The Rotational Barriers of Groups Containing Silicon in Substituted Benzenes. A Theoretical Approach to the Silicon Substituent Effect

Abstract

The potential energy profile for a number of derivatives of benzene substituted with the SiH 3 , SiH 2 CH 3 , and CH 2 SiX 3 (X ) H, F, Cl, CH 3 ) groups was calculated at ab initio (HF/6-31G*//HF/6-31G*) level, and the energy barriers were estimated. The rotational barrier is low for the molecules with silicon directly bonded to the phenyl ring, but higher values are found for the CH 2 SiX 3 groups, which depend on X as well. The hyperconjugative effect of the substituents was estimated from the natural bond orbital (NBO) theory of donor acceptor properties: the SiH 2 CH 3 group was found to be electron-acceptor in character, and the CH 2 SiX 3 were found to be electron-donor groups. The rotational barrier of these molecules is mainly determined by hyperconjugation, steric effects being smaller than in the analogous carbon derivatives. The SisH and SisC bonds display an electron-acceptor character of similar magnitude when silicon is attached to the phenyl ring, but the SisC bond has a prevailing donor character when carbon is bonded to the phenyl ring.