Peroxynitric acid

Peroxynitric acid (H02N02) is a relatively unstable species that is important as a reservoir for N02 at lower temperatures via the reversible reaction (16, -16)  

There are a variety of potential photodissociation pathways, including the following  

TABLE 4.14 Recommended Absorption Cross Sections (Base e) for Gaseous H02N02u 

At 248 nrn, the quantum yield for OH is 0.34 + 0.16, and the formation of electronically excited NO with yields of 30% was observed (MacLeod et al., 1988). Reaction (17a), where some of the NO is formed in an electronically excited state, and reaction (17e), which is the most direct path to OH, are thought to be the important paths. In the absence of additional studies, MacLeod et al. (1988) recommend for application to atmospheric situations 4 n.a = 0.65 and f l7c = 0.35. 

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Peroxyacids Peroxydisulphuric acid Peroxynitric acid Peroxy ditungstic acid... 

The plot of In A, versus time gives k = 3.76 s 1. The pattern may seem to suggest that the reaction occurs with fast formation of the peroxynitrous acid intermediate (k = 3.76 s-1) and slower consumption of it (k = 0.0854 s-1). This is not necessarily so, however, as will be considered in Section 4.2. Rather than committing oneself as to... 

Kikugawa, K., Hiramoto, K., Tomiyama, S., and Asano, Y. 1997. P-Carotene effectively scavenges toxic nitrogen oxides Nitrogen dioxide and peroxynitrous acid. FEBS Lett. 404 175-178. 

At physiological pH, ONOO- protonates to peroxynitrous acid (ONOOH) which disappears within a few seconds, the end product being largely nitrate. The chemistry of peroxynitrite/peroxynitrous acid is extremely complex, although addition of ONOO to cells and tissues leads to oxidation and nitration of proteins, DNA and lipids with a reactivity that is comparable to that of hydroxyl radicals. 

In the last 10 to 15 years, many experimental and theoretical studies have been dedicated to the study of peroxynitrite reactions. Free radical and non-free radical mechanisms of peroxynitrite action have been proposed, which were discussed in numerous studies (see for example, Refs. [103-110]). In accord with non-radical mechanism an activated form of peroxynitrous acid is formed in the reaction of superoxide with nitric oxide, which is able to react with biomolecules without the decomposition to HO and N02 radicals. 

It should be noted that Reaction (4) is not a one-stage process.) Both free radical N02 and highly reactive peroxynitrite are the initiators of lipid peroxidation although the elementary stages of initiation by these compounds are not fully understood. (Crow et al. [45] suggested that trans-ONOO is protonated into trans peroxynitrous acid, which is isomerized into the unstable cis form. The latter is easily decomposed to form hydroxyl radical.) Another possible mechanism of prooxidant activity of nitric oxide is the modification of unsaturated fatty acids and lipids through the formation of active nitrated lipid derivatives. 

The gas-phase stability of a variety of possible products—such as the olefin ozonide, peroxyformyl nitrate, and peroxynitric acid—is not known. 

Pemitric (or peroxynitric) acid is an example of a compound that could be present in significant quantities. This proposal is very speculative, because there is no evidence of this compound in the gas phase, although there is evidence of some such species in solutions.The reaction of hydroperoxy radical with nitrogen dioxide is usually written... 

Peroxynitrous acid, which has an estimated lifetime of 1-3 s at neutral pH, has been studied through ab initio calculations that suggest that peroxynitrous acid, per-oxyformic acid, and dimethyldioxirane have, despite diverse 0—0 bond energies, similar activation energies for oxygen-atom transfer." The transition-state structures for the epoxidation of ethene and propene with peroxynitrous acid are symmetrical with equal or almost equal bond distances between the spiro oxygen and the carbons of the double bond. 

Irradiation of gaseous formaldehyde containing an excess of nitrogen dioxide over chlorine yielded ozone, carbon monoxide, nitrogen pentoxide, nitryl chloride, nitric and hydrochloric acids. Peroxynitric acid was the major photolysis product when chlorine concentration exceeded the nitrogen dioxide concentration (Hanst and Gay, 1977). Formaldehyde also reacts with NO3 in the atmosphere at a rate of 3.2 x 10 cmVmolecule-sec (Atkinson and Lloyd, 1984). 

Peroxyacyl nitrates, see Acetaldehyde, Butane, 2-Bntanone, 2,3-Dimethylbntane Peroxybenzoic acid, see Toluene Peroxynitric acid, see Formaldehyde Peroxypropionyl nitrate, see 2-Methylpentane, Pentane Phenanthrene, see Anthracene, Bis(2-ethylhexyl) phthalate, Naphthalene Phenanthrene-9,10-dione, see Phenanthrene 9,10-Phenanthrenequinone, see Phenanthrene 4-Phenanthroic acid, see Pyrene... 

Despite a considerable literature on the various modes of reactions induced by peroxynitrite, the kinetic and mechanistic aspects of these transformations have been clarified only recently (Nonoyama et al. 1999). The authors give the following picture of the peroxynitrite chemical behavior. In alkaline solutions, peroxynitrite is a stable anionic species. At physiological pH, it is rapidly protonated to form peroxynitrous acid (ONOOH) ONOO -I- H ONOOH. 

Reactants Peroxynitrous acid (20.3) Peroxyformic acid (46.9) DMDO... 

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. 

B. Peroxynitrite Anion and Peroxynitrous Acid. Ground State Properties... 

The different rotational isomers of peroxynitrous acid ONO OH are labeled by their respective dihedral angles (<0N0 0) and ([Pg.8]

C. Higher-lying Metastable States 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 ... 

The similarity of the structure of peroxynitrous acid to the simplest peroxy acid, per-oxyformic acid, immediately raised the question as to its relative reactivity as an oxygen atom donor. This became particularly relevant when it was recognized that the 0—0 bond dissociation energy (AG° = 21 kcalmoR ) of HO—ONO was much lower than that of more typical peroxides. Consequently, peroxynitrous acid (HO-ONO) can be both a one- and two-electron oxidant. Since the 0-0 bond in HO-ONO is so labile, its chemistry is also consistent in many cases with that of the free hydroxyl radical. 

The relatively short half-life of peroxynitrous acid hampers experimental smdies of its reactions as an oxidizing agent. Therefore, theoretical methods can play a particularly useful role in describing the chemistry of peroxynitrous acid. For comparison of the HO-ONO fragment geometries found in transition structures, the geometric parameters of HO-ONO at various levels of theory are presented in Figure 3. 

For practical reasons the two-electron oxidations of such prototypical substrates as alkenes, sulfides, amines and phosphines with peroxynitrous acid have been smdied since this type of oxygen atom transfer reaction is similar to the corresponding oxidations with peroxyformic acid, where both experimental and computational data are available The oxidative reactions (equations 3-6) of these key substrates can... 

FIGURE 3. The geometry of the lowest-energy cis,cis) conformation of peroxynitrous acid calculated at B3LYP/6-31H-G(3df,2p), QCISD/6-31G (bold numbers), CISD/6-31G (in parentheses), B3LYP/6-311G (in square brackets) and MP2(fuU)/6-31G (in curly brackets) computational levels... 

FIGURE 4. Transition structures for the oxidation of tiimethylamine (a), trimethylphosphine (b) and dimethyl sulfide (c,d) with peroxynitrous acid optimized at the B3LYP/6-311G, MP2(full)/6-31G (in parentheses) and QCISD/6-31G (in brackets) levels... 

FIGURE 5. B3LYP/6-31 lG(d,p) optimized intermediates and the transition stmcture for the reaction of ebselen with peroxynitrous acid. For details see Reference 40a... 

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