What can gas phase reactions tell us about solution reactions?

Abstract

Much of the behavior of simple gas phase reaction dynamics can be understood in terms of simple pictures based on the shapes of the underlying potential energy surfaces and the masses of the reagent atoms. Our aim here is to investigate to what extent such gas phase models can be used to understand properties of solution reactions. In particular, we examine for a solution reaction the validity of the Evans–Polanyi rule that an early potential barrier favors translational excitation of the reactants and vibrational excitation of the products, with the converse holding true for a late barrier. The test is performed by using molecular dynamics simulations for an asymmetric linear transition state A+BC→AB+C atom exchange reaction in argon solvent. We calculate for both gas and solution reactions the partitioning among translational, rotational, and vibrational energy during the reaction process. We find that for a short time period (−65 to 65 fs where t=0 is at the barrier top), in which the forces from the intrinsic gas phase potential dominate, the Evans–Polanyi rule can be carried over into the solution reaction. The gas phase vibrational energy distributions persist in solution over a much longer period. In particular, this calculation illustrates for an early barrier linear transition state potential in solution that a solvent induced fluctuation in the reagent translational energy is considerably more effective than a fluctuation in vibrational energy in prompting reaction. The resulting reaction products are formed highly vibrationally excited. For the reverse late barrier reaction, a solvent induced fluctuation in vibrational energy is needed for reaction and the resulting products are initially highly translationally excited. We expect that on the proper time scale, many other gas phase reaction dynamics rules will also carry over to solution reactions, particularly in cases in which the reagent–solvent interaction is weak.