A proposal for a branching mechanism in medium-sized cycloalkanes: MINDO/3 study of the hypersurface for isomerization of cyclohexylium to 1-methylcyclopentylium ion

Abstract

A MINDO/3 study on the branching rearrangement mechanism for cycloalkanes is presented. Starting from cyclohexylium ion, a reaction path to methylcyclopentylium ion is found. Two different intermediates and several alternative transition states are localized, and then characterized by hessian diagonalization after complete optimization of geometry. The barrier height calculated for the inverse path (methylcyclopentylium to cyclohexylium)(21.7 kcal mol^–1) agrees well with the unique available experimental datum (17 kcal mol^–1) in the range of applicability of the MINDO/3 method. The mechanism proposed is consistent with the empirical mechanism suggested by Brouwer for branching rearrangement of linear alkanes.