Pressure effect on CH3 and C2H3 cross-radical reactions

The effect of pressure on the cross-radical reactions of vinyl and methyl r adicals has been investigated. These radicals were produced by excimer lase r photolysis of methyl vinyl ketone (CH3COC2H3) at 193 nm. The reaction pro ducts were detected and analyzed using a sensitive gas chromatograph and ma ss 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 c ross-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 indic ated that the contribution of the combination reaction C2H3 + CH3 + M --> C 3H6 + M to the total cross-radical reactions is reduced from 78% at high pr essures (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-hexadie ne, the allyl radical combination product. The observed 1,5-hexadiene is st rong evidence for allyl radical formation at low pressures, presumably from the decomposition of the chemically activated C3H6 Macroscopic and microsc opic modeling of product yields and their pressure dependencies were used t o interpret the experimental observations. Results of master equation calcu lations using weak colliders and RRKM theory are in agreement with the obse rved pressure dependence of the combination reactions. It has been shown th at the chemically activated species can undergo unimolecular processes that are competitive with collisional stabilization. The pressure dependence fo r the unimolecular steps appears as a pressure dependence of the combinatio n/disproportionation ratio. The apparent pathological behavior in this unsa turated system is attributed to the formation of a stronger C-C bond as con trasted to the weaker C-C bond formed from combination of saturated hydroca rbon radicals. This C-C bond strength is sufficiently high for the chemical ly activated propylene, produced from the methyl and vinyl cross-combinatio n reaction to cleave the allyl C-H bond or isomerize to cyclopropane.