Solvent effects on the kinetics of heterogeneous electron transfer processes. The TMPD/TMPD ·+ redox couple

Abstract

Solvent effects on the heterogeneous electron exchange between N, N, N′, N′-tetramethyl- p-phenylendiamine (TMPD) and its monocation radical (TMPD ·+) have been studied by chronocoulometry at a platinum electrode in perchlorate solutions in a wide range of aprotic and hydrogen-bonded solvents. Formal potentials, diffusion coefficients, anodic transfer coefficients and formal rate constants have been evaluated. The variation of the formal potential with the solvent can be interpreted by determining the solvent—solvent—solute interactions using Taft theory. Correlations between predicted and experimental formal potentials for the Taft model were an improvement over those obtained using the Gutmann donor number approach. A weak correlation between the anodic transfer coefficient and the Taft β parameter was also found. The variations in the rate constants were analyzed on the basis of current electron-transfer models using a statistical method to separate the effects of the longitudinal relaxation time τ L from those of the solvent permittivity parameter γ. It has been shown that the dynamics of solvent relaxation affect the heterogeneous electron-transfer rate in this case, but the rate constants depend on τ −0.53 instead of τ −1, as theory predicts. The degree of reaction adiabaticity and the relative sizes of the inner- and outer-sphere components of the Gibbs energy of activation were considered.