Product Formation in the Cl-Initiated Oxidation of Cyclopropane

The production of HO 2 and OH in the reaction of c -C 3 H 5 + O 2 is investigated as a function of temperature (296−700 K) using laser photolysis/CW infrared frequency modulation spectroscopy. The cyclopropyl radical is generated by the Cl + cyclopropane reaction following pulsed laser photolysis of Cl 2 . Significant OH and HO 2 production is observed at 296 K, and both [OH]/[Cl] 0 and [HO 2 ]/[Cl] 0 increase slowly with increased temperature until ∼600 K, where a sharper increase with temperature is observed. Relative rate and end product measurements are also performed using a smog chamber FTIR apparatus. The relative reactivity of cyclopropyl radicals toward O 2 and Cl 2 is k c- C 3 H 5 +O 2 / k c - C 3 H 5 +Cl 2 = 0.44 ± 0.02 at 700 Torr, 0.44 ± 0.03 at 75 Torr, and 0.24 ± 0.02 at 10 Torr of N 2 diluent at 296 K. Ethene and oxirane are identified as end products of the cyclopropane oxidation. Molar yields of oxirane are 0.11 ± 0.03 at 6 Torr, 0.08 ± 0.02 at 10 Torr, and 0.06 ± 0.02 at 50 Torr total pressure of N 2 /O 2 diluent; molar yields of ethene are 0.14 ± 0.02 (6 Torr) and 0.15 ± 0.01 (10 Torr) and 0.30 ± 0.06 (50 Torr). The combined experimental data suggest that HO 2 is not a primary product of the cyclopropyl + O 2 reaction but arises from secondary reactions of HCO or HCO 2 products formed in conjunction with oxirane or ethene. Quantum chemical calculations of stationary points on the cyclopropyl + O 2 surface indicate that ring opening and isomerization of c -C 3 H 5 O 2 to form a dioxirane species is possible, with a calculated transition state energy 0.5 kcal mol -1 above that of the reactants. This dioxirane species is a conceivable precursor to HCO 2 + ethene or HCO + oxirane formation; however, the calculations suggest OH + acrolein as the dominant bimolecular products.