Wan Zhengyi, Zhu Chongqin, Francisco Joseph S
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.
College of Chemistry, Key Laboratory of Theoretical & Computational Photochemistry of Ministry of Education, Beijing Normal University, Beijing 100190, People's Republic of China.
J Am Chem Soc. 2023 Aug 9;145(31):17478-17484. doi: 10.1021/jacs.3c06527. Epub 2023 Jul 31.
Chemical processes involving chlorine nitrate (ClONO) at the surface of stratospheric aerosols are crucial to ozone depletion. Herein, we show a reaction route for the formation of ClO, which is a source of stratospheric chlorine, in the ClONO + HOCl reaction at the air-water interface. Our molecular dynamics (AIMD) simulations show that the (ClONO)Cl···O(HOCl) halogen bond plays a key role in the reaction and is the main interaction between ClONO and HOCl both at the air-water interface and in the bulk liquid water. Furthermore, metadynamics-based AIMD simulations reveal two pathways: (i) The OCl fragment of HOCl binds to the Cl atom in ClONO, resulting in the formation of ClO and NO. Simultaneously, the remaining hydrogen atom is transferred to a water molecule to form HO. (ii) HOCl acts as a bridge for Cl atom transfer from ClONO to the O atom of a water molecule, and this water molecule transfers one of its H atoms to another water molecule, forming two HOCl molecules, NO, and HO. Free-energy calculations show that the former is the energetically more favorable process. More importantly, the free-energy barrier for ClO formation at the air-water interface is only ∼0.8 kcal/mol, and the reaction is exothermic. These findings provide insights into the importance of fundamental chlorine chemistry and the broader implications of the aerosol air-water interface for atmospheric chemistry.
平流层气溶胶表面涉及硝酸氯（ClONO₂）的化学过程对臭氧消耗至关重要。在此，我们展示了在气-水界面处ClONO₂ + HOCl反应中形成ClO（平流层氯的一个来源）的反应途径。我们的从头算分子动力学（AIMD）模拟表明，(ClONO₂)Cl···O(HOCl)卤键在该反应中起关键作用，并且是ClONO₂与HOCl在气-水界面以及本体液态水中的主要相互作用。此外，基于元动力学的AIMD模拟揭示了两条途径：（i）HOCl的OCl片段与ClONO₂中的Cl原子结合，导致形成ClO和NO。同时，剩余的氢原子转移到一个水分子上形成HO。（ii）HOCl作为Cl原子从ClONO₂转移到一个水分子的O原子的桥梁，并且这个水分子将其一个H原子转移到另一个水分子上，形成两个HOCl分子、NO和HO。自由能计算表明，前者在能量上更有利。更重要的是，在气-水界面处形成ClO的自由能垒仅约为0.8千卡/摩尔，并且该反应是放热的。这些发现为基础氯化学的重要性以及气溶胶气-水界面在大气化学中的更广泛影响提供了见解。
