Energy diffusion-controlled reactions in solution

Abstract

The energy diffusion-controlled limit for reactions in solution is discussed for anharmonic oscillator models of isomerization and dissociation-recombination. Energy diffusion is described by an equation due to Zwanzig. The vibrational energy diffusion coefficient in this equation is related to the spectrum of solvent forces on the oscillator probed at its unperturbed frequencies. The energy diffusion-controlled rate constants k are calculated. For small barrier heights, k is severely depressed by inefficient, adiabatic regime vibrational energy transfer. For large barrier heights, the important energy flow region lies in the more efficient nonadiabatic vibrational energy transfer regime, and k is much larger. The relevance of our results for solution reactions is discussed.