Homolysis peroxynitrous acid

The second proposal is a bit more imaginative and arises from the above arguments that 0—0 bond homolysis is much too slow to be involved in oxidations by peroxynitrate. Pryor and coworkers invoked the intermediacy of a metastable form of peroxynitrous acid (HO—ONO ) in equilibrium with its ground state. This so-called excited state of peroxynitrous acid has, to date, eluded detection or characterization by the experimental community. However, recent high-level theoretical calculations by Bach and his collaborators have presented plausible evidence for the intermediacy of such a shortlived species with a highly elongated 0—0 bond and have confirmed its involvement in the oxidation of hydrocarbons (see below). The discovery of this novel series of biologically important oxidants has fostered a new area of research in both the experimental and theoretical communities. In this chapter we will describe many of the more pertinent theoretical studies on both the physical properties and chemical reactivity of peroxynitrous acid. 

Density functional theory has also been recently applied to several dissociative pathways of HO—ONO and its anion (ONO—0 ) in aqueous solution. For example, the Gibbs free energy for the homolysis of peroxynitrous acid (HO—ONO HO -h ONO) is calculated to be AG (aq.) = 11.1 kcalmol" , a value in good agreement with experiment (13.6 kcalmoD ). For peroxynitrite homolysis (ONO—O NO2 -b O ) the calculated AG (aq.) = 13.0 kcalmoD was obtained for ion-molecule complexes with watef ... 

While it is well established that HO—ONO can be involved in such two-electron processes as alkene epoxidation and the oxidation of amines, sulfides and phosphines, the controversy remains concerning the mechanism of HO-ONO oxidation of saturated hydrocarbons. Rank and coworkers advanced the hypothesis that the reactive species in hydrocarbon oxidations by peroxynitrous acid, and in lipid peroxidation in the presence of air, is the discrete hydroxyl radical formed in the homolysis of HO—ONO. The HO—ONO oxidation of methane (equation 7) on the restricted surface with the B3LYP and QCISD methods gave about the same activation energy (31 3 kcalmol" ) irrespective of basis set size . ... 

Yang, G., Candy, T., Boaro, M., Wilkin, H., Jones, P., Nazhat, N., Saadalla-Nazhat, R., and Blake, D. (1992). Free radical yields for the homolysis of peroxynitrous acid. Free Radical Biol. Med. 12, 327-330. 

The first-order reaction rate constant for the isomerization of peroxynitrous acid to nitrate is 4.5 s 1 at 37°C therefore, at pH 7.4 and at 37°C the half-life of the peroxynitrite/peroxynitrous acid couple (let both these species be referred to as peroxynitrite for the sake of brevity) is less than 1 s. The reaction mechanism of peroxynitrite decomposition was a subject of controversy. Primarily proposed was that peroxynitrous acid decomposes by homolysis, producing two strong oxidants hydroxyl radical and nitrous dioxide (B15) ... 

Peroxynitrite is converted to its protonated form, peroxynitrous acid (ONOOH) with a pRa of 6.8. This acid form undergoes isomerization to form nitrate anion with a rate constant of 1.3 s [Eq. (12)]. Peroxynitrite is a powerful oxidant and during oxidation reactions with substrates, it undergoes homolysis, generating two powerful oxidants OH and NO2 radicals (-28%) [Eq. (13)], which can mediate oxidation, nitration and nitrosation reactions leading to impaired function of biomolecules. 

The oxidation of thiophene (Th) by peroxynitrous acid (HOONO), generated in the H2O2-HNO2 system, proceeds simultaneously in both the gaseous and the liquid phases of the reactor. The conclusion that the active oxidant in both cases is the HO radical, formed by HO-ONO bond homolysis, is based on the equality of the rate constants of the reaction in the two phases. ... 

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