Promoting Cl₂O Generation from the HOCl + HOCl Reaction on Aqueous/Frozen Air-Water Interfaces

Hypochlorous acid (HOCl) is considered a temporary reservoir of dichlorine monoxide (Cl₂O). Previous studies have suggested that Cl₂O is difficult to generate from the reaction of HOCl + HOCl in the gas phase. Here, we demonstrate that Cl₂O can be generated from the HOCl + HOCl reaction at aqueous/frozen air-water interfaces, which is confirmed by ab initio molecular dynamic calculations. Distinct from the one-step reaction in the gas phase, our results show that Cl₂O generation from HOCl + HOCl on aqueous/frozen interfaces involves two elementary steps, namely, one HOCl deprotonation and one Cl-abstraction from the other HOCl. Specifically, the mechanisms of neutral/acidic catalysis from interfacial water/nitric acid and base catalysis from ammonia, methylamine and dimethylamine have been examined. For the former, HOCl deprotonation is the rate-limiting step, and the total k of Cl₂O generation increases to 9.23 × 10^-9-9.10 × 10^-1 M^-1 s^-1 at the aqueous interface and 3.20 × 10^-7-4.10 × 10^-3 M^-1 s^-1 at the frozen interface, which is at least 23 and 25 orders of magnitude greater than that of gaseous k (3.31 × 10^-32 M^-1 s^-1). For the latter, the rate-limiting step is changed to Cl-abstraction, whose total k dramatically increases to 1.40-8.97 × 10⁷ M^-1 s^-1 at the aqueous interface and 7.12-9.99 × 10⁶ M^-1 s^-1 at the frozen interface. Interestingly, the Cl₂O production rates ranked in the order of dimethylamine < methylamine < ammonia and decreased with increasing catalytic alkalinity. These findings provide new insights for understanding other Cl₂O sources beyond the ClONO₂ + HOCl reaction.