Stopped-flow kinetic study of the formation and decay of the 4,4′-(dimethylamino)diphenylmethane radical cation in aqueous solution

Abstract

The oxidation kinetics of 4,4′-(dimethylamino)diphenylmethane (DMADPM) by Ce^IV, by the oxoanions, MnO₄^– and Cr₂O₇^2–, by peroxides, namely, peroxomonosulfate, peroxodisulfate and H₂O₂, and by halogens viz., Cl₂, Br₂ and I₂, to the radical cation, DMADPM˙⁺ along with further oxidation to the product monocation DMADPM⁺ have been studied by the stopped-flow technique. The first- and second-stage oxidations have been followed by monitoring the formation and decay of DMADPM˙⁺via the absorption at 610 nm. Both formation and decay of DMADPM˙⁺ obey total second-order kinetics, first-order each with respect to [DMADPM] or [DMADPM˙⁺] and [oxidant]. The effects of pH and temperature have also been investigated on the formation and decay of DMADPM˙⁺ and the kinetic and transition-state parameters have been evaluated and discussed with suitable reaction mechanisms. DMADPM˙⁺ was converted back into the DMADPM by the following reducing agents; ascorbic acid, dithionite, metabisulfite, sulfite and thiosulfate. The rate constants for these reactions were estimated. The experimentally determined rate constants for the oxidative electron-transfer reactions were correlated theoretically using Marcus theory and the observed and calculated rate constants show good agreement.