Kinetics of the OH + ClOOCl and OH + Cl₂O Reactions: Experiment and Theory

The rate coefficients for the reactions OH + ClOOCl → HOCl + ClOO (eq ) and OH + Cl₂O → HOCl + ClO (eq ) were measured using a fast flow reactor coupled with molecular beam quadrupole mass spectrometry. OH was detected using resonance fluorescence at 309 nm. The measured Arrhenius expressions for these reactions are k₅ = (6.0 ± 3.5) × 10⁻¹³ exp((670 ± 230)/T) cm³ molecule⁻¹ s⁻¹ and k₆ = (5.1 ± 1.5) × 10⁻¹² exp((100 ± 92)/T) cm³ molecule⁻¹ s⁻¹, respectively, where the uncertainties are reported at the 2σ level. Investigation of the OH + ClOOCl potential energy surface using high level ab initio calculations indicates that the reaction occurs via a chlorine abstraction from ClOOCl by the OH radical. The lowest energy pathway is calculated to proceed through a weak ClOOCl−OH prereactive complex that is bound by 2.6 kcal mol⁻¹ and leads to ClOO and HOCl products. The transition state to product formation is calculated to be 0.59 kcal mol⁻¹ above the reactant energy level. Inclusion of the OH + ClOOCl rate data into an atmospheric model indicates that this reaction contributes more than 15% to ClOOCl loss during twilight conditions in the Arctic stratosphere. Reducing the rate of ClOOCl photolysis, as indicated by a recent re-examination of the ClOOCl UV absorption spectrum, increases the contribution of the OH + ClOOCl reaction to polar stratospheric loss of ClOOCl.