Ab initio study of α- and α- silyl substituent effects on vinyl radicals

Abstract

The geometries of α-and β-silyl substituted vinyl radicals and of α, β-disilylvinyl radical have been optimised with the STO-3G and the STO-3G^∗ basis sets. The relative stabilities of various conformera have been determined at the UMP2/6-31G^∗ level. The stabilisation of vinyl radicals through α-silyl substitution is larger than that due to corresponding alkyl groups. The presence of an α-silyl group also leads to a tendency towards linearisation of the vinyl radical centre and a corresponding reduction in the inversion barrier. In marked contrast, the β-silyl effect is negligible. The geometric, conformational and energetic consequences are insignificant. Overall, the silyl substituent effect at vinyl radicals is very different from that computed earlier for the vinyl cations, but qualitatively similar to that found in carbanions.