Barrierless Isomerization Dynamics in Viscous Liquids: Decoupling of the Reaction Rate from the Slow Frictional Forces

Abstract

Many important chemical and biological reactions do not face a sizable activation barrier in their motion along the reaction coordinate. As a result, these reactions often have time constants in the range of a few hundred femtoseconds (fs) only. The existing theories, on the other hand, assume only the viscous, zero frequency frictional response of the solvent, which is clearly inadequate to describe solvent viscosity effects on such ultrafast reactions. In this article, we present a theory of barrierless chemical reactions that includes the bimodal frictional response of the solvent. The generalized theory is based on a non-Markovian Smoluchowski equation, with a time (t) dependent diffusion coefficient (D(t)) to describe the reactive motion along the reaction surface; the reaction itself is described by a coordinate-dependent sink term. This description is reliable for a harmonic reaction potential energy surface. The time-dependent diffusion coefficient can be obtained from the time-dependent friction ...