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

Abstract

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 γ = 1/n² – 1/ε_s (n = refractive index, ε_s = static relative permittivity of the solvent). The investigated γ-range was 0.270 ≤ γ ≤ 0.530. The diffusion corrected rate constants k_et cover a range of 8.9 to 36.7 × 10⁸ M^–1 s^–1 and clearly show a solvent-dynamical effect, expressed by the longitudinal relaxation time τ_L dependence of the solvents used. The observed dynamical friction solvent effects clearly indicate an adiabatic reaction behaviour of this electron self-exchange reaction.