The role and relevance of the transfer coefficient alpha in the study of dissociative electron transfers: Concepts and examples from the electroreduction of perbenzoates

The electrochemical transfer coefficient alpha is shown to be a sensitive p robe of the mechanism by which electron transfer and bond cleavage may be c oupled in dissociative electron transfers. alpha is particularly useful in detecting the transition between stepwise and concerted dissociative electr on transfers. Whereas linear potential dependencies of alpha are in agreeme nt with either mechanism, a mechanism transition can be evidenced upon obse rvation of a nonlinear alpha pattern. Under favorable circumstances,a wavel ike potential dependence of alpha can be observed. This is a function of ma in parameters describing the mechanism competition such as, in particular, the difference between the two relevant standard potentials, the intrinsic barriers, and the preexponential factors of the two rate-constant equations . The analysis of alpha was applied to study the electroreduction of a seri es of perbenzoates, XC6K4CO3But, in DMF. The reduction leads to the irrever sible cleavage of the O-O bond. The alpha data were obtained by cyclic volt ammetry followed by convolution analysis. For all compounds investigated, t he experimental trend could be simulated satisfactorily by reasonable selec tion of the main parameters. Whereas the analysis showed that the reduction of the unsubstituted peroxide proceeds by a pure concerted mechanism, a st epwise mechanism holds when X = 4-NO2. On the other hand, alpha-wave patter ns were found for X = 4-COMe and 3-NO2, as previously described for X = 4-C N. For the latter compounds, the alpha analysis is in agreement with a diss ociative electron transfer process in which the mechanism changes from step wise to concerted by increasing the applied potential. Finally, although th e reduction of the 4-OCOMe perbenzoate basically occurs by a concerted mech anism, a transition pattern seems to emerge at the most negative potentials explored. Further support to the experimental outcome and conclusions was provided by studying the temperature effect on the reduction of the 4-COMe derivative, which led to the expected shift toward the stepwise mechanism b y lowering the temperature.