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. |