DIGITAL-SIMULATION OF PHOTOMODULATION VOLTAMMOGRAMS - REACTIVITY OF THE DIPHENYLMETHYL CARBANION AND CARBOCATION IN ACETONITRILE

Photomodulation voltammograms of transient free radicals in acetonitrile were simulated by the implicit finite-difference method. In general, it was found that (1) the phase shift of the current was determined only by the lifetime of the radical at potentials above the diffusion limit, (2) the position (E1/2) but not the shape of the wave was determined by the lifetime of the product ion, (3) the quadrature voltammograms were more sensitive than the in-phase voltammograms to changes in the ion lifetime, and (4) the breadth of the voltammetric wave was determined by the rate of the heterogeneous electron transfer. A method that allows the systematic determination of all of the relevant kinetic and thermodynamic parameters is described. The model is tested by use of the redox reactions of the diphenylmethyl radical. It is concluded that the measured lifetime of the carbocation is actually that for the nitrilium ion that is formed by the reaction of the diphenylmethyl cation with the solvent. In anhydrous acetonitrile/trifluoroacetic anhydride (10%), this nitrilium ion has an apparent lifetime of ca. 200 ms.