Role of Polar and Enthalpic Effects in the Addition of Methyl Radical to Substituted Alkenes: A Density Functional Study Including Solvent Effects

Abstract

The addition of methyl radical to mono- and disubstituted alkenes has been studied by a hybrid Hartree−Fock/density functional method taking into account solvent effects by the polarizable continuum model. The reliability of the electronic approach has been verified by comparison with refined post-Hartree−Fock computations and with experimental data. Environmental effects do not alter the trends of in vacuo computations due to the low dielectric constant of the solvent and to the lack of significant charge separation effects. Use of substrates characterized by captodative effects and comparison with a genuine nucleophilic radical (CH2OH) allow one to unequivocally conclude that CH3 does not behave as a nucleophile. As a consequence polar effects are negligible and activation barriers are governed by the stability of the forming radical. These trends are confirmed by electron population analysis and evaluation of charge-transfer energies.