Pressure Effect on CH3 and C2H3 Cross-Radical Reactions

Abstract

The effect of pressure on the cross-radical reactions of vinyl and methyl radicals has been investigated. These radicals were produced by excimer laser photolysis of methyl vinyl ketone (CH3COC2H3) at 193 nm. The reaction products were detected and analyzed using a sensitive gas chromatograph and mass spectrometer. The study covered a pressure range from about 0.28 kPa (2.1 Torr) to 27 kPa (200 Torr) at 298 K. The yield of propylene (C3H6), the cross-combination product of methyl and vinyl radicals, was compared to the yield of ethane (C2H6), the methyl radical combination product. At 27 kPa [C3H6]/[C2H6] = 1.28 was derived. This ratio was reduced to about 0.75 when the pressure was reduced to about 0.28 kPa. Kinetic modeling results indicated that the contribution of the combination reaction C2H3 + CH3 + M → C3H6 + M to the total cross-radical reactions is reduced from 78% at high pressures (27 kPa) to about 39% at low pressures (0.28 kPa). At low pressures an additional reaction channel, C2H3 + CH3 → C3H5 + H, becomes available, producing a host of allyl radical reaction products including 1,5-hexadiene, the allyl radical combination product. The observed 1,5-hexadiene is strong evidence for allyl radical formation at low pressures, presumably from the decomposition of the chemically activated C3H6. Macroscopic and microscopic modeling of product yields and their pressure dependencies were used to interpret the experimental observations. Results of master equation calculations using weak colliders and RRKM theory are in agreement with the observed pressure dependence of the combination reactions. It has been shown that the chemically activated species can undergo unimolecular processes that are competitive with collisional stabilization. The pressure dependence for the unimolecular steps appears as a pressure dependence of the combination/disproportionation ratio. The apparent pathological behavior in this unsaturated system is attributed to the formation of a stronger C−C bond as contrasted to the weaker C−C bond formed from combination of saturated hydrocarbon radicals. This C−C bond strength is sufficiently high for the chemically activated propylene, produced from the methyl and vinyl cross-combination reaction to cleave the allyl C−H bond or isomerize to cyclopropane.