Electron pulse radiolysis a t -298'K of 2 atm Hz containing 5 torr 0 2 produces HOz free radical whose disappearance by reaction (I), HOz + HOz -HzOz + 0 2 , is monitored by kinetic spectrophotometry a t 230.5 nm. Using a literature value for the HOz absorption cross section, the values k = 2.5 X cm3/molec.sec, which is in reasonable agreement with two earlier studies, and G(H) -< G(H02) N 13 are obtained. In the presence of small amounts of added HzO or NH3, the observed second-order decay rate of the HOz 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:l complexes, HOz + H20 s HOrHzO (4a) and HOz + NH3 F? HOrNH3 (4b), which are more reactive than uncomplexed HOz toward a second uncomplexed HOn radical. The following equilibrium constants, which agree with independent theoretical calculations on these complexes, are derived from the data: 2 X 5 Kqa 5 6.3 X cm3/molec at 295'K and Kqb = 3.4 X cm"/molec a t 298 O K . Several deuterium isotope effects are also reported, including ka/ko = 2.8 for reaction (1). The atmospheric significance of these results is pointed out.
The equality is rigorously true if experimental distribution ratios have been extrapolated to infinite dilution.(5) C. Tanford, "The Hydrophobic Effect", Wiley, New York, 1973. (6) One of the earliest is the demonstration of a unique CH2 group contribution to the heat capacity of transfer.
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