Solvation effects on a model S(N)2 reaction in water clusters

We present a series of molecular dynamics experiments for the nucleophilic substitution reaction Cl- + CH3Cl-->ClCH3 + Cl- taking place in liquid simple point charge water nanoclusters containing 6, 16, and 32 solvent molecules at temperatures close to 200 K. A three-dimensional;potential energy for the reagent interatomic interactions is employed. Equilibrium and dynamical aspects of the reactive process are investigated. Solvation effects lead to significant enhancements of the computed free energy barriers even in aggregates containing only six water molecules. The equilibrium spatial and orientational correlations describing the changes in the solvation structure along the reaction path are also presented, The reactive/product states are characterized by a fully solvated Cl- ion embedded within the cluster while the CH3Cl remains on the surface; at the transition state, the complex lies at the cluster surface adopting a linear geometry tangential to the cluster boundary. We have also monitored the time relaxation of the solvation structures as the system evolves from the transition to the stable product states. Our results show the reaction proceeds by a series of highly coordinated motions involving the different components of the reagent. Estimates for the rate constants in clusters of different sizes are also computed using the reactive flux correlation function formalism. (C) 1996 American Institute of Physics.