ESR-investigations on the dynamic solvent effects of degenerate electron exchange reactions. Part I: cyanobenzenes

The rates of degenerate electron exchange (electron self-exchange) of various cyanoben-zenes have been measured by EPR line broadening technique in nine different solvents atroom temperature. The molecules studied comprise besides benzene-1,2-dicarbonitrile,benzene-1,4-dicarbonitrile and benzene-1,2,4,5-tetracarbonitrile, the two isomeric tri-cyanobenzenes, benzene-1,2,3- tricarbonitrile and benzene-1,2,4-tricarbonitrile, the anionradicals of which have not been characterized before.The experimentally observed rates vary from 4.5×108to 44.0×108M−1s−1andshow the pronounced dependence on the longitudinal relaxation times,τL, of the solvents.The solvent dynamical effect so manifested is confirmed with remarkable clarity usingsolvents spanning a wide range ofτL-values, which comprise acetonitrile (0.2ps) ando-dichlorobenzene (6.0 ps) at its extremes. The rate constants are compared with Marcustheory using the continuum model (CM) and the mean spherical approximation (MSA)for the outer sphere reorganization energies and Nelson’s method for the inner spherereorganization energies. Furthermore, an estimation of the resonance splitting energies,VRP, is given based on the experimental rates.