KINETICS OF THE ELECTROREDUCTION AND ELECTROOXIDATION OF TETRAKIS(DIMETHYLAMINO)-P-BENZOQUINONE IN POLAR APROTIC-SOLVENTS

A high-frequency ac admittance technique capable of precisely measurin g rate constants greater than 1 cm s(-1) was used to collect kinetic d ata for both the one-electron reduction and one-electron oxidation of tetrakis-(dimethylamino)-p-benzoquinone. This study was conducted at a gold ultramicroelectrode in seven polar aprotic solvents, with varyin g viscosity, in the presence of 0.1 M tetraethylammonium perchlorate. In addition, the temperature dependence of the standard rate constants was determined in three of these solvents. On the basis of an analysi s of the solvent effects, it is shown that the kinetic data show a var iation in the degree of reaction adiabaticity and the relative sizes o f both the inner- and outer-sphere components of the Gibbs energy of a ctivation. Solvent effects on diffusion coefficients and standard pote ntials were also investigated. The standard reduction potential is a l inear function of the solvent acceptor number, a parameter reflecting molecular solvating properties, whereas the corresponding standard pot ential for oxidation is a linear function of solvent polarizability, a parameter that reflects nonspecific interactions between the reactant and the solvent. The significance of these results is discussed withi n the context of the current models for simple heterogeneous electron transfer reactions.