One-electron transfer equilibria and redox potentials of radicals studied by pulse radiolysis

The pulse radiolysis technique is utilized for measurements of the equilibrium constants for electron transfer between the durosemiquinone radical anion and oxygen, menadione, and indigodisulfonate. These equilibrium constants are in turn used for calculations of one-electron redox potentials for these systems. Each of these equilibrium constants was determined experimentally and independently and found to be self-consistent. Only for the reactions of the semiquinone radical ions with oxygen could the electron transfer reaction be followed directly. For the reactions between the various quinone-semiquinone systems substantial indirect evidence is presented that these equilibria are achieved rapidly. In those cases equilibrium constants were determined from studies of the effect of quinone concentrations on the relative yields of the semiquinones. A method for distinguishing between kinetic competition and equilibrium is outlined and its usefulness is emphasized. The DQ parallel DQ- (DQ = duroquinone) and IDS parallel IDS- (IDS = indigodisulfonate) systems were employed as reference couples as the redox potentials for those systems are either available in the literature (IDS parallel IDS-) or may be calculated from available data (DQ parallel DQ-). Taking E71, the redox potential for the first one-electron reduction step at pH 7, of DQ parallel DQ- as -0.235 V or of IDS parallelIDS- as -0.247 V both yield E71 = -0.325 V for the O2 parallel O2- system (1 atm of O2) and E21 = -0.20 V for the menadione system. (U.S.)