Evidence for the transition between concerted and stepwise heterogeneous electron transfer bond fragmentation mechanisms

The electrochemical reduction of tert-butyl p-cyanoperbenzoate in DMF leads to the cleavage of the oxygen-oxygen bond. A subsequent reaction yielding tert-butyl p-cyanobenzoate could be avoided by the addition of a mild acid, with the result that the main reduction peak was affected only by the kinetics of the electron transfer bond fragmentation process. Unusual features of the heterogeneous electron transfer mechanism were evidenced by the conventional voltammetric criteria, which pointed to a nonlinear potential dependence of the transfer coefficient alpha. The fine details of the electron transfer mechanism were obtained by convolution analysis of the voltammetric curves, a procedure that led to a bell-shaped potential dependence of alpha. The observed behavior is explained by the assumption that when the electrode potential is decreased, the process changes from a concerted electron transfer bond cleavage to a partially stepwise mechanism. The observed potential dependence of alpha is in agreement with the results from simple calculations aimed to obtain the potential dependences of alpha expected for the two pure mechanisms.