Using a recently developed empirical valence bond model for the nucleophilic substitution reactionSN2 in solution, we examine microhydration effects on the benchmark Cl-+CH3Cl reaction inliquid chloroform. Specifically, the effect of the hydration of the reactive system by one to five watermolecules on the reaction-free energy profile and the rate constant is examined. We find that theactivation-free energy is highly sensitive to the number of water molecules hydrating thenucleophile, increasing the barrier by about 4 kcal/mol by the first water molecule. With five watermolecules, the barrier height is 10 kcal/mol larger than the barrier in bulk chloroform and only 3kcal/mol below the barrier in bulk water. A number of properties vary monotonically with thenumber of water molecules, including the rate of change in the system’s electronic structure and thesolvent stabilization of the transition state. These and other properties are a rapidly varying functionof the reaction coordinate. Deviation from transition state theory due to barrier recrossing is notlarge and falls between the behavior in bulk water and bulk chloroform.
展开▼
