Kinetic isotope effect in the photoreduction of o-benzoquinones in the presence of N,N-dimethylaniline

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 Delta G(e), the effective rate constants k(H) and k(D) and the quantum yields phi(H) and phi(D) of the reaction varied nonmonotonically with a maximum at Delta G(e), of approximately +0.1 eV. For the reactant pairs relevant to the ascending (left-hand) branch of the plot k(H) = f(Delta G(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(Delta G(e)), there is no isotope effect or its value is insignificant.