Electron-Transfer Component in Hydroxyl Radical Reactions Observed by Time Resolved Resonance Raman Spectroscopy

The existence of an electron-transfer pathway in the reaction of •OH radical with aromatic molecules in water has been established, for the first time, using time-resolved resonance Raman spectroscopy as a diagnostic tool and p-dimethoxybenzene as a model system. In the currently accepted mechanism, the cation radical is produced by •OH addition to the ring, followed by loss of OH-. The present work demonstrates that this process competes with direct electron transfer. A generalized reaction mechanism has been proposed in terms of potential energy diagrams to explain two-step formation of the cation radical. In this reaction mechanism, the electron-transfer component and the rate of OH- elimination from the •OH adduct both depend on the ionization potential (IP) of the molecule. The cation radical yield by electron transfer increases from 6% in p-dimethoxybenzene to 30% in p-anisidine and 85% in p-phenylenediamine. For neutral molecules with IP > 8 eV, the •OH addition is the first step in the chemistry, and for IP < 7 eV, it is the electron transfer. In the intermediate IP range, both processes occur simultaneously.