The time-resolved formation of OH from ethyl + O 2 and propyl + O 2 reactions has been measured by OH laser-induced fluorescence in pulsed-photolytic Cl-initiated oxidation of ethane and propane between 296 and 700 K. The propane oxidation produces more OH at each temperature than does ethane oxidation. Above 600 K, the peak amplitudes of the OH signals from both reactions increase sharply with increasing temperature. Solutions to the time-dependent master equation for the C 2 H 5 + O 2 , i-C 3 H 7 + O 2 , and n-C 3 H 7 + O 2 reactions, employing previously published ab initio characterizations of the stationary points of the systems, have been used to produce temperature-dependent parameterizations that predict the rate constants for formation of all of the products (R + O 2 , RO 2 , QOOH, OH + aldehydes, OH + O-heterocycles, HO 2 + alkene). These parameterizations are utilized in rate equation models to compare to experimental results for HO 2 and OH formation in Cl-initiated ethane and propane oxidation. The models accurately describe the time behavior and amplitude of the HO 2 from both oxidation systems. However, the model underpredicts the amount of OH observed at high temperatures (>600 K) and overpredicts the amount of OH observed at lower temperatures (e600 K).

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The production of HO2 in the reaction of ethyl radicals with molecular oxygen has been investigated using laser photolysis/cw infrared frequency modulation spectroscopy. The ethyl radicals are formed by reaction of photolytically produced Cl atoms with ethane, initiated via pulsed laser photolysis of Cl2, and the progress of the reaction is monitored by frequency-modulation spectroscopy of the HO2 product. The yield of HO2 in the reaction is measured by comparison with the Cl2/CH3OH/O2 system, which quantitatively converts Cl atoms to HO2. At low temperatures stabilization to C2H5O2 dominates, but at elevated temperatures (> 575 K) dissociation of the ethylperoxy radical begins to contribute. Biexponential time behavior of the HO2 production allows separation of prompt, “direct” HO2 formation from HO2 produced after thermal redissociation of an initial ethylperoxy adduct. The prompt HO2 yield exhibits a smooth increase with increasing temperature, but the total HO2 yield, which includes contributions from the redissociation of ethylperoxy radicals, rises sharply from ∼10% to 100% between 575 and 675 K. Because of the separation of time scales in the HO2 production, this rapid rise can unambiguously be assigned to ethylperoxy dissociation. No OH was observed in the reaction, and an upper limit of 6% can be placed on direct OH formation from the C2H5 + O2 reaction at 700 K. The time behavior of the HO2 production is at variance with the predictions of Wagner et al.'s RRKM-based parameterization of this reaction (J. Phys. Chem. 1990, 94, 1853). However, a simple ad hoc correction to that model, which takes into account a recent reinterpretation of the equilibrium constant for C2H5 + O2 ↔ C2H5O2, predicts yields and time constants consistent with the present measurements. The reaction mechanism is further discussed in terms of recent quantum chemical and master equation studies of this system, which show that the present results are well described by a coupled mechanism with HO2 + C2H4 formed by direct elimination from the C2H5O2 adduct.

The production of HO2 from the reaction of C3H7 and O2 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 Cl2/CH3OH/O2 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 ∼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 + O2 is similar to that previously observed in studies of the C2H5 + O2 reaction. The temperature dependence of the prompt yield for the two reactions is very similar, with the C3H7 + O2 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 + O2 is slightly larger than that for C2H5 + O2 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 + O2 and C3H7 + O2, respectively. These results suggest similar coupled mechanisms for HO2 production in the C2H5 + O2 and C3H7 + O2 reactions, with similar concerted HO2 elimination pathways from the RO2 species.

The time-resolved production of HO 2 and OH has been measured in pulsed-photolytic Cl-initiated oxidation of neopentane (2,2-dimethylpropane) between 573 and 750 K. The initial reaction in the oxidation process is the reaction of the neopentyl radical (R), formed by Cl + neopentane, with O 2 . The neopentylperoxy radical (RO 2 ) formed can then isomerize to a hydroperoxyalkyl radical (QOOH) and dissociate. The production of HO 2 in the neopentane oxidation is attributed to secondary reaction of the RO 2 or QOOH radicals, since the neopentylperoxy radical cannot form a conjugate alkene + HO 2 , and formation of 1,1dimethylcyclopropane + HO 2 is energetically inaccessible. Significant HO 2 formation is measured above 623 K, and the formation of HO 2 increases with increasing temperature. The overall amount of HO 2 produced increases with increasing O 2 at 673 K, consistent with the proposed role for QOOH + O 2 in chain branching for this system. A simple kinetic model is constructed based on comparison with previous time-dependent master equation calculations of analogous processes in the reaction of n-propyl with O 2 . The present experimental results require inclusion of formally direct pathways for several chemical activation reactions, especially direct production of OH from R + O 2 . Estimation of these direct components by analogy with n-propyl + O 2 is reasonably successful. The isomerization from RO 2 to QOOH is found to be significantly larger than previously proposed. A possible course towards a generally applicable theoretically-based model for alkyl radical oxidation is suggested.