Product formation in the Cl-initiated oxidation of cyclopropane

The production of HO2 and OH in the reaction Of c-C3H5 + 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 HO2 production is observed at 296 K, and both [OH]/[Cl](0) and [HO2]/[Cl](0) increase slowly with increased temperature until similar to600 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 kc-C3H5+O-2/kc-C3H5+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 Tort) and 0.15 +/- 0.01 (10 Torr) and 0.30 +/- 0.06 (50 Torr). The combined experimental data suggest that HO2 is not a primary product of the cyclopropyl + O-2 reaction but arises from secondary reactions of HCO or HCO2 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-C3H5O2 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 HCO2 + ethene or HCO + oxirane formation; however, the calculations suggest OH + acrolein as the dominant bimolecular products.