Poskrebyshev Gregory A, Shafirovich Vladimir, Lymar Sergei V
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
J Am Chem Soc. 2004 Jan 28;126(3):891-9. doi: 10.1021/ja038042l.
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.
对亚硝酸根水合自由基（ONNO⁻ 和 ONNOH）以及一氧化氮（NO）与硝酰基（HNO⁻ 和 HNO）加合物的所有主要性质进行了修正。在这项工作中，通过在 2 - 14 的 pH 范围内用 OH、N₃ 或 SO₄⁻ 自由基氧化各种亚硝酸根物种来产生这些自由基。对于 HONNOH、HONNO⁻ 和 ONNO₂⁻ 的氧化反应，估计其与 OH 的反应速率常数分别为 4×10⁷、4.2×10⁹ 和 8.8×10⁹ M⁻¹ s⁻¹。N₃ 与 ONNO₂⁻ 以及 SO₄⁻ 与 HONNO⁻ 的反应速率常数分别为 1.1×10⁹ 和 6.4×10⁸ M⁻¹ s⁻¹。ONNO⁻ 自由基呈现出强烈的特征吸收光谱，在 280 和 420 nm 处有最大值（ε₂₈₀ = 7.6×10³ 和 ε₄₂₀ = 1.2×10³ M⁻¹ cm⁻¹）。该光谱与先前报道的光谱有很大差异，表明先前工作中自由基的归属有误。ONNOH 自由基呈弱酸性；其 pKa 为 5.5，是通过随 pH 变化的光谱得出的。结果表明，相对于 NO 的消除，ONNO⁻ 和 ONNOH 的稳定性都比先前认为的高出 3 个数量级以上。通过气相从头算计算、对 ONNOH 水合的合理估计及其 pKa，推导出了 ONNO⁻ 和 ONNOH 自由基的水相热力学性质。发现这些自由基既具有强氧化性，E°(ONNO⁻/ONNO₂⁻) = 0.96 V 和 E°(ONNOH, H⁺/HONNOH) = 1.75 V，又具有中等还原性，E°(2NO/ONNO⁻) = -0.38 V 和 E°(2NO, H⁺/ONNOH) = -0.06 V，均相对于标准氢电极（NHE）。总体而言，这些性质使亚硝酸根自由基成为水相氧化还原化学中导致或源自一氧化氮的重要中间体。
