Reduction of the naphthazarin molecule as studied by pulse radiolysis. Part 1.—Addition of a single electron

Abstract

Optical absorption characteristics have been determined for naphthazarin and the semiquinone radicals formed from it by one-electron reduction. The parent quinone has pK_a values of 7.85 and 10.7 and the semiquinone radicals have pK_a= 2.7 and 13.8. Second-order rate constants have been determined for the reactions of e^– _aq, CO^˙– ₂, H˙ and O^˙– ₂ with naphthazarin and also for the reaction of the semiquinone radicals with O₂. From the equilibrium reactions involving the quinone, the semiquinone and either oxygen, other quinones or other reference compounds, equilibrium constants and hence one-electron reduction potentials have been determined over the pH range 5–11. The one-electron reduction potential of naphthazarin is E¹ ₇=–110 mV at pH 7 (vs. NHE). This potential varies with pH in accordance with the pK_a values of the parent quinone and the semiquinone.