KINETICS OF DIABATIC AND ADIABATIC ELECTRON EXCHANGE IN ORGANIC-SYSTEMS COMPARISON OF THEORY AND EXPERIMENT

The rate constants k(et) of electron self exchange between neutral molecules and the corresponding radical ions, measured by ESR line broadening, are discussed for seven organic pi-systems in 18 aprotic solvents of a wide range of relaxation times, tau-L (0.2 < tau-L/PS < 10), and polarities, gamma(0.05 < gamma < 0.53) in terms of the theoretical approaches of Marcus and Sumi and of Jortner and Bixon, both valid for comparable magnitudes of inner and outer reorganization energies. (gamma = 1/n2-1/epsilon; n = refractive index; epsilon = dielectric constant.) The electron transfer is diabatic for four substituted p-phenylenediamines, it is adiabatic relaxation overdamped for tetracyanoquinodimethane (TCNQ), tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE). By changing the solvents no transition between both mechanisms is found. To explain the results, the reactants are represented by ellipsoids. Their centre to centre distance in the transition state is derived from slopes of lnk(et)tau-L-gamma--1/2 vs. gamma (for adiabatic reactions) or of lnk(et)-gamma-1/2 vs. gamma (for diabatic reactions). The mutual orientation of the reactants follows the minimum of electrostatic energy. The resonance splitting V for this orientation, calculated from the overlap integrals agrees well for all systems with the observed diabatic or adiabatic behaviour of the reaction, except for TCNE. The resonance splitting and the equilibrium constant of the precursor complex are derived experimentally from the temperature dependence of the electron transfer rate constant k(et). Negative association energies of the complex are due to ion-dipole interactions of the reactants. An additional reorganization term is observed for the aliphatic TCNE system in ethereal solvents. To describe the experiments quantitatively, both theories used are nearly equivalent.