| Abstract
| - All major properties of the aqueous hyponitrite radicals (ONNO- and ONNOH), the adducts ofnitric oxide (NO) and nitroxyl (3NO- and 1HNO), are revised. In this work, the radicals are produced byoxidation of various hyponitrite species in the 2−14 pH range with the OH, N3, or SO4- radicals. Theestimated rate constants with OH are 4 × 107, 4.2 × 109, and 8.8 × 109 M-1 s-1 for oxidations of HONNOH,HONNO-, and ONNO2-, respectively. The rate constants for N3 + ONNO2- and SO4- + HONNO- are 1.1× 109 and 6.4 × 108 M-1 s-1, respectively. The ONNO- radical exhibits a strong characteristic absorptionspectrum with maxima at 280 and 420 nm (ε280 = 7.6 × 103 and ε420 = 1.2 × 103 M-1 cm-1). This spectrumdiffers drastically from those reported, suggesting the radical misassignment in prior work. The ONNOHradical is weakly acidic; its pKa of 5.5 is obtained from the spectral changes with pH. Both ONNO- andONNOH are shown to be over 3 orders of magnitude more stable with respect to elimination of NO thanit has been suggested previously. The aqueous thermodynamic properties of ONNO- and ONNOH radicalsare 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°(ONNO-/ONNO2-) = 0.96 V and E°(ONNOH, H+/HONNOH) = 1.75 V, and moderately reducing, E°(2NO/ONNO-) = −0.38 V and E°(2NO,H+/ONNOH) = −0.06 V, all vs NHE. Collectively, these properties make the hyponitrite radical an importantintermediate in the aqueous redox chemistry leading to or originating from nitric oxide.
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