Absorption Spectra, pKa, and Reduction Potential of Phenoxyl Radical from 4,4‘-Biphenol in Aqueous Medium

The optical absorption spectra, p K a, and one-electron reduction potential ( E 1 ) of the phenoxyl radical of 4,4‘-biphenol (HOArArOH; Ar C 6 H 4 ) in aqueous solution are measured by the pulse radiolysis technique. At pH 4.8 the absorption spectrum of the HOArArO · radical shows two peaks at 385 nm (ε = 2223 m 2 mol -1 ) and 620 nm (ε = 867 m 2 mol -1 ). At pH 12.0, the respective peaks due to the - OArArO · radical appear at 445 nm (ε = 2850 m 2 mol -1 ) and 730 nm (ε =1680 m 2 mol -1 ). The radical p K a (HOArArO · + H 2 O ⇔ H 3 O + + - OArArO · ) is measured to be 7.5 ± 0.1. At pH 12, a reduction potential = 0.448 V vs NHE for the - OArArO ·,H + /HOArArO - couple is obtained from measurements against the Fe(CN) 6 3- /Fe(CN) 6 4- reference couple. Subsequently, from the parent p K a (9.4, 14.1) and the radical p K a, the reduction potential values in the pH range 0−14 are obtained. Some representative values (in V) are: 1.265 (at pH = 0), 0.805 (pH = 7.5), 0.616 (pH = 9.4), and 0.339 (pH >14). From the reduction potential value at pH = 0, the upper limit of its first O−H bond dissociation energy in the gas phase is estimated to be 357.7 kJ mol -1 .