Kinetics of electron exchange in N,N,N′,N′-tetramethyl-p-phenylenediamine in aprotic solvents and the semiclassical theory of electron transfer

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The homogeneous electron exchange of the N,N,N′N′-tetramethyl-p-phenylenediamine radical (S˙+) with the reduced (R) and the totally oxidized (T2+) forms has been measured in aprotic solvents by means of e.s.r. line-broadening. Solvent polarity and temperature were varied. For each temperature the logarithm of the rate constant k1(S˙+⇌ R) is a linear function of the solvent parameter γ= 1/n2–1/ε(where n is the refractive index and ε the dielectric constant) and at constant γ(0.27 ⩽γ⩽ 0.53) a linear function of 1/T. The outer reorganization energy can be calculated using an ellipsoidal model of the reactants and a reaction distance equal to the molecular distance in the radical dimer. The inner reorganization energy is calculated from i.r. and crystallographic data. Nuclear tunnelling is considered since the intramolecular frequencies are high. The experimental function ln k1(γ, T–1) cannot be reproduced by the theory unless additional interaction energies such as ion association S˙+⋯ClO–4 and especially ion-molecule interactions S˙+⋯R are allowed for. The preexponential factors of the theoretical expressions are considerably greater than the experimental values, especially if a precursor equilibrium is assumed. The rate constant k2(S˙+⇌ T2+) is small compared with k1. The difference is mainly caused by the Coulomb repulsion term.