Stepwise and concerted pathways in thermal and photoinduced electron-transfer/bond-breaking reactions

Bond cleavages triggered by electron transfer may follow a stepwise or a co ncerted mechanism. A proper description of the competition between the two reaction pathways requires a more detailed analysis of the reaction coordin ate than usual. Such an analysis is presented for systems in which nuclear reorganization involves mostly the stretching of the cleaving bond and the reorganization of the solvent. It is first applied to thermal reactions suc h as electrochemical or homogeneous bond cleavages and then to photoinduced reactions. The quantum yields predicted for stepwise mechanisms are derive d under conditions of practical interest and compared with the quantum yiel ds predicted for concerted mechanisms. It is shown that quantum yields for stepwise mechanisms may approach unity and are not necessarily smaller than quantum yields for concerted mechanisms. The fragments resulting from bond cleavage may interact within the solvent cage where they are formed. The i nfluence of such interactions on the dynamics of stepwise and concerted rea ctions is discussed.