Mechanistic Insights into Chloric Acid Production by Hydrolysis of Chlorine Trioxide at an Air-Water Interface.

Chlorine oxides play crucial roles in ozone depletion, and the final oxidation steps of chlorine oxide potentially result in the formation of chloric acid (HClO 3 ) or perchloric acid (HClO 4 ). Herein, the solvation and reactive uptake of three stable isomers of chlorine trioxide (Cl 2 O 3 ), namely, ClOCl(O)O, ClClO 3 , and ClOOOCl, at the air-water interface were investigated using classical and hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) coupled with advanced free energy methods. Two distinct mechanisms were revealed for the hydrolysis of ClOCl(O)O and ClClO 3 : molecular and ionic mechanisms. A comparison of the computed free-energy profiles for the gaseous and air-water interfacial systems indicated that the air-water interface could markedly lower the free-energy barrier for ClO 3 - or HClO 3 formation while stabilizing the product state. In particular, the hydrolysis of ClClO 3 at the air-water interface was barrierless. In contrast, our calculations showed that the hydrolysis of ClOOOCl was very slow, indicating that ClOOOCl was inert to water at the air-water interface. This study provides theoretical evidence for the hypothesis that HClO 3 is a sink for chlorine oxides and for the widespread distributions of HClO 3 recently observed in the Arctic region.