On the acidity and reactivity of HNO in aqueous solution and biological systems

The gas phase and aqueous thermochemistry and reactivity of nitroxyl (nitro syl hydride, HNO) were elucidated with multiconfigurational self-consistent field and hybrid density functional theory calculations and continuum solv ation methods. The pK(a) of HNO is predicted to be 7.2 +/- 1.0, considerabl y different from the value of 4.7 reported from pulse radiolysis experiment s. The ground-state triplet nature of NO- affects the rates of acid-base ch emistry of the HNO/NO- couple. HNO is highly reactive toward dimerization a nd addition of soft nucleophiles but is predicted to undergo negligible hyd ration (K-eq = 6.9 x 10(-5)). HNO is predicted to exist as a discrete speci es in solution and is a viable participant in the chemical biology of nitri c oxide and derivatives.