Molecular Insights into the Spontaneous Generation of Cl₂O in the Reaction of ClONO₂ and HOCl at the Air–Water Interface

Chemical processes involving chlorine nitrate (ClONO2) at the surface of stratospheric aerosols are crucial to ozone depletion. Herein, we show a reaction route for the formation of Cl2O, which is a source of stratospheric chlorine, in the ClONO2 + HOCl reaction at the air–water interface. Our ab initio molecular dynamics (AIMD) simulations show that the (ClONO2)Cl···O(HOCl) halogen bond plays a key role in the reaction and is the main interaction between ClONO2 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 ClONO2, resulting in the formation of Cl2O and NO3–. Simultaneously, the remaining hydrogen atom is transferred to a water molecule to form H3O+. (ii) HOCl acts as a bridge for Cl atom transfer from ClONO2 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, NO3–, and H3O+. Free-energy calculations show that the former is the energetically more favorable process. More importantly, the free-energy barrier for Cl2O 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