Molecular dynamics of a model S N 2 reaction in water

Abstract

Molecular dynamics are computed for a model SN2 reaction Cl−+CH3Cl→ClCH3+Cl− in water and are found to be strongly dependent on the instantaneous local configuration of the solvent at the transition state barrier. There are significant deviations from the simple picture of passage over a free energy barrier in the reaction coordinate, and thus, a marked departure from transition state theory occurs in the form of barrier recrossings. Factors controlling the dynamics are discussed, and, in particular, the rate of change of atomic charge distribution along the reaction coordinate is found to have a major effect on the dynamics. A simple frozen solvent theory involving nonadiabatic solvation is presented which can predict the outcome of a particular reaction trajectory by considering only the interaction with the solvent of the reaction system at the gas-phase transition barrier. The frozen solvent theory also gives the transmission coefficient κ needed to make the transition state theory rate agree with the outcome of the molecular dynamics trajectories. This theoretical κ value, which is the implementation for the SN2 reaction of the van der Zwan–Hynes nonadiabatic solvation transmission coefficient, is in good agreement with the trajectory results. In contrast, a Kramers theory description fails dramatically.