Asymmetrically distorted structure of radical cations of ethylsilanes, studied by electron spin resonance spectroscopy, abinitio and density functional theories

Abstract

ESR spectra for the radical cations of SiEt₂H₂, SiEt₃H, SiEt₄ and SiEt₂MeH, generated in halocarbon matrices by ionizing radiation at 77 K, consist of a doublet with a coupling constant of ca. 6.3 mT to a ¹H nucleus. Accumulation of our data for the cations with C–Si–C bonds suggests that the cations adopt asymmetrically distorted structures with an unpaired electron largely in one of the elongated Si–C bonds and that the ¹H nucleus in the “trans’' position to the bond results in the large hyperfine (hf) splitting observed. Calculations at abinitio theory and density functional theory (DFT) levels strongly support this conclusion. The reversible spectral changes, observed for SiEt₂H₂⁺ and SiEt₂MeH⁺ in CF₂BrCF₂Br matrices, have been analyzed with a three-site jumping model for the internal rotation of the methyl group having the ¹H nucleus assigned and a two-site jumping model for a selective bond length alternation between two adjacent Si–C bonds.

References

1. M. C. R. Symons, Chem. Soc. Rev., 1984, 393.
2. T. Shida, E. Haselbach and T. Bally, Acc. Chem. Res., 1984, 17, 180.
3. L. B. Knight, Acc. Chem. Res., 1986, 19, 313.
4. M. Shiotani, Magn. Reson. Rev., 1987, 12, 333.
5. K. Toriyama, in Radical Ionic Systems, ed. A. Lund and M. Shiotani, Kluwer, Dordrecht, 1991, ch. I-4, p. 99.
6. M. Lindgren and M. Shiotani, in Radical Ionic Systems, ed. A. Lund and M. Shiotani, Kluwer, Dordrecht, 1991, ch. I-5, p. 125; M. Shiotani and A. Lund, ibid., ch. I-6, p. 151.
7. M. Iwasaki, K. Toriyama and K. Nunome, J. Am. Chem. Soc., 1981, 103, 3591.
8. K. Nunome, K. Toriyama and M. Iwasaki, Tetrahedron, 1986, 42, 6315.
9. K. Toriyama, K. Nunome and M. Iwasaki, J. Chem. Phys., 1982, 77, 5891.
10. M. Shiotani, N. Ohta and T. Ichikawa, Chem. Phys. Lett., 1988, 149, 185.
11. M. Lindgren, M. Shiotani, N. Ohta, T. Ichikawa and L. Sjöqvst, Chem. Phys. Lett., 1989, 161, 127.
12. L. Sjöqvst, M. Lindgren, A. Lund and M. Shiotani, J. Chem. Soc., Faraday Trans., 1990, 86, 3377.
13. M. Shiotani, M. Lindgren, N. Ohta and T. Ichikawa, J. Chem. Soc., Perkin Trans. 2, 1991, 711.
14. K. Toriyama, K. Matsuura and K. Nunome, Chem. Phys. Lett., 1990, 169, 311.
15. M. Shiotani, M. Matsumoto and M. Lindgren, J. Chem. Soc., Perkin Trans. 2, 1993, 1995.
16. M. Lindgren, K. Komaguchi, M. Shiotani and K. Sasaki, J. Phys. Chem., 1994, 98, 8331.
17. M. Shiotani, K. Komaguchi, J. Ohshita and M. Ishikawa, Chem. Phys. Lett., 1992, 188, 93.
18. K. Komaguchi, M. Shiotani, M. Ishikawa and K. Sasaki, Chem. Phys. Lett., 1992, 200, 580.
19. T. Fängström, S. Lunell, B. Engels, L. A. Eriksson, M. Shiotani and K. Komaguchi, J. Chem. Phys., 1997, 107, 1.
20. K. Komaguchi and M. Shiotani, J. Phys. Chem. A, 1997, 101, 6983.
21. K. Toriyama, Chem. Phys. Lett., 1991, 177, 39.
22. K. Toriyama, M. Okazaki and K. Nunome, J. Chem. Phys., 1991, 95, 3955.
23. K. Toriyama and M. Okazaki, J. Chem. Phys., 1992, 96, 6986.
24. M. Okazaki and K. Toriyama, J. Phys. Chem., 1993, 97, 8212.
25. K. Toriyama and M. Okazaki, Acta. Chem. Scand., 1997, 51, 167.
26. A. E. Finholt, A. C. Bond, Jr and H. I. Schlesinger, J. Am. Chem. Soc., 1947, 69, 1199.
27. J. Heinzer, Mol. Phys., 1971, 22, 167.
28. QCPE Program, no. 209, Indiana University, 1972.
29. B. W. Walther and F. Williams, J. Chem. Soc., Chem. Commun., 1982, 270.
30. L. Bonazzola, J. P. Michaut and J. Roncin, New J. Chem., 1992, 16, 489.
31. J. N. Murrel and W. Schmidt, J. Chem. Soc., Faraday Trans. 2, 1992, 88, 1709.
32. R. G. Dromey and J. B. Peel, J. Mol. Struct., 1974, 23, 53.
33. M. Iwasaki, K. Toriyama and K. Nunome, Faraday Discuss. Chem. Soc., 1984, 78, 19.