Ab initio MO study of benzylic cations—2. Steric effects on the resonance interaction and on the resonance demand in the Yukawa-Tsuno equation

Abstract

Geometries of secondary and tertiary benzylic cations, which have bulky substituents at α positions, were optimized by ab initio MO calculations at the RHF/6-31G* and, in part, MP2/6-31G* levels. Calculated øcalc, which is the dihedral angle of the α-C (SINGLE BOND) C bond with respect to the aromatic plane, is compared with the relative resonance demand r/rmax obtained from solvolysis reactivity and also cation stability; r and rmax are the resonance demand in the Yukawa(SINGLE BOND)Tsuno equation for any given system examined and the corresponding ideal full-resonance stabilized demand, respectively. The results suggest that there is a close relationship between the experimental and calculated relative resonance demands expressed by r/rmax = cos2 øcalc, which is suggested by HMO theory for the resonance interaction in the benzyl cation. Thus the r value is a good parameter indicating the degree of resonance interaction between benzylic 2p π-orbital and the benzene π-system.