THE INHIBITING EFFECTS OF TETRAALKYLAMMONIUM CATIONS ON SIMPLE HETEROGENEOUS ELECTRON-TRANSFER REACTIONS IN POLAR APROTIC-SOLVENTS

Kinetic data for the electroreduction of nitromesitylene at mercury in propylene carbonate have been determined in the presence of tetraalkylammonium salts of varying chain length, both as a function of electrode potential and temperature. Although the standard rate constant decreases with increase in cation size, the experimental activation enthalpy is independent of the cation for variation in this ion from tetraethylammonium to tetraoctylammonium. These results indicate that the tetraalkylammonium ions am adsorbed on the electrode forming a blocking layer whose thickness increases with alkyl chain length. Electron transfer to the redox center takes place by tunneling through this film. Theoretical estimates of the activation enthalpy support this conclusion and suggest that the redox reaction occurs in a region where solvent reorganization determines the largest portion of its magnitude.