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((1)HNO/(3)NO(−)) ≈ 11.4, than previously believed. These conclusions are in accord with the observed large differences between (1)HNO and (3)NO(−) in their reactivities toward O(2) and NO. Laser flash photolysis was used to generate (1)HNO and (3)NO(−) by photochemical cleavage of trioxodinitrate (Angeli's anion). The spin-allowed addition of (3)O(2) to (3)NO(−) produced peroxynitrite with nearly diffusion-controlled rate (k = 2.7 × 10(9) M(−1)⋅s(−1)). In contrast, the spin-forbidden addition of (3)O(2) to (1)HNO was not detected (k ≪ 3 × 10(5) M(−1)⋅s(−1)). Both (1)HNO and (3)NO(−) reacted sequentially with two NO to generate N(3)O [Formula: see text] as a long-lived intermediate; the rate laws of N(3)O [Formula: see text] formation were linear in concentrations of NO and (1)HNO (k = 5.8 × 10(6) M(−1)⋅s(−1)) or NO and (3)NO(−) (k = 2.3 × 10(9) M(−1)⋅s(−1)). Catalysis by the hydroxide ion was observed for the reactions of (1)HNO with both O(2) and NO. This effect is explicable by a spin-forbidden deprotonation by OH(−) (k = 4.9 × 10(4) M(−1)⋅s(−1)) of the relatively unreactive (1)HNO into the extremely reactive (3)NO(−). Dimerization of (1)HNO to produce N(2)O occurred much more slowly (k = 8 × 10(6) M(−1)⋅s(−1)) than previously suggested. The implications of these results for evaluating the biological roles of nitroxyl are discussed.