Cyclic Versus Linear Isomers Produced by Reaction of the Methylidyne Radical (CH) with Small Unsaturated Hydrocarbons
Goulay, Fabien × Trevitt, Adam J Meloni, Giovanni Selby, Talitha M Osborn, David L Taatjes, Craig A Vereecken, Luc Leone, Stephen R #
Amer chemical soc
Journal of the American Chemical Society vol:131 issue:3 pages:993-1005
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 CHBr3 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 + C2H4 reaction the CA 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 C3H2 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 + C2H2 reaction presented in this manuscript predict a higher contribution of the Cyclic C3H2 (27.0%) versus triplet propargylene (63.5%) than earlier predictions. Extensive calculations on the CA and CAD system combined with experimental isotope ratios for the CD + C2H2 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.