Electron transfer in the Marcus inverted region: Experiment and adiabatic tunneling mechanism

Abstract

This paper presents an adiabatic tunneling model for electron transfer reactions in the Marcus inverted region. The model is used to address the role vibrational modes in determining the magnitude of the reaction rate constants for the inverted charge transfer reaction: (benzene+)Cl–O−→(benzene)Cl–O in acetonitrile solution. Two frequency ranges for the accepting vibrational mode were studied; 1500 cm−1 representative of the stretching mode of the aromatic ring, and 208 cm−1, used to model the intermolecular stretching mode of the donor–acceptor complex. The temperature dependence is different for the two calculated cases. In the high-frequency case the tunneling rate constant is independent of temperature (270–330 K) while the activated rate constant changes by one order of magnitude. For the low-frequency mode, the situation is more complicated. The temperature dependence is a function of the magnitude of the matrix coupling element, H12. The calculated temperature behavior for a tunneling mechanism using either class of vibrational modes is consistent with experiment. While electronic coupling and reorganization energy are not available from experiment or quantum chemical calculation, the approach used allows for the determination of the range for these parameters that fit the experimental values.