Solvent Dynamic in Electron Transfer Reactions: An Application of Kramers Theory to Homogeneous Organic Electron Self Exchange

Using ESR spectroscopy the rate constants of homogeneous electron self exchange in the TCNQ/TCNQ − ., TCNE/TCNE − . and TTF/TTF + .‐couples (TCNQ = tetracyanoquinonedimethane, TCNE = tetracyanoethylene, TTF = tetrathiofulvalene) were measured in 14 different solvents at 293 K. The rate constants were expressed by the preequilibrium encounter model for adiabatic processes applying Kramers theory. “Friction” dependent preexponential factors expressed by the longitudinal relaxation time τ L of the solvents are found. The solvent dependence of the rate constant is explained in terms of Marcus theory by using an outer‐sphere reorganization energy for ellipsoidal molecules. From these solvent dependencies the geometric term g(r̄,d′) = 1/ r̄‐1/d′ is obtained leading to an experimental reaction distance d between both molecules in the transition state.