Peroxynitrous acid: controversy and consensus surrounding an enigmatic oxidant

The isomerisation of ONOOH to NO3- 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 center dot and NO2 center dot to an extent of ca. 30%. We review the evidence pro and contra homolysis in studies that involve (1) thermochemical considerations, (2) isomerisation to NO3- and H+, (3) decomposition to NO2- and O-2, (4) HO center dot scavenger studies, (5) deuterium isotope effects, (6) O-18-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 NO2- 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.