Radiolytically-induced one-electron reduction of methyl viologen in aqueous solution: Stability of the radical cation in acidic and highly alkaline media (1)

Abstract

Pulse and continuous radiolysis have been used to investigate the stability of the reduced methyl viologen radical cation (MV +.) in acidic and highly alkaline aqueous solution. The reaction of the methyl viologen dication (MV 2+) with (CH 3) 2ĊOH and (CH 3) 2ĊO - radicals generates MV +. rapidly ( k = 2.9±0.2 × 10 9 and 6.7±0.3 × 10 9 M -1 s -1, respectively). The absorption spectrum of MV +. is the same at pH 1, natural pH, and pH 13 suggesting that MV +. is not involved in acid-base equilibria in that pH range. Between pH 0 and 2, MV +. disappears via second-order kinetics with k obs an inverse function of [MV 2+] and pH. The decay of MV +. occurs via H +-assisted disproportionation and yields ultimately a hydrogenated species with λ max 220, 255 nm (∈ max 8.2 × 10 3, 4.1 × 10 3 M -1cm -1, respectively). At pH 13, MV +. is infinitely stable in the absence of O 2; further reduction of MV +. yields the moderately stable MV° species with λ max 368 nm(∈ max3.6 × 10 4 M -1 cm -1) and a shoulder at 370 nm(∈3.0 × 10 4 M -1cm -1). Reaction of MV° with O 2 yields MV 2+ quantitatively via two one-electron oxidation steps. Acidification of MV° in the absence of O 2 yields the same air-insensitive hydrogenation product as is obtained from the disproportionation of MV +. in acidic solution. The relationship of these observations to the use of MV 2+ as an electron relay species in photochemical solar energy conversion schemes is examined.