ESR‐Spectroscopic Investigation of the Homogeneous Electron Transfer Reactions between Substituted p‐Phenylenediamines and Quinonediimines, and the Validity of Marcus' Theory. II. Temperature Dependence and Activation Parameters

Using ESR‐line broadening the rate constants of electron self exchange between five substituted p‐phenylenediamines, semiquinonediimines, and quinonediimines in six aprotic solvents are measured in the temperature range 230 K < T < 360 K. A linear dependence of ln k on the solvent parameter γ = 1/n2 ‐ 1/ϵ (n = refractive index, ϵ = permittivity) is observed for each temperature as expected from Marcus' theory, also a linear dependence of ln kT−1/2 on T−1 for each solvent. The experimental surface ln k(γ, T), however, is not in agreement with Marcus' theory, the resulting activation energies and ‐entropies being proportional to γ and increasing from negative values at low γ (extrapolated) to positive at high γ. The deviations are explained by a temperature‐dependent reaction distance and a preceding association between the radical cations and the counterions which increases strongly with decreasing polarity of the solvent.