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(-)) approximately 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 x 10(9) M(-1) x s(-1)). In contrast, the spin-forbidden addition of (3)O(2) to (1)HNO was not detected (k << 3 x 10(5) M(-1) x s(-1)). Both (1)HNO and (3)NO(-) reacted sequentially with two NO to generate N(3)O as a long-lived intermediate; the rate laws of N(3)O formation were linear in concentrations of NO and (1)HNO (k = 5.8 x 10(6) M(-1) x s(-1)) or NO and (3)NO(-) (k = 2.3 x 10(9) M(-1) x 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 x 10(4) M(-1) x 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 x 10(6) M(-1) x s(-1)) than previously suggested. The implications of these results for evaluating the biological roles of nitroxyl are discussed.
Shafirovich
Vladimir
V
Chemistry Department and Radiation and Solid-State Laboratory, New York University, New York 10003, USA. vs5@nyu.edu
Lymar
Sergei V
SV
D006878:Hydroxides; D007700:Kinetics; D009569:Nitric Oxide / Q000737:chemistry*; D009589:Nitrogen Oxides / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D030421:Peroxynitrous Acid; D012996:Solutions; D014867:Water
D006878:Hydroxides; D009589:Nitrogen Oxides; D012996:Solutions; D014867:Water; D030421:Peroxynitrous Acid; D009569:Nitric Oxide; C031356:hydroxide ion; C039900:nitroxyl; D010100:Oxygen