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

Abstract

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 ε_392.5= 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.