Reactions of methyl viologen dication (MV2+) with H atoms in aqueous solution:: Mechanism derived from pulse radiolysis measurements and ab initio MO calculations

The mechanistic details of a H atom (H-. radical) reaction with the methyl viologen (Paraquat) dication, MV2+, in aqueous solutions is presented, based on pulse radiolysis measurements in acid (HClO4) solution and supported by ab initio molecular orbital calculations. H atom-initiated reduction follows three different paths. About 76% of reactions follow the H atom addition to one of the ring C atom positioned meta to a N atom. The resulting cyclohexadienyl-type radical (MVHC.2+) shows an absorption peak at 350 nm (fwhm = 40 nm, epsilon = 4470 +/- 300 M-1 cm(-1)), and its second-order decay rate (2k(350nm) = 2.6 +/- 1.1 x 10(9) M-1 s(-1)) suggests a radical-radical disproportionation reaction. About 10% of H atom reactions proceed with abstraction of one H atom from a >N-CH3 group, resulting in the formation of a >N-CH2. radical. Subsequently, its transformation into the N-hydroxycyclohexadienyl-type radical (MVOHN.2+) in the presence of water is indicated. The MVOHN.2+ radical is exclusively generated during the (OH)-O-. radical reaction with MV2+. It shows two absorption peaks at 315 nm (fwhm = 45 nm, epsilon = 7400 400 M-1 cm(-1)) and 470 nm (fwhm = 60 nm, epsilon = 15 300 +/- 700 M-1 cm(-1)); a mixed-order radical decay in this case suggests a complex kinetics. The remaining H atom reaction proceeds with an H adduct formation at one of the two N atoms. The resulting MVHN.2+ radical shows a pK(a) value of -0.2 +/- 0.1 corresponding to the MVHN.2+ + H2O double left right arrow MV.+ + H3O+ equilibrium. Thus, in solution of pH > 1, deprotonation of the MVHN.2+ radical results in the appearance of the well-known intense blue MV.+ radical absorption signal (lambda(max) = 393 nm, epsilon = 42000 +/- 1200 M-1 cm(-1); lambda(max) = 605 nm, epsilon = 13 300 +/- 550 M-1 cm(-1)), also obtained directly during MV2+ reduction by e(aq)(-), CO2.-, etc. These results also reconfirm that a previous report of the pK(a) value of 2.9 for MVH.2+ adduct radical was merely a reflection of e(aq)(-) scavenging in acidic solution.