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

The bimolecular gas-phase reactions of the phenylethynyl radical (C₆H₅CC, X²A₁) with allene (H₂CCCH₂), allene-d₄ (D₂CCCD₂), and methylacetylene (CH₃CCH) were studied under single-collision conditions utilizing the crossed molecular beams technique and merged with electronic structure and statistical calculations. The phenylethynyl radical was found to add without an entrance barrier to the C1 carbon of the allene and methylacetylene reactants, resulting in doublet C₁₁H₉ collision complexes with lifetimes longer than their rotational periods. These intermediates underwent unimolecular decomposition via atomic hydrogen loss through tight exit transition states in facile radical addition─hydrogen atom elimination mechanisms forming predominantly 3,4-pentadien-1-yn-1-ylbenzene (C₆H₅CCCHCCH₂) and 1-phenyl-1,3-pentadiyne (C₆H₅CCCCCH₃) in overall exoergic reactions (−110 kJ mol^-1 and −130 kJ mol^-1) for the phenylethynyl-allene and phenylethynyl-methylacetylene systems, respectively. These barrierless reaction mechanisms mirror those of the ethynyl radical (C₂H, X²Σ⁺) with allene and methylacetylene forming predominantly ethynylallene (HCCCHCCH₂) and methyldiacetylene (HCCCCCH₃), respectively, suggesting that in the aforementioned reactions the phenyl group acts as a spectator. These molecular mass growth processes are accessible in low-temperature environments such as cold molecular clouds (TMC-1) or Saturn’s moon Titan, efficiently incorporating a benzene ring into unsaturated hydrocarbons.