Influence of quantum energy flow and localization on molecular isomerization in gas and condensed phases

Abstract

Just as collisions between a reactant and its environment affect thermal unimolecular reaction rates, as described by the Lindemann mechanism, energy flow between the reaction mode and other modes of the reactant analogously influences microcanonical rates. Conformational isomerization typically proceeds over a relatively low-energy barrier, and the influence of slow quantum energy flow or localization on the microcanonical rate can be dramatic. We briefly review a theory describing quantum energy flow in moderate-sized to large molecules and how that picture can be used to understand and predict the influence of intramolecular energy flow on unimolecular reaction rates in gas and condensed phases. This theory locates a transition to global energy flow, the quantum ergodicity transition (QET), and predicts relatively slow flow rates at energies not far above the transition. We then apply the theory to predict rates of conformational isomerization of 2-fluoroethanol and allyl fluoride, each with a barrier between 1000 and 2000 cm−1. We find the QET of each to lie at energies near or above 3000 cm−1, consistent with recent experimental findings. How the thermal rate varies with pressure or viscosity is seen to depend sensitively on the QET and the rate of quantum energy flow. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 523–531, 1999