Journal of the American Chemical Society, Vol.120, No.17, 4161-4166, 1998
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)-O-. 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)-O-. 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)-O-. 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)-O-. 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.
Keywords:CONDUCTOMETRIC PULSE-RADIOLYSIS;AQUEOUS-SOLUTION;SPIN RESONANCE;OH RADICALS;ORGANIC-MOLECULES;CATION RADICALS;RATE CONSTANTS;VIBRATIONAL-RELAXATION;ADDITION-ELIMINATION;METHOXYLATED PHENOLS