ELECTRON-TRANSFER KINETICS AND ACTIVATION BARRIERS FOR THE REDUCTIONSOF NITROSONIUM AND NITRONIUM IONS IN APROTIC-SOLVENTS

Using the cyclic voltammetry (CV), the electron-transfer kinetics for the reductions of NO+ and NO2+ cations have been studied at the Pt ele ctrode in nitromethane, sulfolane, and propylene carbonate. The hetero geneous rate constants have been determined by two independent procedu res from the transfer coefficient alpha, the diffusion coefficient D, from a detailed examination of the CV-peak separations, and from an in spection of the values of the cathodic peak potentials at different sc an rates. The results have been compared to those reported in the lite rature, and discussed. In the classical model, outer-sphere electron-t ransfer reactions are considered subject to an activation energy arisi ng from solvent reorganization and bond reorganization processes. The solvent and molecular reorganizational barriers for these electroreduc tions have been assessed in aprotic media. The Marcus-Hush theory has been applied to the self-exchange reactions of the NO2+/NO2 and NO+/NO couples in an attempt to predict the rate of electron transfer. The f indings indicate some improvement between theory and experiment. Howev er, it should be noted that the experimental values of k(s) found for the NO2+ reduction in the solvents used are still too high in comparis on with those determined theoretically. In view of the fairly strong c oordination of the solvent molecule(s) as ligand(s) to NO2+ and NO+ ca tions, we believe that such discrepancies should stem, to some extent, from the involvement of an inner-sphere pathway by generation of an a ctivated complex on the surface of the Pt electrode.