Qualitative kinetics of electron transfer reactions

Abstract

We revise the conventional theory of electron transfer reactions and give a qualitative picture of the turnover from non-adiabatic to adiabatic regime consistent with the general principle that equilibrium properties can depend on no kinetic parameters. It is shown that if the solvent polarization fluctuations are fast enough then the reaction is non-adiabatic and the solvent follows the instantaneous charge distribution. No kinetic characteristics of the solvent enters the expression for the rate constant which in this case is determined by the total reorganization energy of the system. If a slow component of polarization fluctuations can be identified then a high frequency dielectric permittivity of the solvent can be introduced self-consistently. In the normal region the reaction is adiabatic and the charge distribution follows the solvent polarization relaxation in the potential which has the form of the free energy of the two level system with no interaction between the states at fixed value of the slow reaction coordinate. In this regime the reaction rate is determined by the reorganization energy of the slow degrees of freedom. It is also shown that in the inverted region the reaction can never be adiabatic.