Self‐Reaction of HO₂ and DO₂: Negative temperature dependence and pressure effects

The negative temperature dependence, pressure dependence, and isotope effects of the self‐reaction of HO 2 are modeled, using RRKM theory, by assuming that the reaction proceeds via a cyclic, hydrogen‐bonded intermediate. The negative temperature dependence is due to a tight transition state, with a negative threshold energy relative to reactants, for decomposition of the intermediate to products. A symmetric structure for this transition state reproduces the observed isotope effect. The weak pressure dependence for DO 2 self‐reaction is due to the approach to the high‐pressure limit. Addition of a polar collision partner, such as ammonia or water vapor, enhances the rate by forming an adduct that reacts to produce deexcited intermediate. A detailed model is presented to fit the data for these effects. Large ammonia concentrations should make it possible to reach the high‐pressure limit of the self‐reaction of HO 2 .