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

Abstract

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( 1 HNO/ 3 NO − ) ≈ 11.4, than previously believed. These conclusions are in accord with the observed large differences between 1 HNO and 3 NO − in their reactivities toward O 2 and NO. Laser flash photolysis was used to generate 1 HNO and 3 NO − by photochemical cleavage of trioxodinitrate (Angeli's anion). The spin-allowed addition of 3 O 2 to 3 NO − produced peroxynitrite with nearly diffusion-controlled rate ( k = 2.7 × 10 9 M −1 ⋅s −1 ). In contrast, the spin-forbidden addition of 3 O 2 to 1 HNO was not detected ( k ≪ 3 × 10 5 M −1 ⋅s −1 ). Both 1 HNO and 3 NO − reacted sequentially with two NO to generate N 3 O \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 N 3 O \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 1 HNO ( k = 5.8 × 10 6 M −1 ⋅s −1 ) or NO and 3 NO − ( k = 2.3 × 10 9 M −1 ⋅s −1 ). Catalysis by the hydroxide ion was observed for the reactions of 1 HNO with both O 2 and NO. This effect is explicable by a spin-forbidden deprotonation by OH − ( k = 4.9 × 10 4 M −1 ⋅s −1 ) of the relatively unreactive 1 HNO into the extremely reactive 3 NO − . Dimerization of 1 HNO to produce N 2 O occurred much more slowly ( k = 8 × 10 6 M −1 ⋅s −1 ) than previously suggested. The implications of these results for evaluating the biological roles of nitroxyl are discussed.