Infrared frequency-modulation probing of product formation in alkyl plus O2 reactions:: II.: The reaction of C3H7 with O2 between 296 and 683 K

The production of HO2 from the reaction of C3H7 and O-2 has been investigated as a function of temperature (296-683 K) using laser photolysis/CW infrared frequency-modulation spectroscopy. The HO2 yield is derived by comparison with the Cl-2/CH3OH/O-2 system and is corrected to account for HO2 Signal loss due to competing reactions involving HO2 radical and the adduct C3H7O2 The time behavior of the HO2 signal following propyl radical formation was observed to have two separate components. The first component is a prompt production of HO2, which increases with temperature and is the only HO2 production observed between 296 and 550 K. This prompt yield increases from less than 1% at 296 K to similar to 16% at 683 K. At temperatures above 550 K, a second, slower rise in the HO2 signal is also observed. The production of HO2 on a slower time scale is attributable to propylperoxy radical decomposition. The total HO2 yield, including the contribution from the slower rise, increases rapidly with temperature from 5% at 500 K to 100% at 683 K. The second slower rise accounts for nearly all of the product formation at these higher temperatures. The biexponential time behavior of the HO2 production from C3H7 + O-2 is similar to that previously observed in studies of the C2H5 + O-2 reaction. The temperature dependence of the prompt yield for the two reactions is very similar, with the C3H7 + O-2 reaction having a slightly lower yield at each temperature. The temperature dependence of the total HO2 yield is also very similar for the two reactions, with the sharp increase in the total HO2 yield at high temperatures occurring in very similar temperature ranges. The phenomenological rate constant for delayed HO2 production from C3H7 + O-2 is Slightly larger than that for C2H5 + O-2 at each temperature. Apparent activation energies, obtained from an Arrhenius plot of the inverse of the time Constants for delayed HO2 production are similar for the two systems, being 24.6 and 26.0 kcal mol(-1) for C2H5 + O-2 and C3H7 + O-2 respectively. These results suggest similar coupled mechanisms for HO2 production in the C2H5 + O-2 and C3H7 + O-2 reactions, with similar concerted HO2 elimination pathways from the RO2 species.