Radiolytically-induced one-electron reduction of methyl viologen in aqueous solution

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 (MV2+) with (CH3)2ĊOH and (CH3)2ĊO- radicals generates MV+. rapidly (k = 2.9±0.2 × 109 and 6.7±0.3 × 109 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 kobs an inverse function of [MV2+] and pH. The decay of MV+. occurs via H+-assisted disproportionation and yields ultimately a hydrogenated species with λmax 220, 255 nm (∈max 8.2 × 103, 4.1 × 103 M-1cm-1, respectively). At pH 13, MV+. is infinitely stable in the absence of O2; further reduction of MV+. yields the moderately stable MV° species with λmax 368 nm(∈max3.6 × 104 M-1 cm-1) and a shoulder at 370 nm(∈3.0 × 104 M-1cm-1). Reaction of MV° with O2 yields MV2+ quantitatively via two one-electron oxidation steps. Acidification of MV° in the absence of O2 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 MV2+ as an electron relay species in photochemical solar energy conversion schemes is examined.