Nitroxyl and its anion in aqueous solutions: Spin states, protic equilibria, and reactivities toward oxygen and nitric oxide

The thermodynamic properties of aqueous nitroxyl (HNO) and its anion (NO-) have been revised to show that the ground state of NO- is triplet and that HNO in its singlet ground state has much lower acidity, pKa((HNO)-H-1/(NO-)-N-3) approximate to 11.4, than previously believed. These conclusions are in accord with the observed large differences between (HNO)-H-1 and (NO-)-N-3 in their reactivities toward O-2 and NO. Laser flash photolysis was used to generate (HNO)-H-1 and (NO-)-N-3 by photochemical cleavage of trioxodinitrate (Angeli's anion). The spin-allowed addition of O-3(2) to (NO-)-N-3 produced peroxynitrite with nearly diffusion-controlled rate (k = 2.7 x 10(9) M(-1.)s(-1)). In contrast, the spin-forbidden addition of O-3(2) to (HNO)-H-1 was not detected (k much less than 3 x 10(5) M(-1.)s(-1)). Both (HNO)-H-1 and (NO-)-N-3 reacted sequentially with two NO to generate N3O3- as along-lived intermediate; the rate laws of N3O3- formation were linear in concentrations of NO and (HNO)-H-1 (k = 5.8 x 10(6) M(-1.)s(-1)) or NO and (NO-)-N-3 (k = 2.3 x 10(9) M(-1.)s(-1)). Catalysis by the hydroxide ion was observed for the reactions of (HNO)-H-1 with both O-2 and NO. This effect is explicable by a spin-forbidden deprotonation by OH- (k = 4.9 x 10(4) M(-1.)s(-1)) of the relatively unreactive (HNO)-H-1 into the extremely reactive (NO-)-N-3. Dimerization of (HNO)-H-1 to produce N2O occurred much more slowly (k = 8 x 10(6) M(-1.)s(-1)) than previously suggested. The implications of these results for evaluating the biological roles of nitroxyl are discussed.