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 CeIV, by the oxoanions, MnO4– and Cr2O72–, 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.