Cyclic versus linear isomers produced by reaction of the methylidyne radical (CH) with small unsaturated hydrocarbons

Fabien Goulay; Adam J Trevitt; Giovanni Meloni; Talitha M Selby; David L Osborn; Craig A Taatjes; Luc Vereecken; Stephen R Leone

doi:10.1021/ja804200v

Cyclic versus linear isomers produced by reaction of the methylidyne radical (CH) with small unsaturated hydrocarbons

J Am Chem Soc. 2009 Jan 28;131(3):993-1005. doi: 10.1021/ja804200v.

Authors

Fabien Goulay¹, Adam J Trevitt, Giovanni Meloni, Talitha M Selby, David L Osborn, Craig A Taatjes, Luc Vereecken, Stephen R Leone

Affiliation

¹ Department of Chemistry, University of California, Berkeley, California 94720, USA. fgoulay@sandia.gov

PMID: 19123915
DOI: 10.1021/ja804200v

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 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.