A spin statistical factor in electron transfer to oxygen molecules.

The oxygen molecule in its ground triplet state (O) is a strong electron acceptor. Electron transfer to O to form a superoxide anion is an important elementary step in many chemical and biological processes. If this transfer occurs from a spin 1/2 paramagnetic particle where the total spin of the reactants is equal to 3/2, the reaction is spin-forbidden. In liquids, the significant dipole-dipole electron spin interaction in O is supposed to mix the non-reactive quartet and reactive doublet states at a time scale of ∼10 ps, thus avoiding the barrier. To elucidate the role of spin effects in the electron transfer to O, we studied this reaction over a range of more than three orders of magnitude of the relative diffusion coefficient () of the reactants. It was found that spin effects during electron transfer to O become insignificant when < 10 m s. In the range of intermediate values (10 m s < < 10 m s) - which corresponds to some reactions of oxygen with small radicals in aqueous solutions - the effective spin factor decreases with increasing value. If > 10 m s, the electron transfer is spin-selective with the spin factor of 1/3 as determined by the spin statistics. At such values, the reaction encounter time may exceed the expected quartet-doublet mixing time by almost an order of magnitude. The reduced rate of quartet-doublet transitions within the encounter complex in the reaction with O has been explained by the spin-exchange interaction and chemical Zeno effect.