The structure and stability of silyl-substituted carbonium ions

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Ab-initio SCFMO calculations on silyl-substituted alkanes, alkyl-radicals and carbonium ions show that the carbonium ion is destabilized by an α-silyl group but stabilized by a β-silyl group in comparison with the carbon analogues. In contrast, replacement of α or β-methyl by silyl has little effect on the stability of the radicals or alkanes. These results are consistent with observations on the rates of S N1 solvolysis of trimethylsilylalkyl halides, and indicate that the relative rates are largely determined by the relative stabilities of the carbonium ions. A non-classical bridged structure for SiH 3CH 2CH + 2 has been found to have an energy approximately 0.5 eV (50kJ/mole) lower than that of a classical open structure.