Infrared Frequency-Modulation Probing of Product Formation in Alkyl + O2 Reactions: II. The Reaction of C3H7 with O2 between 296 and 683 K

The production of HO 2 from the reaction of C 3 H 7 and O 2 has been investigated as a function of temperature (296−683 K) using laser photolysis/CW infrared frequency-modulation spectroscopy. The HO 2 yield is derived by comparison with the Cl 2 /CH 3 OH/O 2 system and is corrected to account for HO 2 signal loss due to competing reactions involving HO 2 radical and the adduct C 3 H 7 O 2 . The time behavior of the HO 2 signal following propyl radical formation was observed to have two separate components. The first component is a prompt production of HO 2, which increases with temperature and is the only HO 2 production observed between 296 and 550 K. This prompt yield increases from less than 1% at 296 K to ∼16% at 683 K. At temperatures above 550 K, a second, slower rise in the HO 2 signal is also observed. The production of HO 2 on a slower time scale is attributable to propylperoxy radical decomposition. The total HO 2 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 HO 2 production from C 3 H 7 + O 2 is similar to that previously observed in studies of the C 2 H 5 + O 2 reaction. The temperature dependence of the prompt yield for the two reactions is very similar, with the C 3 H 7 + O 2 reaction having a slightly lower yield at each temperature. The temperature dependence of the total HO 2 yield is also very similar for the two reactions, with the sharp increase in the total HO 2 yield at high temperatures occurring in very similar temperature ranges. The phenomenological rate constant for delayed HO 2 production from C 3 H 7 + O 2 is slightly larger than that for C 2 H 5 + O 2 at each temperature. Apparent activation energies, obtained from an Arrhenius plot of the inverse of the time constants for delayed HO 2 production, are similar for the two systems, being 24.6 and 26.0 kcal mol -1 for C 2 H 5 + O 2 and C 3 H 7 + O 2, respectively. These results suggest similar coupled mechanisms for HO 2 production in the C 2 H 5 + O 2 and C 3 H 7 + O 2 reactions, with similar concerted HO 2 elimination pathways from the RO 2 species.