One-Electron Reduction of Aqueous Nitric Oxide:  A Mechanistic Revision

• Published in: Inorganic chemistry 2005-07, Vol.44 (15), p.5212-5221
• Main Authors: Lymar, Sergei V, Shafirovich, Vladimir, Poskrebyshev, Gregory A
• Format: Article
• Language: English
• Subjects: Electrons; Nitric Oxide - chemistry; Oxidation-Reduction; Pulse Radiolysis - methods; Time Factors; Water - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0501317

The pulse radiolysis of aqueous NO has been reinvestigated, the variances with the prior studies are discussed, and a mechanistic revision is suggested. Both the hydrated electron and the hydrogen atom reduce NO to yield the ground-state triplet 3NO- and singlet 1HNO, respectively, which further react with NO to produce the N2O2 - radical, albeit with the very different specific rates, k(3NO- + NO) = (3.0 ± 0.8) × 109 and k(1HNO + NO) = (5.8 ± 0.2) × 106 M-1 s-1. These reactions occur much more rapidly than the spin-forbidden acid−base equilibration of 3NO- and 1HNO under all experimentally accessible conditions. As a result, 3NO- and 1HNO give rise to two reaction pathways that are well separated in time but lead to the same intermediates and products. The N2O2 - radical extremely rapidly acquires another NO, k(N2O2 - + NO) = (5.4 ± 1.4) × 109 M-1 s-1, producing the closed-shell N3O3 - anion, which unimolecularly decays to the final N2O + NO2 - products with a rate constant of ∼300 s-1. Contrary to the previous belief, N2O2 - is stable with respect to NO elimination, and so is N3O3 -. The optical spectra of all intermediates have also been reevaluated. The only intermediate whose spectrum can be cleanly observed in the pulse radiolysis experiments is the N3O3 - anion (λmax = 380 nm, εmax = 3.76 × 103 M-1 cm-1). The spectra previously assigned to the NO- anion and to the N2O2 - radical are due, in fact, to a mixture of species (mainly N2O2 - and N3O3 -) and to the N3O3 - anion, respectively. Spectral and kinetic evidence suggests that the same reactions occur when 3NO- and 1HNO are generated by photolysis of the monoprotonated anion of Angeli's salt, HN2O3 -, in NO-containing solutions.