One- and two-electron reduction of quinizarin and 5-methoxyquinizarin: a pulse radiolysis study
Abstract
Absorption characteristics of the semiquinone free radicals formed by one-electron reduction of quinizarin (QH2), 5-methoxyquinizarin (MQH2) and quinizarin 2-sulphonate (QSH2) have been studied by pulse radiolysis in a mixed solvent system consisting of water, isopropyl alcohol and acetone. Second-order rate constants have been determined for the reactions of (CH3)2ĊOH with the quinones, of the semiquinones with O2 and of the semiquinones with each other. The one-electron reduction potentials (vs. NHE) are E17=–269 mV for QH2, –333 mV for MQH2 and –298 mV for QSH2. They vary with pH in accordance with the pKa values of the parent quinones and the semiquinones. The radicals are stable within the approximate pH range 5–11. The stability constant is highest at pH 8.5 (Ks≈ 0.09) for QH2, at pH ≈ 9.5 for QSH2(Ks≈ 10) and pH ≈ 10.8 for MQH2(Ks≈ 4.8), respectively. The one-electron reduction potentials of the semiquinones and the two-electron reduction potentials of the quinones are calculated to be E27=–188, –192 and –216 mV, and Em7=–229, –263 and –257 mV for QH2, MQH2 and QSH2, respectively. The effect of solvent on the properties of the semiquinones is discussed.