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Inorg. Chem. 1994, 33, 5086-5093
Variable Photon Energy Photoelectron Spectroscopic and Theoretical Investigations of the Electronic Structure of Tic14 Bruce E. Bursten,? Jennifer C. Green,*($Nikolas Kaltsoyannis,"' Michael A. MacDonald," Kong H. Sze,S and John S. Tse" Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K., SERC Daresbury Laboratory, Warrington WA4 4AD, U.K., and National Research Council of Canada, Ottawa K1A OR6, Canada Received February 24, 1994@
The photoelectron spectrum of T i c 4 has been measured using synchrotron radiation over the incident photon energy range 17-69 eV, and relative partial photoionization cross sections have been derived for the valence bands. Band A (lt1-l) and band B (3t2-l) display cross section variations typical of C13p atomic orbital-localized levels. Bands C+D show cross section enhancement between 40 and 60 eV and are assigned to le-' and 2t2-l. Band E (2al-l) shows no such feature and has cross section behavior which is appreciably different from that of any of the other bands. The small cross section maximum at 40 eV is attributed to a molecular shape resonance. Discrete variational Xa calculations support this orbital ordering, and quantitative agreement is found between the experimental and calculated ionization energies. Photoionization cross sections were calculated using the multiple-scattering Xa method. Good agreement was found between experimental and theoretical branching ratios, confirming the assignment and suggesting that the resonances found in the 2t2 and 2al orbital cross sections are due to multiple scattering from the localized C1 electrons. The minimum in the 3t2 cross section is associated with the Cooper minimum of the C1 3p orbital.
Introduction Tetrahedral coordination is one of the geometries central to inorganic chemistry. A thorough knowledge of the electronic structures of tetrahedral molecules is therefore of key importance in understanding a wide range of chemical systems. The structural simplicity, high symmetry, and volatility of T i c 4 have led to extensive experimental investigations via photoelectron spectroscopy and numerous calculational approaches have also been e m p l ~ y e d . ' ~ It ~ ~is~ perhaps -~~ surprising, t The Ohio State University.
* University of Oxford.
SERC Daresbury Laboratory. Research Council of Canada. Abstract published in Advance ACS Abstracts, September 15, 1994. (1) Burroughs, P.; Evans, S.; Hamnett, A.; Orchard, A. F.; Richardson, N. V. J. Chem. Soc., Faraday Trans. 2 1974, 70, 1895. (2) Egdell, R. G. Thesis, University of Oxford, 1977. (3) Egdell, R. G.; Orchard, A. F.; Lloyd, D. R.; Richardson, N. V. J. Electron Spectrosc. Relat. Phenom. 1977, 12, 415. (4) Cox, P. A.; Evans, S.; Hamnett, A,; Orchard, A. F. Chem. Phys. Lett. 1970, 7, 414. (5) Green, J. C.; Green, M. L. H.; Joachim, P. J.; Orchard, A. F.; Tumer, D. W. Philos. Trans. R. Soc. London, A 1970, 268, 111. (6) Egdell, R. G.; Orchard, A. F. J. Chem. Soc., Faraday Trans. 2 1978, 74, 485. (7) Bancroft, G. M.; Pellach, E.; Tse, J. S. Inorg. Chem. 1982, 21, 2950. (8) Wetzel, H. E. Thesis, University of Hamburg, 1987. (9) Becker, C. A. L.; Dahl, J. P. Theor. Chim. Acta 1969, 14, 26. (10) Becker, C. A. L.: Ballhausen, C. J.; Trajberg, I. Theor. Chim. Acra 1969, 13, 355. (11) Choplin, F.; Kaufmann, G. Theor. Chim. Acta 1972, 25, 54. (12) Truax, D. R.: Geer, J. A.; Ziegler, T. J. Chem. Phys. 1973, 59, 6662. (13) Fenske, F. R.; Radtke, D. D. Inorg. Chem. 1968, 7, 479. (14) Ellis, D. E.; Parameswan, T. Int. J. Quantum Chem., Symp. 1971, 5, 443. (15) Parameswan, T.; Ellis, D. E. J. Chem. Phys. 1973, 58, 2088. (16) Tossell, J. A. Chem. Phys. Lett. 1979, 65, 371. (17) Foti, A. E.; Smith, V. H.; Whitehead, M. A. Mol. Phys. 1982, 45, 385. (18) Hillier, I. H.; Kendnck, J. Inorg. Chem. 1976, 15, 520. (19) von Niessen, W. Inorg. Chem. 1987, 26, 567. 8
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therefore, that there is still no general agreement as to the assignment of the photoelectron (PE) spectrum of this prototypical molecule. Clearly, this remains a highly desirable goal-an understanding of the bonding in these simple systems is vital in view of the fundamental position which they occupy in coordination chemistry. Furthermore, it is only through an unequivocal assignment of their PE spectra that the success or otherwise of the wide range of theoretical approaches that have been brought to bear can be properly gauged. From the experimental standpoint, it is important that we understand these simple tetrahedral molecules if we are to have confidence in applying our techniques to more complicated transition metal systems. We recently published a combined theoretical and experimental study of 0 ~ 0 4 in, which ~ ~ ASCF and Green's function methods were applied together with synchrotron radiation-based PES to produce a consistent interpretation of the valence PE spectrum. In this paper, we present a similar study of TiC14, although in this instance local density functional theory (in its discrete variational (DV) form) is the method used for calculation of the ionization energies and ionization cross sections are also calculated using the multiple-scattering OMS) Xa m e t h ~ d . ~ l - ~ ~ Variable photon energy PE spectroscopy (PES) is now established as a powerful tool in electronic structure determination. We and others have investigated a wide range of chemical systems, with examples drawn from both classical c o ~ r d i n a t i o n ~ ~and ~ *organometallic ~-~~ hemi is try,^^,^^-^* and have found the technique to be a sensitive probe of the (20) Green, J. C.; Guest, M. F.: Hillier, I. H.; Jarrett-Sprague, S. A,; Kaltsoyannis, N.; MacDonald, M. A,; Sze, K. H. Inorg. Chem. 1992, 31, 1588. (21) Johnson, K. H. Adv. Chem. Phys. 1973, 7, 143. (22) Dill, D.; Dehmer, J. J. Chem. Phys. 1974, 61, 692. (23) Davenport, J. W. Phys. Rev. Lett. 1976, 36, 945. (24) Electron-Molecule and Photon-Molecule Collisions; Rescigno, T., McKoy, V., Schneider, B., Eds.; Plenum: New York, 1979.
0 1994 American Chemical Society
Electronic Structure of TiCL localization properties of valence electrons. In particular, the very different partial cross section behavior of d and f electrons with respect to s and p provides extensive information on metal-ligand covalency. The empirical He II/He I intenisty ratio r ~ l e s have ~ ~ -been ~ ~examined more fully, and many of the limitations of the discharge lamp method have been exposed. A variable photon energy investigation of T i c 4 was therefore a logical choice in pursuit of unequivocal spectral assignment, particularly as much of the dispute in the literature is based on experimentally observed He II/He I intensity changes.’ Further interest stems from the opportunity to compare transition metal with main group compounds, as a detailed investigation of the valence molecular orbital cross sections of CC449 and SiC450 has already been done. It was hoped that halogen- and central atom-derived effects would be distinguishedmore easily through this comparison.
Experimental Section The PE spectra of TiCh were obtained using the synchrotron radiation source at the Science and Engineering Research Council Daresbury Laboratory. A full account of our experimental method has been given?* and the apparatus and its performance are described e l s e ~ h e r e . Hence ~~ only a brief account of experimental procedures is given here. Synchrotron radiation from the 2 GeV electron storage ring at the SERC Daresbury Laboratory was monochromated using a toroidal grating monochromator and was used to photoionize gaseous samples in a cylindrical ionization chamber. The photoelectrons were energyanalyzed using a three-element zoom lens in conjunction with a hemispherical electron energy analyzer, which was positioned at the “magic angle” so as to eliminate the effects of the PE asymmetry parameter, p, on signal intensity. Multiple-scan PE spectra were collected at each photon energy required. The decay of the storage ring beam current was corrected for by linking the scan rate with the output from a photodiode positioned to intersect the photon beam after it had passed through the ionization region. The sensitivity of the Green. J. C.: Kaltsovannis.. N.:. MacDonald. M. A,:. Sze.. K. H. Chem. Phys. Lett. i990,175, 359. Green, J. C.: Kaltsovannis, N.: MacDonald, M. A.; Sze, K. H. J. Chem. Soc., Dalton. Trans. 1991,2371. Brennan, J. B.; Green, J. C.; Redfem, C. M.; MacDonald, M. A. J. Chem. Soc., Dalton Trans. 1990, 1907. Cooper, G.; Green, J. C.; Payne, M. P.; Dobson, B. R.; Hillier, I. H. J. Am. Chem. Soc. 1987,109, 3836. Cooper, G.; Green, J. C.; Payne, M. P. Mol. Phys. 1988,63, 1031. Brennan, J . B.: Green, J. C.; Redfem, C. M. J. Am. Chem. SOC. 1989, 111, 2373. Brennan, J. G.; Cooper, G.; Green, J. C.; Kaltsoyannis, N.; MacDonald, M. A,; Payne, M. P.; Redfem, C. M.; Sze, K. H. Chem. Phys. 1992, 164, 276. Davies, C. E.; Green, J. C.; Kaltsoyannis, N.; MacDonald, M. A,; Qin, J.; Rauchfuss, T. B.; Redfem, C. M.; Stringer, G. H.; Woolhouse, M. G. Inorg. Chem. 1992,31, 3779. Didziulis, S . V.; Cohen, S. L.; Gewirth, A. A,; Solomon, E. I. J. Am. Chem. Soc. 1988,110, 250. Butcher, K. D.; Didziulis, S.V.; Briat, B.; Solomon, E. I. J. Am. Chem. Soc. 1990,112, 2231. Butcher, K. D.; Gebhard, M. S.; Solomon, E. I. Inorg. Chem. 1990, 29, 2067. Yang, D. S.; Bancroft, G. M.; Puddephatt, R. J.; Tan, K. H.; N., C. J.; Bozek, J. D. Inorg. Chem. 1990,29, 4956. Li, X . R.; Bancroft, G. M.; Puddephatt, R. J.; Hu, Y. F.; Liu, Z.; Tan, K. H. Inorg. Chem. 1992,31, 5162. Li, X . R.; Bancroft, G. M.; Puddephatt, R. J.; Hu, Y. F.; Liu, Z.; Sutherland, D. G. J.; Tan, K. H. J. Chem. Soc. Chem. Commun. 1993, 67. Eland, J. H. D. Photoelectron Spectroscopy; Buttenvorths: London, 1984. Tumer, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Spectroscopy; Wiley-Interscience: London, 1970. Rabalais, J. W. Principles of Ultraviolet Photoelectron Spectroscopy; Wiley-Interscience: New York, 1977. Carlson, T. A. Photoelectron and Auger Spectroscopy; Plenum: New York, 1975. Briggs, D. Handbook of X-ray and ultra-violet photoelectron spectroscopy; Heyden: London, 1977.
Inorganic Chemistry, Vol. 33, No. 22, 1994 5087 photodiode to different radiation energies was determined by measuring the np-’ PE spectra of Ne, Ar, and Xe. This procedure ensures that the band intensity of the final spectrum divided by the number of scans gives a measure of the cross section at any particular photon energy. The rare gas data were also used to characterize and correct for a falloff in analyzer collection efficiency at PE kinetic energies
