title: Peroxynitrous acid: controversy and consensus surrounding an enigmatic oxidant.
abstract: The isomerisation of ONOOH to NO(3)(-) and H(+), some oxidations and all hydroxylations and nitrations of aromatic compounds are first-order in ONOOH and zero-order in the compounds that are modified. These reactions are widely believed to proceed via homolysis of ONOOH into HO˙ and NO(2)˙ to an extent of ca. 30%. We review the evidence pro and contra homolysis in studies that involve (1) thermochemical considerations, (2) isomerisation to NO(3)(-) and H(+), (3) decomposition to NO(2)(-) and O(2), (4) HO˙ scavenger studies, (5) deuterium isotope effects, (6) (18)O-scrambling studies, (7) electrochemistry, (8) CIDNP NMR, and (9) photolysis. Our conclusion is that homolysis may be involved to a minor extent of ca. 5%. The initiation of ONOOH isomerisation may be visualised as an out-of-plane vibration of the terminal HO-group relative to the nitrogen. At ONOO(-) concentrations exceeding 0.1 mM and near neutral pH, disproportionation to NO(2)(-) and O(2) occurs; such disproportionations are typical for peroxy acids. For oxidation and nitration of organic substrates, we favour a mechanism involving initial formation of an adduct between the compound to be oxidised or nitrated and ONOOH.
authors.affiliation: Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Switzerland. koppenol@inorg.chem.ethz.ch
authors.forename: Willem H
authors.initials: WH
authors.lastname: Koppenol
authors.forename: Patricia L
authors.initials: PL
authors.lastname: Bounds
authors.forename: Thomas
authors.initials: T
authors.lastname: Nauser
authors.forename: Reinhard
authors.initials: R
authors.lastname: Kissner
authors.forename: Heinz
authors.initials: H
authors.lastname: Rüegger
mesh_terms: D015394:Molecular Structure; D016877:Oxidants / Q000737:chemistry*; D030421:Peroxynitrous Acid / Q000737:chemistry*; D013237:Stereoisomerism; D013696:Temperature
chemical_list: D016877:Oxidants; D030421:Peroxynitrous Acid