Reactivity of OH and O– with aqueous methyl viologen studied by pulse radiolysis

The behaviour of aqueous MV2+ towards oxidizing radicals (OH and O–) has been investigated in the pH range from 6 to 14 by means of pulse radiolysis. A semi-linear optimization method was applied for resolving the complex reaction mechanism. In the pH range from 6 to 8 the rate constant for attack by OH is k=(2.5±0.2)× 108 dm3 mol–1 s–1. The resulting transient absorbs at λ max= 470 nm (ε 470= 1600±70 m2 mol–1) and decays with 2k=(1.3±0.2)× 108 dm3 mol–1 s–1. In strongly alkaline solutions (pH ⩾ 13.8) the O– radical anion reacts preferentially by hydrogen abstraction from the methyl group, k=(1.4±0.2)× 109 dm3 mol–1 s–1, forming a radical which then decays by reaction with OH–(k= 2.8 × 106 dm3 mol–1 s–1) to produce a modified radical cation; this has absorption maxima at 392 and 605 nm (ε 392= 4300 m2 mol–1, ε 605= 1500 m2 mol–1) and is relatively long lived. The remaining part (<10%) of O– attacks MV2+ at the ring carbon atom, k=(1.0±0.4)× 108 dm3 mol–1 s–1, resulting in an O– adduct, which has λ max= 470 nm (ε 470= 2200±100 m2 mol–1).