Kinetic Isotope Effect in the Photoreduction of o-Benzoquinones in the Presence of N,N-Dimethylaniline

Abstract

The kinetics of photoinduced reduction of a series of 3,5- and 3,6-di-tert-butylbenzoquinones-1,2 was studied in the presence of N,N-dimethylaniline, its deuterated analog, and a series of para-substituted N, N-dimethylanilines. On passing from the endothermic to the exothermic range of the free energy of electron transfer ΔG e , the effective rate constants k H and k D and the quantum yields ϕH and ϕD of the reaction varied nonmonotonically with a maximum at ΔG e of approximately +0.1 eV. For the reactant pairs relevant to the ascending (left-hand) branch of the plot k H = f(ΔG e ), the H/D isotope effect takes place, varying from two to five in magnitude. For the reactant pairs at the maximum and in the descending (right-hand) branch of the plot k H = f(ΔG e ), there is no isotope effect or its value is insignificant.