The Dependence on H₂O and on NH₃ of the Kinetics of the self‐reaction of HO₂ in the gas‐phase formation of HO₂·H₂O and HO₂·NH₃ complexes

Electron pulse radiolysis at ⋍298°K of 2 atm H 2 containing 5 torr O 2 produces HO 2 free radical whose disappearance by reaction (1), HO 2 + HO 2 →H 2 O 2 + O 2 , is monitored by kinetic spectrophotometry at 230.5 nm. Using a literature value for the HO 2 absorption cross section, the values k 1 = 2.5×10 −12 cm 3 /molec·sec, which is in reasonable agreement with two earlier studies, and G (H) G (HO 2 ) ⋍13 are obtained. In the presence of small amounts of added H 2 O or NH 3 , the observed second‐order decay rate of the HO 2 signal is found to increase by up to a factor of ⋍2.5. A proposed kinetic model quantitatively explains these data in terms of the formation of previously unpostulated 1:1 complexes, HO 2 + H 2 O ⇋ HO 2 ·H 2 O (4a) and HO 2 + NH 3 ⇋ HO 2 ·NH 3 (4b), which are more reactive than uncomplexed HO 2 toward a second uncomplexed HO 2 radical. The following equilibrium constants, which agree with independent theoretical calculations on these complexes, are derived from the data: 2×10 −20 ≲ K 4a ≲6.3 × 10 −19 cm 3 /molec at 295°K and K 4b = 3.4 × 10 −18 cm 3 /molec at 298°K. Several deuterium isotope effects are also reported, including k H / k D = 2.8 for reaction (1). The atmospheric significance of these results is pointed out.