Reduction potentials of naphthoxyl and pyridoxyl radicals in aqueous solutions

Reduction potentials of 1- and 2-naphthoxyl, 2-, 3-, and 4-pyridoxyl, and 4-phenylphenoxyl radicals in alkaline aqueous solutions are determined by pulse radiolysis. The potential of 1-naphthoxyl is determined to be 0.59 V versus NHE from the redox equilibrium involving l-naphthol, 3-methylphenol, and their radicals. The reduction potential of the 3-methylphenoxyl radical (0.73 V) is obtained from its equilibrium with the phenoxyl radical. The potential for 2-naphthoxyl (0.69 V) is obtained from equilibrium with the phenoxyl radical. The reduction potentials of the three pyridoxyl radicals are measured from equilibria with the 4-cyanophenoxyl radical. The values are found to be 1.18 V for 2-pyridoxyl, 1.06 V for 3-pyridoxyl, and 1.24 V for 4-pyridoxyl. The reduction potential of 4-phenylphenoxyl radical (0.93 V) is obtained from equilibrium with the I-2(.-) radical. From these reduction potentials and the known pK(a) values of the phenolic compounds, we estimate the O-H bond dissociation energies.