Modeling SN2 reactions in methanol solution by ab initio calculation of nucleophile solvent-substrate clusters

Abstract

Ab initio calculations were used to study the S N2 reactions of the CH 3OCH 2I molecule with a methoxide ion (CH 3O -) and a methanol molecule by systematically building up the reaction system with explicit incorporation of the methanol solvent molecules. For the reaction of CH 3OCH 2I with a methoxide ion, the explicit incorporation of the methanol molecules to better solvate the methoxide ion led to an increase in the barrier to reaction. For the reaction of CH 3OCH 2I with a methanol molecule, the explicit incorporation of the methanol molecules led to a decrease in the barrier to reaction because of an inclination of this reaction to proceed with the nucleophilic displacements accompanied by proton transfer through the H-bonding chain. The H-bonding chain served as both acid and base catalysts for the displacement reaction. A ca. 10 15-fold acceleration of the methanol tetramer incorporated S N2 reaction was predicted relative to the corresponding methanol monomer reaction. The properties of the reactions examined are discussed briefly. © 2005 American Chemical Society.