Electron self-exchange kinetics between 2,3-dicyano-5,6-dichloro-p-benzoquinone (DDQ) and its radical anion.: Part 1.: Solvent dynamical effects

Rate constants of the electron self-exchange between 2,3-dicyano-5,6-dichloro-p-benzoquinone (DDQ) and its radical anion (DDQ(.-)) are measured by means of EPR-line-broadening effects in different solvents at T = 293 K. Solvents of different polarity like CHCl3, CH2Cl2, CH3CN, benzonitrile and acetone were used. No correlation is found in the sense of the classical Marcus theory, where ln k(et) should depend linearly on the solvent parameter gamma = 1/n(2) -1/epsilon(s) (n = refractive index, epsilon(s) = static relative permittivity of the solvent). The investigated gamma-range was 0.270 less than or equal to gamma less than or equal to 0.530. The diffusion corrected rate constants k(et) cover a range of 8.9 to 36.7 x 10(8) M-1 s(-1) and clearly show a solvent-dynamical effect, expressed by the longitudinal relaxation time tau(L) dependence of the solvents used. The observed dynamical friction solvent effects clearly indicate an adiabatic reaction behaviour of this electron self-exchange reaction.