Fundamental aspects of chemical kinetics in condensed phases

Abstract

Solvents can exert strong effects on chemical reaction rates. The interaction of solvent with reactant species causes shifts in transition states, replacing the gas phase saddle point with a distribution of saddle points differing in height, extent of asymmetry, curvatures, etc. The effect of such a distribution on measured reaction rates can be assessed with an extension of the Stillinger-Weber inherent structure theory. This theory uses a mass-weighted descent mapping to partition the multidimensional configuration space into distinct potential energy “basins”; in the present work these are classified by the numbers of reactant and product species present at the minima. Chemical transition states are flanked by pairs of “gateway” basins. We have implemented this formalism numerically and located the actual chemical transition states for a molecular dynamics model of the exchange reaction F + F 2 ⇌ F 2 + F in liquid argon. In the dense solution, the frequency of trajectory recrossings through the transition state exceeds that in the gas phase reaction. Most of this difference stems from the changes in geometry of reactants at the distribution of reactive saddle points.