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

Abstract

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 calculation that predicted linear triplet propargylene to be 90% of the total H-atom co-products. High-level CBS-APNO quantum calculations and RRKM calculation 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 these 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 discrepancy between the 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.3

Chemical dynamics, molecular energetics, and kinetics at the synchrotron

“…In the CH + C 2 H 2 reaction, the cyclic C 3 H 2 product is primarily observed, in contradiction to previous calculations, which suggested that linear triplet propargylene (HCCCH) accounts for 90% of the product formation. New calculations 59 indicate that a somewhat higher production of the cyclic product (27%) should be expected, however, from these calculations together with D atom isotope studies, it was shown that subsequent H-atom assisted isomerization of the triplet propargylene is most likely responsible for the additional large conversion to the observed c-C 3 H 2 . Cyclic isomers are also observed for the reaction of CH with methylacetylene.…”

Direct detection of pyridine formation by the reaction of CH (CD) with pyrrole: a ring expansion reaction

“…A small amount of N 2 is added to the main flow to quench any vibrationally excited CH radicals. 5 All flows are metered using calibrated mass flow controllers. In our experiment, the photolysis fluence is typically ~15 mJ/cm 2 in a ~20 ns pulse duration inside the flow tube.…”

Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection

“…When the ions are time-tagged on detection, secondary chemistry can be identified. 44 Comparing measured spectra to reference absolute ionization cross-sections for each individual isomer can ultimately provide product branching ratios.…”