Short range caging effects for reactions in solution. I. Reaction rate constants and short range caging picture

The effects of short range solvent structure and short range dynamical correlations are investigated for the steady state rate constant k for solution reactions influenced by diffusion. The description is in terms of a Smoluchowski equation describing relative motion of two molecules in an outer spatial translational region, supplemented by a sink term that accounts for dynamics in an inner reaction zone. In the outer region, solvent structural effects are included by a potential of mean force, which exhibits a short range well and barrier combination leading to ’’potential caging.’’ Outer region short range dynamical correlations are included via a separation-dependent diffusion coefficient, leading to ’’dynamical caging’’ as relative motion is showed at small separations. These two short range effects are neglected in standard diffusion treatments. We find that k is only modestly influenced by the above short range effects. In order to expose short range structure and correlation influence in a more sensitive way, we formulate the short range caging picture. This information focuses on events occuring at the small separations usually implied by the term ’’solvent cage.’’ At this level of description, short range effects are clearly revealed. We compare our formulation with the encounter formulation of Noyes.