I5. Time-Resolved Infrared Probing of HO₂ Formation in Alkyl + O₂ Reactions

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Sandia National Laboratories
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The mechanisms of R + O2 reactions have been investigated by measuring the time-resolved production of HO2 in Cl-initiated oxidation reactions. The oxidation of ethane, propane, cyclopentane, i-butane, and n-butane have been studied using CW infrared frequency modulation spectroscopy between 298 and 700 K following pulsed photolysis of Cl2. The branching fraction to HO2 + alkene in the alkyl + O2 reactions displays a dramatic rise with increasing temperature between about 550 K and 700 K (the "transition region") which is accompanied by a qualitative change in the time behavior of the HO2 production. At low temperatures the HO2 is formed promptly; a second, slower production of HO2 is responsible for the bulk of the increased yield in the transition temperature region. This slower production is attributable to thermal dissociation of the alkylperoxy radical. The total HO2 yield in the butyl radical reactions appears to remain significantly below 1 up to 700 K, implying a significant role for OH-producing channels. The effective rate constants for the slower HO2 production in the Cl-initiated alkane oxidation reactions display similar apparent activation energies, suggesting that the energetics of the HO2 elimination transition state are similar for a broad range of R + O2 systems. The experimentally observed HO₂ production is explained in the framework of recent ab initio and master equation calculations.

This work is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, the Office of Basic Energy Sciences, the U. S. Department of Energy