Nitrate as a potential surrogate parameter for abatement of micropollutants, oxidant exposure, and characterizing oxidant-reactive DON during ozonation has attracted extensive attention, however, understanding of its formation mechanisms is still limited. In this study, nitrate formation mechanisms from amino acids (AAs) and amines during ozonation were investigated by the DFT method. The results indicate that N-ozonation initially occurs to produce competitive nitroso- and N,N-dihydroxy intermediates, and the former is preferred for both AAs and primary amines. Then, oxime and nitroalkane are generated during further ozonation, which are the important last intermediate products for nitrate formation from the respective AAs and amines. Moreover, the ozonation of the above important intermediates is the nitrate yield-controlling step, where the relatively higher reactivity of the CN moiety in the oxime compared to the general C_α atom in the nitroalkane explains why the nitrate yields of most AAs are higher than those from general amines, and it is the larger number of released C_α⁻ anions, which are the real reaction sites attacked by ozone, that leads to the higher nitrate yield for nitroalkane with an electron-withdrawing group bound to the C_α atom. The good relationship between nitrate yields and activation free energies of the rate-limiting step (ΔG^≠_rls) and nitrate yield-controlling step (ΔG^≠_nycs) for the respective AAs and amines verifies the reliability of the proposed mechanisms. Additionally, the bond dissociation energy of C_α–H in the nitroalkanes formed from amines was found to be a good parameter to evaluate the reactivity of the amines. The findings here are helpful for further understanding nitrate formation mechanisms and predicting nitrate precursors during ozonation.

中文翻译：

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臭氧化过程中游离氨基酸和胺的硝酸盐形成机制比较：计算研究

硝酸盐作为减少微污染物、氧化剂暴露和表征臭氧化过程中氧化剂反应性呕吐毒素的潜在替代参数引起了广泛关注，然而，对其形成机制的了解仍然有限。在这项研究中，通过 DFT 方法研究了臭氧化过程中氨基酸 (AAs) 和胺的硝酸盐形成机制。结果表明，N -臭氧化最初发生以产生竞争性亚硝基 - 和N，N-二羟基中间体，前者是氨基酸和伯胺的首选。然后，在进一步臭氧化过程中产生肟和硝基烷烃，它们是从相应的氨基酸和胺形成硝酸盐的重要的最后中间产物。_{此外，上述重要中间体的臭氧化是硝酸盐产率控制步骤，与硝基烷烃中的一般 C α}原子相比，肟中 C N 部分的反应性相对较高，这解释了为什么大多数氨基酸的硝酸盐产率较高比一般胺类释放的 C _α ⁻_{阴离子是受到臭氧攻击的真实反应位点，这导致硝基烷烃的硝酸盐产率更高，吸电子基团与 C α}原子结合。硝酸盐产率与限速步骤 (Δ G ^≠ _rls ) 和硝酸盐产率控制步骤 (Δ G ^≠ _nycs ) 对于各自的 AAs 和胺的活化自由能之间的良好关系验证了所提出机制的可靠性。_{此外，C α}的键解离能发现由胺形成的硝基烷烃中的 –H 是评估胺反应性的良好参数。这里的发现有助于进一步了解硝酸盐形成机制和预测臭氧化过程中的硝酸盐前体。