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 S i 4 bond and the plane formed by Si with the other two carbons is found to be 51.7O. The inversion barrier through the higher energy planar C3" structure is calculated to be 13.3 kcal/mol. The planar CSh structure 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 and interpretations for the various conformations offered. Substituent effects on the degree of pyramidalization and on the barrier to inversion in silyl radicals are substantial. Substitution of three fluorines for hydrogens in .SM3 raises the barrier from 5 to 68 kcal/mol. Successive substitution of methyl groups for hydrogens also increases the pyramidal-planar barrier.

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