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

Publisher Website
Google Scholar
Abstract
Electron pulse radiolysis at ⋍298°K of 2 atm H2 containing 5 torr O2 produces HO2 free radical whose disappearance by reaction (1), HO2 + HO2 →H2O2 + O2, is monitored by kinetic spectrophotometry at 230.5 nm. Using a literature value for the HO2 absorption cross section, the values k1 = 2.5×10−12 cm3/molec·sec, which is in reasonable agreement with two earlier studies, and G(H) G(HO2) ⋍13 are obtained. In the presence of small amounts of added H2O or NH3, the observed second‐order decay rate of the HO2 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, HO2 + H2O ⇋ HO2·H2O (4a) and HO2 + NH3⇋ HO2·NH3 (4b), which are more reactive than uncomplexed HO2 toward a second uncomplexed HO2 radical. The following equilibrium constants, which agree with independent theoretical calculations on these complexes, are derived from the data: 2×10−20≲K4a≲6.3 × 10−19 cm3/molec at 295°K and K4b = 3.4 × 10−18 cm3/molec at 298°K. Several deuterium isotope effects are also reported, including kH/kD = 2.8 for reaction (1). The atmospheric significance of these results is pointed out.
Keywords