Calculated energy barriers for the identity S(N)2 reaction H2O+CH3OH2+->+H2OCH3+OH2 in the gas phase, in water clusters, and in aqueous solution

The bimolecular nucleophilic substitution reaction H2O + CH3OH2+ --> +H2OCH 3 + OH2 has been studied using various quantum chemical methods. Accurate b arriers for the reaction in the gas phase are presented and discussed. The effect of microsolvation by water molecules in small clusters has been inve stigated. Extrapolation of the barrier obtained in the small clusters, usin g a linear relationship between the activation energy and the proton affini ty of water clusters, gives a barrier for the reaction in aqueous solution which is in good agreement with that obtained in separate model calculation s (polarized continuum model of a super molecule with the first solvation s hell included). (C) 1999 Elsevier Science B.V.