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

Abstract

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 (ε₄₇₀= 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 (ε₃₉₂= 4300 m² mol^–1, ε₆₀₅= 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 (ε₄₇₀= 2200±100 m² mol^–1).