Hydrogen-atom attack on methyl viologen in aqueous solution studied by pulse radiolysis

Using hydrogen at high pressures of up to 150 bar as an OH scavenger in aqueous MV2+ solutions (pH 1) it is possible to differentiate between two kinds of transient formed simultaneously by H-atom attack on methyl viologen. One of them is assigned to an H adduct on the N atom, MV+H+, with absorption bands identical to those of the radical cation, MV+. The MV+H+ species deprotonates forming the long-lived radical cation, MV+. The second type of transient produced is attributed to an H-adduct on the ring carbon, MV2+H, decaying by second-order kinetics. The formation of MV+ by electron transfer from the propan-2-ol radical has been reinvestigated (pH 0 to 7); its absorption spectrum does not change in this pH range. Rate constants and molar extinction coefficients are presented. (U.K.)