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

AUTOR(ES)

Shafirovich, Vladimir

FONTE

The National Academy of Sciences

RESUMO

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.

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