CHAPTER 18: Extension of Marcus Rate Theory to Electron Transfer Reactions with Large Solvation Changes

The Marcus theory for charge transfer reaction in solution is reviewed using a molecular statistical mechanics language. This theory is based on the so-called microscopic energy-gap coordinate and relies on a Gaussian solvation picture or, equivalently, on a linear response approximation. It involves two parameters, the reorganization energy and the reaction free energy parameters. However, it may fail when the solvation has a different character in the reactant and product state. For that situation, we describe a complementary theoretical extension, based on a non-Gaussian solvation picture, and we discuss its implications for electron transfer rate constants and rate constant/reaction free energy relationships. Relying on molecular simulation results, we show that such situation arise even for simple half oxido-reduction reactions involving the Cu⁺/Cu²⁺ or Ag⁰ /Ag⁺ couples or for generic charge transfer reactions involving high polarizability changes. The non-Gaussian theory exhibits the correct non-linear response behavior and reproduces the simulation results quantitatively, whereas a simple one-Gaussian-state Marcus description breaks down.