Kinetic and thermodynamic properties of the aminoxyl (NH2O•) radical

The product of one-electron oxidation of (or H-atom abstraction from) hydroxylamine is the H2NO center dot radical. H2NO center dot is a weak acid and deprotonates to form HNO-center dot; the pK(a)(H2NO center dot) value is 12.6 +/- 0.3. Irrespective of the protonation state, the second-order recombination of the aminoxyl radical yields N-2 as the sole nitrogen-containing product. The following rate constants were determined: k(r)(2H(2)NO(center dot)) = 1.4 x 10(8) M-1 s(-1), k(r)(H2NO center dot + HNO-center dot) = 2.5 x 10(9) M-1 s(-1), and k(r)(2HNO(-center dot)) = 4.5 x 10(8) M-1 s(-1). The HNO-center dot radical reacts with 02 in an electron-transfer reaction to yield nitroxyl (HNO) and superoxide (O-2(-center dot)), with a rate constant of k(e)(HNO-center dot + O-2 -> HNO + O-2(-center dot)) = 2.2 x 10(8) M-1 s(-1). Both O-2 and O-2(-center dot) seem to react with deprotonated hydroxylamine (H2NO-) to set up an autoxidative chain reaction. However, closer analysis indicates that these reactions might not occur directly but are probably mediated by transition-metal ions, even in the presence of chelators, such as ethylenediamine tetraacetic acid (EDTA) or diethylenetriamine pentaacetic acid (DTPA). The following standard aqueous reduction potentials were derived: E degrees(H2NO center dot,2H(+)/H3NOH+) = 1.25 +/- 0.01 V; E degrees(H2NO center dot,H+/H2NOH) = 0.90 +/- 0.01 V; and E degrees(H2NO center dot/H2NO-) = 0.09 +/- 0.01 V. In addition, we estimate the following: E degrees(H2NOH+center dot/H2NOH) = 1.3 +/- 0.1 V, E degrees(HNO, H+/H2NO center dot) = 0.52 +/- 0.05 V, and E degrees(HNO/HNO-center dot) = -0.22 +/- 0.05 V. From the data, we also estimate the gaseous O-H and N-H bond dissociation enthalpy (BDE) values in H2NOH, with BDE(H2NO-H) = 75-77 kcal/mol and BDE(H-NHOH) = 81-82 kcal/mol. These values are in good agreement with quantum chemical computations.