A Theoretical Study of Silicon-Centered Radicals (Ab Initio, Perturbation, Molecular Orbitals, Inversion Barriers, Pyramidalization).
Date of Award
Doctor of Philosophy (PhD)
Energies and structures for a series of silicon-centered radicals were calculated by ab initio methods. The lowest energy conformation of trimethylsilyl radical is found to be pyramidal. The angle formed by an Si-C bond and the plane formed by silicon with the two other carbons is 52.6 (3-21G(,*)). The inversion barrier through the higher energy planar C(,3v) structure is calculated to be 17.8 kcal/mol. The planar C(,3h) geometry lies still higher in energy. In the planar forms of the methyl-substituted radicals, the methyl groups are slightly distorted so that the CH bond more eclipsed with the SOMO is bent away from this MO. The results are compared to the tert-butyl radical. Interpretations for the various conformations are offered. In silacycloalkyl radicals with the radical center at silicon, the barriers to pyramidal inversion at the radical center increase with decreasing ring size. In the four- and five-membered rings, ring inversion is a lower energy pathway than pyramidal inversion at silicon. Substituent effects on the degree of pyramidalization and on the barrier to inversion in silicon-centered radicals are substantial. Substitution of three fluorines for hydrogens in (,(.))SiH(,3) raises the barrier from 7.6 to 80 kcal/mol. Substitution by methyl groups is less extreme but nevertheless increases the degree of pyramidalization of the radical over that of (,(.))SiH(,3). For the most part the barrier to pyramidal inversion can be addressed in terms of SOMO/LUMO interactions in the corresponding planar radical. Radicals which have a smaller SOMO/LUMO gap generally have a higher inversion barrier.
Piccione, Raymond V., "A Theoretical Study of Silicon-Centered Radicals (Ab Initio, Perturbation, Molecular Orbitals, Inversion Barriers, Pyramidalization)." (1985). LSU Historical Dissertations and Theses. 4069.