We have performed molecular dynamics calculations on a model Cl- + CH3Cl SN2 reaction in water in order to elucidate how the reactants obtain sufficient energy from the solvent to climb the potential energy barrier to reaction. This system, consisting of ionic and dipolar reagents in a polar solvent, is representative of a large class of chemical reactions with strong Coulombic reagent-solvent coupling. We find that the change in internal energy of the reactants during the barrier-climbing process involves three distinct epochs: (i) vibrational activation of the methyl chloride in the initial Cl-CH3Cl ion-dipole complex, (ii) gradual increase in kinetic and potential energies of the reactants, and (iii) fast dumping of reactant kinetic energy into reactant potential energy resulting in the reactants reaching the top of the potential energy barrier, with the symmetric structure CL-delta-CH3-delta-+CL-delta-. The energy that the reagents gain during this process comes primarily from the potential energy of the water solvent. We also show that many water molecules are involved in this energy transfer, but almost as much energy is removed from the reactants by these water molecules as is deposited in them over the course of the barrier climbing. The critical change in the charge distribution as the reactants climb the barrier occurs over a very short time, and we present evidence that the total energy of the water solvent molecules remains essentially constant, consistent with the frozen solvent nonadiabatic solvation model used previously to understand the deviations from the transition-state rate for this system (Bergsma et al. J. Chem. Phys. 1987, 86, 1356; Gertner et al. J. Chem. Phys. 1987, 86, 1377; 1989, 90, 3537). We also find that the water solvent undergoes a substantial, though not complete, reorganization well before the change in the charge distribution of the reactants. This reorganization is crucial, although not sufficient, for the success of the barrier climbing. Many of these results for this strongly coupled system contrast starkly with those found by Benjamin e al. (J. Am. Chem. Soc. 1990, 112, 524) for a neutral symmetric atom exchange reaction in a rare gas solvent (Bergsma et al. Chem. Phys. Lett. 1986, 123, 394; J. Chem. Phys. 1986, 85 5625) where the forces between solvent and reagents are short range and the coupling is much weaker. Thus, there is a rich variety of energy flow phenomena and solvent dynamics that must be considered in order to understand the detailed molecular dynamics of how chemical reactions take place in solution and how these dynamics arise from the particular system's reagent, solvent, and solvent-reagent forces.
