 905. 20 A.W. Potts and W.C. Price, Proc. Roy. Sot. London,
J. Electron
Spectrosc.,
1
(1972/1973)
A326 (1972) 165.
499
Preliminary communication On the assignment of the He I photoelectron
spectrum of silicon tetrafluoride
MB. HALL, M.F. GUEST and I.H. HILLIER Chemistry Department,
University of Manchester, Manchester Ml3 9PL (England)
D.R. LLOYD Chemisrv
Depnrtment,
The University, P.0. Box 363, Birmingham BI5 2TT (Engknd)
A.F. ORCHARD Inorganic Chemistry Laboratory,
University of Oxford. Oxford OXI 3QZ (England)
A.W. POTTS Physics Department,
Kings College, Strand, London
WC2R 2LS (England)
(Received 8 December 1972)
Recently,
high-resolution He I photoelectron (p.e.) spectra have been reported for CF4, SiF4 and GeF4 by Jonas et al.‘, following an earlier report of Bull et al.‘. In these papers assignments for the spectra of SiF4 and GeF4 are proposed which differ markedly from earlier assignments based on low-resolution spectra3, and we wish to comment on the new interpretation. In such tetrahedral AB4 molecules the classical view of the bonding is that it arises mainly from interaction of the valence s and p orbitals of A with “u” orbitals of B, those directed along the bond axes. This description as an sp’ hybrid set may be modified in some cases by m interaction of the empty high lying d orbitals of A with lone pair orbit& of B. In terms of delocaliscd orbitals, the main u bonding occurs through orbitals o1 (s) and ts (p), the n lone pairs giving rise to a set e + tl + t2 ;-any d orbital interaction stabilises e through a pure rr interaction, and to a lesser extent the r2 orbitals through an interaction which is mainly n. It has been shown from He I p.e. studies4 that in tetrachlorides and tetrabromides of Group IVB elements the orbital sequence is tl > fi > e > tz > al. The same sequence has been deduced for tetrahedral oxy-anions on the basis of x-ray p.e. spectra and ab initio molecular orbital calculations; there is evidence from int-ensities that the lowest valence m.o. is the al W. Several ab initi~ calculations upon CF4 indicate this same sequence, and it is generally agreed that the p.e. spectra of CF, are in accord with this’-4,7’ 9 _ In earlier work3 the same sequence was proposed for SiF4 and GeF4, but the new assignment132 for SiF4 brings the al orbital above e, into the lone pair region, while e is strongly stabilised along with the lower t2 orbital.. For GeF4 the stabilisation proposed for e brings it below the lower t2 J. Hectron
Spectrosc.,
1 (1972/1973)
497
while u1 remains in the lone pair region’$ . This means that for both SiF4 and GeF4 there is virtually no (Tbonding interactiou with the valence s orbital, and that in GeF4 the major bonding interaction is with the d orbitah and is of n type. A priori it seems most unlikely that a CJinteraction of fluorine with an s orbital should be very much less than the ff interaction with the much higher lying set of d orbitals. Jonas et al.’ justify the stabilisation of the e orbital set by reference to CNDO/2 calculations withand without d orbitals, though Bull et al.* remark that the d orbital participation is probably over-emphasised in the calculations. The rise in energy of al is justified by Jonas et al. in terms of a reduction in calculated F 2s character, but we note that in this orbital F 2F enters with an mttibonding interaction with the central atom orbital, so that a reduction in F 2s character would be expected to stabilise al rather than destabilize it. Even in transition metal tetrachlorides, where there must be strong d orbital participation, al is &ill the lowest orbita14. By comparison of the p.e. spectra and nb initio calculations we have shown that in the molecules isoelectronic with SiF4 (POF3, SO:! F2, C103 F), the central atom charactei of the lowest valence orbital above -30 eV is predominantly 3s, i.e. the lowest orbital corrlates with a1 of SiF4 3T10.Also, internally consistent correlation of ionisation potentials is possible through the moiecules SiF 4, POF3, PF3 if the SiF4 orbital sequence is assumed tobet >t, >e>fz >a1 ll. The set of ab initio calculations has now been completed . with one upon SiF 4. This calculation was carried out using an extended basis of Slater type orbitab @TO) each expanded in three Gaussian type functiuns” _ The core orbitals (Is, 2s, 2p on Si and 1s on F) were represented by one best atom ST013, while the valence orbitals (3s, 3p on Si and 2s, 2p on F) were represented by two STO’s14. In addition, two 36 polarisation STO’s with the same exponents as the 3p STO’s, and both 4s and 4p STO’s with exponents of 1.0, were added to achieve a very accurate description of the valence -molecular orbitals. In contrast to the CND0/2 calculations, the participation by d orbitals is low; the total Si 3d population is only 0.63 e- , compared with 1.44 e- in the CND0/2 approximation’ . The eigenvalues have been mu1tiplied by -0.92, and are given below, in eV, with experimental values’ ?‘y16 in parentheses: tl 17.5 (16.5); t2 18.4(17.5);e 18.9 (18.1); r2 20.0 (19.5); al 21.7 (21.5). The very good agreement of calculated and experimental values is similar to that observed for the isoelectronic molecules; exactly parallel correlations can be drawn for both theory and experiment through the set SiF4, POFJ , SO2 F2 and ClOsF. We believe that this is very strong evidence for the 21.5 eV band of SiF4 being assigned as al . Further support for this assignment of the 21 S eV band as CI~comes from the low intensity; the ratio of intensities for the last two bands in the He II spectrum is approximately 3: 1 15716,the same ratio as is observed for Sic14 in the He I spectrum3~17. Given the same assignment for CF4 and SiF+ there seems no reason to invoke a different assignment for GeF 4 ; if the lowest orbital in GeF4 is also al , then the similarity of ionisation potentials for this bandI and for that in SiF4 is readily explained in terms of the Si 3s and Ge 4s atomic ionisation potentials3’4. Similar high stabilisations of the e orbital and destabilisations of the a, orbital 498
J. Electron
Spectrosc..
1 (1972/1973)
have been suggested by Jonas et al. for the Group IVB tetramethyls’ , which are isoelectronic with the corresponding fluorides. Since on electronegativity grounds there is much less likelihood of extensive d orbital participation in tetramethyls than in tetrafluorides, or even than in tetrachlorides, it seems to us that this proposed stabilisation of the e orbitals should be treated with reserve. The band in Pb(CH3)4 at ,l 1 eV which Jonas et al. assign as the al orbital is absent in the spectrum published by Evans et al.19, so there is no reason to suppose that a1 rises rapidly on going down Group IVB. The assignments of Evans et al. are analogous to, though not identical with, the assignments proposed above for the tetrafluorides, and the behaviour of the orbital which they assign as 2~~ parallels that of la1 in the tetrahydrides”. The work was supported by the S.R.C. of Great Britain. REFERENCES 1 A_E. Jonas, G.K. Schweitzer, F.A. Grimm and T.A. Carlson, J. Electron Spectrosc., 1 (1972) 29. 2 W.E. Bull, B.P. Pullen, F.A. Grimm, W-E. Moddeman, G.K. Schweitzer and T.A. Carlson, Inorg. Chem.. 9 (1970) 2474.
3 P.J. Bassett and D.R. Lloyd,J. C&em. Sot. (A), (1971) 641. 4 J.C. Green, M.L.H. Green, P.J. Joachim, A.F. Orchard and D.W. Turner, Phil. Trans. Roy. Sot. London, A268 (1970) 111. 5 J.A. Connor, I.H. Hillier, V.R. Saunders and M. Barber, Mol. Phys.. 23 (1972) 81. 6 R. PrinsandT. Novakov, Chem fiys. Lea., 8 (1971) 593. 7 K. Siegbahn et al., ESCA Applied to Free Molecules, North-Holland Publ. CO., Amsterdam, 1969. 8 C.R Brundle, M.B. Robin and H. Basch, J. Chem. Ph.ys.,’53 (1970) 2196. 9 A.W. Potts, H.J. Lempka, D.G. Streets and W.C. Price, Phil. Trans. Roy. Sot. London, A268 (1970) 59. 10 RL. DeKock, D.R. Lloyd, 1-H. Hillier and V.R. Saunders,fioc. Roy. Sot. London, A328 (1972) 401. 11 P.J. Bassett and D-R Lloyd, J. Chem Sot. (A), (1972) 24%. 12 RF. Stewart,J. Chem Phys., 52 (1970) 431. 13 E. Qementi and D.L. Raimondi,J. Chem. Phys., 38 (1963) 2686. 14 E C!le~enti,IBMJ. Res. Dev., 9 (1965) 2. 15 W.C. Price, kW. Potts and D.G. Streets, in D.A. Smley (editor), Electron Spectroscopy, North-Holland Publ. Co., Amsterdam, 1972, p.187. 16 AW. Potts, unpublished observations. 17 The intensities given in ref. 4 need to be corrected for the effect of analyser transmission with electron energy. 18 S. Cradock, Chem Phys. Left., 10 (1971) 291. 19 S. Evans, J.C. Green, P.J. Joachim, A.F. Orchard, D.W. Turner and J.P. Maier, KS Frrra&y II, 6% (1972) 905. 20 A.W. Potts and W.C. Price, Proc. Roy. Sot. London,
J. Electron
Spectrosc.,
1
(1972/1973)
A326 (1972) 165.
499