Molecular Insights into the Spontaneous Generation of Cl 2 O in the Reaction of ClONO 2 and HOCl at the Air-Water Interface.

Chemical processes involving chlorine nitrate (ClONO 2 ) at the surface of stratospheric aerosols are crucial to ozone depletion. Herein, we show a reaction route for the formation of Cl 2 O, which is a source of stratospheric chlorine, in the ClONO 2 + HOCl reaction at the air-water interface. Our ab initio molecular dynamics (AIMD) simulations show that the (ClONO 2 )Cl···O(HOCl) halogen bond plays a key role in the reaction and is the main interaction between ClONO 2 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 2 , resulting in the formation of Cl 2 O and NO 3 - . Simultaneously, the remaining hydrogen atom is transferred to a water molecule to form H 3 O + . (ii) HOCl acts as a bridge for Cl atom transfer from ClONO 2 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 3 - , and H 3 O + . Free-energy calculations show that the former is the energetically more favorable process. More importantly, the free-energy barrier for Cl 2 O 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.