Hyponitrite radical, a stable adduct of nitric oxide and nitroxyl

All major properties of the aqueous hyponitrite radicals (ONNO- and ONNOH), the adducts of nitric oxide (NO) and nitroxyl (3NO- and 1HNO), are revised. In this work, the radicals are produced by oxidation of various hyponitrite species in the 2-14 pH range with the OH, N3, or SO4- radicals. The estimated rate constants with OH are 4 x 10(7), 4.2 x 10(9), and 8.8 x 10(9) M(-1) s(-1) for oxidations of HONNOH, HONNO-, and ONNO2-, respectively. The rate constants for N3 + ONNO2- and SO4- + HONNO- are 1.1 x 10(9) and 6.4 x 10(8) M(-1) s(-1), respectively. The ONNO- radical exhibits a strong characteristic absorption spectrum with maxima at 280 and 420 nm (epsilon280 = 7.6 x 10(3) and epsilon420 = 1.2 x 10(3) M(-1) cm(-1)). This spectrum differs drastically from those reported, suggesting the radical misassignment in prior work. The ONNOH radical is weakly acidic; its pKa of 5.5 is obtained from the spectral changes with pH. Both ONNO- and ONNOH are shown to be over 3 orders of magnitude more stable with respect to elimination of NO than it has been suggested previously. The aqueous thermodynamic properties of ONNO- and ONNOH radicals are derived by means of the gas-phase ab initio calculations, justified estimates for ONNOH hydration, and its pKa. The radicals are found to be both strongly oxidizing, E degrees (ONNO-/ONNO2-) = 0.96 V and E degrees (ONNOH, H+/HONNOH) = 1.75 V, and moderately reducing, E degrees (2NO/ONNO-) = -0.38 V and E degrees (2NO, H+/ONNOH) = -0.06 V, all vs NHE. Collectively, these properties make the hyponitrite radical an important intermediate in the aqueous redox chemistry leading to or originating from nitric oxide.