The rate constant for the reaction OH+ClO →HO2+Cl (1a) GFIX→HCl+O2 (1b) has been measured in a discharge‐flow system with far infrared laser magnetic resonance detection of OH, ClO, and HO2. Measurements were made at low pressure (1.0–1.1 Torr) and at temperatures between 219 and 373 K under pseudo‐first‐order conditions. The measured rate constant is k1=(8.0±1.4)×1012 exp[(235±46)/T] cm3 molecule1 s1. The branching ratio at 293 K is, k1a/k1=0.86±0.14. The rate constant to channel (1a) was used with that for the reverse reaction to calculate the heat of formation of HO2. ΔH°f,298 (HO2)=3.0±0.4 kcal mol1.

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