Hydrogen-Atom Attack on Methyl Viologen in Aqueous Solution Studied by Pulse Radiolysis

Using hydrogen at high pressures of up to 150 bar (0.12 mol dm–3 H2) 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+(k= 3.1 × 108 dm3 mol–1 s–1), with absorption bands identical to those of the radical cation, MV˙+, but with ε392.5= 3200 m2 mol–1 and ε600= 1100 m2 mol–1. The MV˙+H+ species deprotonates with k= 2 × 104 s–1, forming the long-lived radical cation, MV˙+. The second type of transient produced, with k= 2.9 × 108 dm3 mol–1 s–1, is attributed to an H-adduct on the ring carbon, MV˙2+H, with ε310= 570 m2 mol–1 and ε470= 920 m2 mol–1, decaying by second-order kinetics with 2k=(6.0±1)× 108 dm3 mol–1 s–1. The formation of MV˙+ by electron transfer from the propan-2-ol radical has been reinvestigated (pH 0–7); its absorption spectrum does not change in this pH range.