Cyclic Versus Linear Isomers Produced by Reaction of the Methylidyne Radical (CH) with Small Unsaturated Hydrocarbons

The reactions of the methylidyne radical (CH) with ethylene, acetylene, allene, and methylacetylene are studied at room temperature using tunable vacuum ultraviolet (VUV) photoionization and time-resolved mass spectrometry. The CH radicals are prepared by 248 nm multiphoton photolysis of CHBr(3) at 298 K and react with the selected hydrocarbon in a helium gas flow. Analysis of photoionization efficiency versus VUV photon wavelength permits isomer-specific detection of the reaction products and allows estimation of the reaction product branching ratios. The reactions proceed by either CH insertion or addition followed by H atom elimination from the intermediate adduct. In the CH + C(2)H(4) reaction the C(3)H(5) intermediate decays by H atom loss to yield 70(+/-8)% allene, 30(+/-8)% methylacetylene, and less than 10% cyclopropene, in agreement with previous RRKM results. In the CH + acetylene reaction, detection of mainly the cyclic C(3)H(2) isomer is contrary to a previous RRKM calculations that predicted linear triplet propargylene to be 90% of the total H-atom coproducts. High-level CBS-APNO quantum calculations and RRKM calculations for the CH + C(2)H(2) reaction presented in this manuscript predict a higher contribution of the cyclic C(3)H(2) (27.0%) versus triplet propargylene (63.5%) than earlier predictions. Extensive calculations on the C(3)H(3) and C(3)H(2)D system combined with experimental isotope ratios for the CD + C(2)H(2) reaction indicate that H-atom-assisted isomerization in the present experiments is responsible for the remaining discrepancy between the new RRKM calculations and the experimental results. Cyclic isomers are also found to represent 30(+/-6)% of the detected products in the case of CH + methylacetylene, together with 33(+/-6)% 1,2,3-butatriene and 37(+/-6)% vinylacetylene. The CH + allene reaction gives 23(+/-5)% 1,2,3-butatriene and 77(+/-5)% vinylacetylene, whereas cyclic isomers are produced below the detection limit in this reaction. The reaction exit channels deduced by comparing the product distributions for the aforementioned reactions are discussed in detail.