Nitroxyl and its anion in aqueous solutions: Spin states, protic equilibria, and reactivities toward oxygen and nitric oxide

The thermodynamic properties of aqueous nitroxyl (HNO) and its anion (NO−) have been revised to show that the ground state of NO− is triplet and that HNO in its singlet ground state has much lower acidity, pKa(1HNO/3NO−) ≈ 11.4, than previously believed. These conclusions are in accord with the observed large differences between 1HNO and 3NO− in their reactivities toward O2 and NO. Laser flash photolysis was used to generate 1HNO and 3NO− by photochemical cleavage of trioxodinitrate (Angeli's anion). The spin-allowed addition of 3O2 to 3NO− produced peroxynitrite with nearly diffusion-controlled rate (k = 2.7 × 109 M−1⋅s−1). In contrast, the spin-forbidden addition of 3O2 to 1HNO was not detected (k ≪ 3 × 105 M−1⋅s−1). Both 1HNO and 3NO− reacted sequentially with two NO to generate N3O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{3}^{-}}}\end{equation*}\end{document} as a long-lived intermediate; the rate laws of N3O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{3}^{-}}}\end{equation*}\end{document} formation were linear in concentrations of NO and 1HNO (k = 5.8 × 106 M−1⋅s−1) or NO and 3NO− (k = 2.3 × 109 M−1⋅s−1). Catalysis by the hydroxide ion was observed for the reactions of 1HNO with both O2 and NO. This effect is explicable by a spin-forbidden deprotonation by OH− (k = 4.9 × 104 M−1⋅s−1) of the relatively unreactive 1HNO into the extremely reactive 3NO−. Dimerization of 1HNO to produce N2O occurred much more slowly (k = 8 × 106 M−1⋅s−1) than previously suggested. The implications of these results for evaluating the biological roles of nitroxyl are discussed.