Radical reactivity of radical ions in solution. Radical-radical and radical-substrate coupling mechanisms

The linear sweep voltammetry response to competitive radical ion-substrate coupling and radical ion dimerization mechanisms was determined by digital simulation. The simulations were carried out to mimic the conditions under which experimental studies had previously shown that the radical ion-substrate coupling mechanism is the preferred reaction pathway. It was observed that in order for the dependence of the peak potential on substrate concentration (delta Ep/delta log CA) to be in the experimentally observed range (36-40 mV/decade) that the relative rate constants for radical ion substrate coupling and radical ion dimerization (ki/kii) must be greater than about 10. It is pointed out that since the reactants, the transition states and the products differ by only a single electron that these competitive reactions represent an ideal test case for the configuration mixing (CM) model. The CM model predicts an electronic reaction barrier for reaction (i) but not for reaction (ii). The difference in standard free energy changes for reactions (i) and (ii) were estimated to be of the order of 7 kcal mol-1 or greater with (ii) being energetically more favorable than (i). It is concluded that the experimental data for the relative rates of reactions (i) and (ii) do not conform to the CM model predictions in the cases discussed. ArH(.+) + ArH-->+ ArH-ArH.(i) ArH(.+) + APH .+ --> + ArH-ArH+(ii).