Nitrate anion (NO3-) is a ubiquitous species in aqueous phases in the environment, including atmospheric particles, aerosol droplets, surface waters, and snow. The photolysis of nitrate is a 'renoxification' process, which converts \nitrate solvated in water or deposited on surfaces back into NOx to the atmosphere. Nitrate photolysis under environmental conditions can follow two channels: (1) NO2 and O-; (2) nitrite and O. Despite the well-studied macroscopic kinetics of the two channels, the microscopic picture of the photolysis still needs to be explored. Furthermore, previous experiments have shown that nitrate photolysis in aqueous solutions has a low quantum yield of ~1% leading to a solvation cage effect hypothesis. A previous theoretical study has indicated that the low quantum yield may be due to the direct spin-forbidden absorption of \nitrate to its triplet state. Here, we employ first-principles molecular dynamics simulations at the level of hybrid DFT with enhanced sampling to explore the two channels in an aqueous solution to unravel the atomistic and electronic structure details of the photolysis, as well as investigate the causes of its low quantum yield under a solvation environment. The direct spin-forbidden absorption to T1 state is viable through spin-orbit coupling and is ~15 times weaker than the spin-allowed absorption to S1 state. A solvation cage complex is identified as a metastable state that requires additional thermal energy to complete the dissociation of the N-O bond at the triplet state. This metastable state allows the photo fragments to recombine or deactivate through non-radiative processes. Our simulations also qualitatively explain the temperature dependence of the two channels observed in experiments based on the rearrangement of H-bonds. This work provides a novel molecular picture illustrating the significantly low quantum yield and temperature dependence of nitrate photolysis under environmental conditions and a starting point for future studies of environmental nitrate photochemistry.

中文翻译：

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通过第一性原理模拟揭示水中硝酸盐的光化学途径


硝酸盐阴离子 （NO3-） 是环境中水相中普遍存在的物质，包括大气颗粒、气溶胶液滴、地表水和雪。硝酸盐的光解是一个“再氧化”过程，它将溶剂化在水中或沉积在表面的 \nit酸盐转化为 NOx 回到大气中。环境条件下的硝酸盐光解可以遵循两个通道：（1） NO2 和 O-;（2） 亚硝酸盐和 O。尽管对两个通道的宏观动力学进行了充分的研究，但光解的微观图像仍然需要探索。此外，以前的实验表明，硝酸盐在水溶液中的光解具有 ~1% 的低量子产率，导致溶剂化笼效应假设。先前的理论研究表明，低量子产率可能是由于 \nitrate 直接自旋禁止吸收到其三重态。在这里，我们在混合 DFT 水平上采用第一性原理分子动力学模拟和增强采样来探索水溶液中的两个通道，以揭示光解的原子和电子结构细节，并研究其在溶剂化环境下量子产率低的原因。通过自旋-轨道耦合，对 T1 状态的直接自旋禁止吸收是可行的，并且比对 S1 状态的自旋允许吸收弱 ~15 倍。溶剂化笼络合物被确定为一种亚稳态，需要额外的热能来完成三重态下 N-O 键的解离。这种亚稳态允许光碎片通过非辐射过程重新组合或失活。 我们的模拟还定性地解释了在基于 H 键重排的实验中观察到的两个通道的温度依赖性。这项工作提供了一张新的分子图，说明了在环境条件下硝酸盐光解的量子产率和温度依赖性显着降低，并为未来环境硝酸盐光化学研究提供了起点。