Using a recently developed empirical valence bond (EVB) model for the nucleophilic substitution reaction (SN2) in solution, we study the benchmark Cl− + CH3Cl reaction at the water/chloroform liquid/liquid interface. The reaction free energy profile is determined as a function of the reagents’ location relative to the interface. We find that the activation free energy is very sensitive to the reagents’ location and to the orientation of the nucleophilic attack. The barrier height at the interface is equal or slightly larger than the barrier in bulk water and approaches the value in bulk chloroform only when the solute is a few nanometers deep into the organic phase. We show that this is due to the ability of the nucleophile to keep part of its hydration shell. This suggests that for the catalytic effect of the nonpolar solvent to be appreciable, the nucleophile must be transferred away from the interface. The dynamical correction to the rate, the variation in the system’s electronic structure and other system properties as a function of the location with respect to the interface, provide additional insight into the system’s behavior.

The effect of a tetramethylammonium cation phase transfer catalyst on the benchmark Cl− + CH3Cl reaction at the water/chloroform liquid/liquid interface is investigated by a molecular dynamics-empirical valence bond (EVB) model. The effect of the catalyst on the reaction free energy profile at different interface locations and in bulk chloroform is examined. We find that, because of significant water “pollution”, activation of the nucleophilic attack is limited to the bulk organic region. The barrier height at the interface is equal to or slightly larger than the barrier in bulk water and is unaffected by the presence of the catalyst. In bulk chloroform, our calculations suggest that the barrier height, which is much lower than in bulk water, moderately increases when a few water molecules interact with the system and when the catalyst forms an ion-pair with the nucleophile. Thus, the catalyst is most effective if its role is limited to bringing the nucleophile to the bulk organic phase.

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Using a recently developed empirical valence bond model for the nucleophilic substitution reaction (S(N)2) in solution, we examine microhydration effects on the benchmark Cl(-) + CH(3)Cl reaction in liquid chloroform. Specifically, the effect of the hydration of the reactive system by one to five water molecules on the reaction-free energy profile and the rate constant is examined. We find that the activation-free energy is highly sensitive to the number of water molecules hydrating the nucleophile, increasing the barrier by about 4 kcal/mol by the first water molecule. With five water molecules, the barrier height is 10 kcal/mol larger than the barrier in bulk chloroform and only 3 kcal/mol below the barrier in bulk water. A number of properties vary monotonically with the number of water molecules, including the rate of change in the system's electronic structure and the solvent stabilization of the transition state. These and other properties are a rapidly varying function of the reaction coordinate. Deviation from transition state theory due to barrier recrossing is not large and falls between the behavior in bulk water and bulk chloroform.

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