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.

Articles you may be interested inTemperature dependent kinetics of the formation and selfreactions of FC(O)O2 and FC(O)O radicals J. Chem. Phys. 98, 9522 (1993);

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Exploratory ab initio calculations using a minimal basis set support the existence of the 02• 20 and H02• NH3 complexes, as proposed earlier by experiment. For the minimum energy configurations having H02 as the H donor within a linear H-bond structure, electronic stabilization energies of 9.1 and 12.0 kcal/mol are calculated for H02-H20 and HC^-NHb, respectively, compared with 5.3 kcal/mol for (H20)2. Values of AH°a nd AS°for the complex formations are estimated and found consistent with available experimental data. A plausible model is proposed to explain the reactivity of these complexes. These calculations indicate that in the troposphere a significant fraction of the H02 is complexed with H20.

A kinetic and mechanistic study of the autoxidation of liquid pentaerythrityl tetraheptanoate (PETH) a t 180-22OOC has been carried out utilizing a stirred-flow reactor. The results are consistent with the occurrence of a chain reaction scheme similar to that proposed for n -hexadecane autoxidation, namely, the formation of monohydroperoxides by the intermolecular abstraction reaction (3), the formation of a,?-and cu,d-dihydroperoxides and a,?-and a,b-hydroperoxyketones by intramolecular peroxy radical abstraction reactions (4) and (4*), the bimolecular termination of peroxy radicals, reaction (61, and the rapid conversion of a,?-hydroperoxyketones to the corresponding cleavage acids and methyl ketones, reaction (7). Comparisons ofvarious rate parameters for the n-hexadecane and PETH systems reveal that the values of k7 and (k&l atom)/(2 hfi)1/2 are within experimental uncertainties identical for the two systems a t 180°C. The proposed reaction scheme includes the concurrent formation of hydroxy radicals and hydroperoxyketone species. The results of kinetic analysis and the experimentally observed isomer distributions of primary and secondary monohydroperoxide products a t high and low oxygen pressures suggest that ~6 0 % of the hydrogen abstractions from PETH a t high oxygen pressures occur by hydroxy radicals.