Abstract

The reaction force F(R) of a chemical or physical process is given by the negative derivative of the potential energy V(R) along the intrinsic reaction coordinate R. F(R) unambiguously and naturally divides the activation barrier in each direction into two contributions, one of which has been found to reflect preparative structural factors, E (act,prep), and the other corresponds to the first part of the transition to products, E (act,trans). We have analyzed F(R) for an S N2 substitution reaction in both the gas and aqueous phases. Although the overall forward and reverse activation barriers are significantly lowered by the solvent, the E (act,trans) are very little affected. Thus the increased rates that are predicted for this process in aqueous solution can be attributed to the solvent facilitating the structural effects in the preparative stages, decreasing the E (act,prep.) This example shows how the reaction force decomposition of activation barriers can help to elucidate the roles played by external factors, e.g., solvents.