Journal of the Chemical Society - Faraday Transactions p. 1483 - 1491 (1990)
Update date:2022-08-11
Topics:
Mukherjee, Tulsi
Swallow, A. John
Guyan, Patricia M.
Bruce, J. Malcolm
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)2COH with the quinones, of the semiquinone 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.
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