Exploring the Chemical Dynamics of Phenylethynyl Radical (C6H5CC; X2A1) Reactions with Allene (H2CCCH2; X1A1) and Methylacetylene (CH3CCH; X1A1)

The bimolecular gas-phasereactions of the phenylethynyl radical(C6H5CC, X(2)A(1)) with allene(H2CCCH2), allene-d (4) (D2CCCD2), and methylacetylene (CH3CCH) were studied under single-collision conditions utilizing thecrossed molecular beams technique and merged with electronic structureand statistical calculations. The phenylethynyl radical was foundto add without an entrance barrier to the C1 carbon of the alleneand methylacetylene reactants, resulting in doublet C11H9 collision complexes with lifetimes longer than theirrotational periods. These intermediates underwent unimolecular decompositionvia atomic hydrogen loss through tight exit transition states in facileradical addition hydrogen atom elimination mechanisms formingpredominantly 3,4-pentadien-1-yn-1-ylbenzene (C6H5CCCHCCH2) and 1-phenyl-1,3-pentadiyne (C6H5CCCCCH3) in overall exoergic reactions (-110kJ mol(-1) and -130 kJ mol(-1)) for the phenylethynyl-allene and phenylethynyl-methylacetylenesystems, respectively. These barrierless reaction mechanisms mirrorthose of the ethynyl radical (C2H, X-2 & sigma;(+)) with allene and methylacetylene forming predominantly ethynylallene(HCCCHCCH2) and methyldiacetylene (HCCCCCH3),respectively, suggesting that in the aforementioned reactions thephenyl group acts as a spectator. These molecular mass growth processesare accessible in low-temperature environments such as cold molecularclouds (TMC-1) or Saturn's moon Titan, efficiently incorporatinga benzene ring into unsaturated hydrocarbons.