MAGNITUDE AND ORIGIN OF THE beta -SILICON EFFECT ON CARBENIUM IONS.

Article abstract of DOI:10.1021/ja00292a008

Ab initio molecular orbital calculations have been carried out on alpha - and beta -substituted methyl and vinyl cations to obtain a quantitative measure of the substituent effect of a silyl group relative to a methyl group and hydrogen. Geometries optimized with the 3-21G **(***) basis set were used in calculations at the MP3/6-31G* level. The stabilization energies due to various substituents were determined by means of isodesmic reactions involving the parent methyl and classical vinyl cations. alpha -Methyl substitution of the methyl cation leads to a stabilization energy of 34. 0 kcal/mol compared to 17. 8 kcal/mol obtained through alpha -silyl substitution. The stabilization due to alpha -methyl and alpha -silyl groups is comparable for the vinyl cation (27. 2 and 24. 1 kcal/mol), suggesting that the inductive effect of silicon is more effective in this case.