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 R is shown to be a sensitive probe of the mechanism by which electron transfer and bond cleavage may be coupled in dissociative electron transfers. R is particularly useful in detecting the transition between stepwise and concerted dissociative electron transfers. Whereas linear potential dependencies of R are in agreement with either mechanism, a mechanism transition can be evidenced upon observation of a nonlinear R pattern. Under favorable circumstances, a wavelike potential dependence of R can be observed. This is a function of main 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 R was applied to study the electroreduction of a series of perbenzoates, XC6H4CO3But, in DMF. The reduction leads to the irreversible cleavage of the O-O bond. The R data were obtained by cyclic voltammetry followed by convolution analysis. For all compounds investigated, the experimental trend could be simulated satisfactorily by reasonable selection of the main parameters. Whereas the analysis showed that the reduction of the unsubstituted peroxide proceeds by a pure concerted mechanism, a stepwise mechanism holds when X ) 4-NO2. On the other hand, R-wave patterns were found for X ) 4-COMe and 3-NO2, as previously described for X ) 4-CN. For the latter compounds, the R analysis is in agreement with a dissociative electron transfer process in which the mechanism changes from stepwise to concerted by increasing the applied potential. Finally, although the reduction of the 4-OCOMe perbenzoate basically occurs by a concerted mechanism, 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 by lowering the temperature.