Anglada Josep M, Martins-Costa Marilia T C, Francisco Joseph S, Ruiz-López Manuel F
Departament de Química Biològica, Institut de Química Avançada de Catalunya (IQAC - CSIC), c/Jordi Girona 18, E-08034 Barcelona, Spain.
Laboratoire de Physique et Chimie Théoriques, UMR CNRS 7019, University of Lorraine, BP 70239, 54506 Vandoeuvre-lès-Nancy, France.
J Am Chem Soc. 2021 Jan 13;143(1):453-462. doi: 10.1021/jacs.0c11841. Epub 2020 Dec 23.
Recent experiments and theoretical calculations have shown that HNO may exist in molecular form in aqueous environments, where in principle one would expect this strong acid to be completely dissociated. Much effort has been devoted to understanding this fact, which has huge environmental relevance since nitric acid is a component of acid rain and also contributes to renoxification processes in the atmosphere. Although the importance of heterogeneous processes such as oxidation and photolysis have been evidenced by experiments, most theoretical studies on hydrated molecular HNO have focused on the acid dissociation mechanism. In the present work, we carry out calculations at various levels of theory to obtain insight into the properties of molecular nitric acid at the surface of liquid water (the air-water interface). Through multi-nanosecond combined quantum-classical molecular dynamics simulations, we analyze the interface affinity of nitric acid and provide an order of magnitude for its lifetime with regard to acid dissociation, which is close to the value deduced using thermodynamic data in the literature (∼0.3 ns). Moreover, we study the electronic absorption spectrum and calculate the rate constant for the photolytic process HNO + ν → NO + OH, leading to 2 × 10 s, about twice the value in the gas phase. Finally, we describe the reaction HNO + OH → NO + HO using a cluster model containing 21 water molecules with the help of high-level calculations. A large number of reaction paths are explored, and our study leads to the conclusion that the most favorable mechanism involves the formation of a pre-reactive complex (HNO)(OH) from which product are obtained through a coupled proton-electron transfer mechanism that has a free-energy barrier of 6.65 kcal·mol. Kinetic calculations predict a rate constant increase by ∼4 orders of magnitude relative to the gas phase, and we conclude that at the air-water interface, a lower limit for the rate constant is = 1.2 × 10 cm·molecule·s. The atmospheric significance of all these results is discussed.
最近的实验和理论计算表明，在水性环境中，HNO可能以分子形式存在，原则上人们会认为这种强酸会完全解离。人们已付出诸多努力来理解这一事实，鉴于硝酸是酸雨的成分之一且对大气中的再氧化过程也有贡献，这一事实具有重大的环境意义。尽管诸如氧化和光解等非均相过程的重要性已通过实验得到证实，但大多数关于水合分子HNO的理论研究都集中在酸解离机制上。在本工作中，我们在不同理论水平上进行计算，以深入了解液态水表面（气 - 水界面）的分子硝酸的性质。通过多纳秒的量子 - 经典联合分子动力学模拟，我们分析了硝酸的界面亲和力，并给出了其酸解离寿命的量级，该量级与文献中使用热力学数据推导的值（约0.3纳秒）相近。此外，我们研究了电子吸收光谱，并计算了光解过程HNO + ν → NO + OH的速率常数，得到2×10 s，约为气相值的两倍。最后，我们借助包含21个水分子的团簇模型，通过高水平计算描述了反应HNO + OH → NO + HO。我们探索了大量的反应路径，研究得出结论，最有利的机制涉及形成预反应复合物(HNO)(OH)，通过耦合质子 - 电子转移机制从该复合物中获得产物，该机制具有6.65千卡·摩尔的自由能垒。动力学计算预测速率常数相对于气相增加约4个数量级，我们得出结论，在气 - 水界面，速率常数的下限为 = 1.2×10 cm·分子·秒。我们还讨论了所有这些结果的大气意义。
