Proton Coupled Electron Transfer in the Reaction of Hydroxybenzenes with Hydrazyl Radical in Aqueous Media

Abstract

Possible electron transfer mechanisms have been studied for the reaction of 2,2′-diphenyl-1-picrylhydrazyl with a number of natural hydroxybenzenes in aqueous buffer solutions at pH = 2–9 using the methods of spectrophotometry and quantum chemical calculations. In acidic media, electron transfer occurs from the molecular form of hydroxybenzene to the radical with subsequent loss of proton, whereas in alkaline media the electron is transferred from the phenolate ion to the radical, preceded by the proton elimination step. Realization of a specific mechanism is indicated by the dependence of the reaction rate constant on the pH of the media and by the presence of correlation between the Gibbs free activation energy, calculated using the Marcus equation and that determined experimentally.