Reactions of Methyl Viologen Dication (MV2+) with H Atoms in Aqueous Solution: Mechanism Derived from Pulse Radiolysis Measurements and ab Initio MO Calculations

Abstract

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, ε = 4470 ± 300 M-1 cm-1), and its second-order decay rate (2k350nm = 2.6 ± 1.1 × 109 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 radical reaction with MV2+. It shows two absorption peaks at 315 nm (fwhm = 45 nm, ε = 7400 ± 400 M-1 cm-1) and 470 nm (fwhm = 60 nm, ε = 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 pKa value of −0.2 ± 0.1 corresponding to the MVHN•2+ + H2O ⇔ 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 (λmax = 393 nm, ε = 42000 ± 1200 M-1 cm-1; λmax = 605 nm, ε = 13 300 ± 550 M-1 cm-1), also obtained directly during MV2+ reduction by eaq-, CO2•-, etc. These results also reconfirm that a previous report of the pKa value of 2.9 for MVH•2+ adduct radical was merely a reflection of eaq- scavenging in acidic solution.