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^• radical. H₂NO^• is a weak acid and deprotonates to form HNO^-•; the pK_a(H₂NO^•) value is 12.6 ± 0.3. Irrespective of the protonation state, the second-order recombination of the aminoxyl radical yields N₂ as the sole nitrogen-containing product. The following rate constants were determined:  k_r(2H₂NO^•) = 1.4 × 10⁸ M^-1 s^-1, k_r(H₂NO^• + HNO^-•) = 2.5 × 10⁹ M^-1 s^-1, and k_r(2HNO^-•) = 4.5 × 10⁸ M^-1 s^-1. The HNO^-• radical reacts with O₂ in an electron-transfer reaction to yield nitroxyl (HNO) and superoxide (O₂^-•), with a rate constant of k_e(HNO^-• + O₂ → HNO + O₂^-•) = 2.2 × 10⁸ M^-1 s^-1. Both O₂ and O₂^-• 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°(H₂NO^•,2H⁺/H₃NOH⁺) = 1.25 ± 0.01 V; E°(H₂NO^•,H⁺/H₂NOH) = 0.90 ± 0.01 V; and E°(H₂NO^•/H₂NO^-) = 0.09 ± 0.01 V. In addition, we estimate the following:  E°(H₂NOH^+•/H₂NOH) = 1.3 ± 0.1 V, E°(HNO, H⁺/H₂NO^•) = 0.52 ± 0.05 V, and E°(HNO/HNO^-•) = −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.