Non-equilibrium solvation in SN1 and SN2 reactions in polar solvents

Abstract

In the standard transition-state theory (TST) view of the role of the polar solvent in ionic reactions, the solvent is implicitly assumed always to be in equilibrium with the intrinsic reaction system at each point along the reaction coordinate. However, if there is insufficient time for the solvent molecules to so equilibrate, there will be non-equilibrium solvation effects. These cause a breakdown in the TST predictions for the reaction rate. These effects are analytically described via van der Zwan–Hynes theory and Grote–Hynes theory and examined via a molecular-dynamics simulation of a model Cl^–+CH₃Cl S_N2 reaction in water, and analytically for a model S_N1 dissociation in water. Finally, a solution-phase reaction-path Hamiltonian theory is described to investigate anharmonic effects on model S_N2 reactions.