A Combined Experimental and Computational Study on the Reaction Dynamics of the 1-Propynyl (CH₃CC)-Acetylene (HCCH) System and the Formation of Methyldiacetylene (CH₃CCCCH)

We investigated the 1-propynyl (CH₃CC; X²A₁) plus acetylene/acetylene- d₂ (HCCH/DCCD; X¹Σ_g⁺) under single-collision conditions using the crossed molecular beams method. The reaction was found to produce C₅H₄ plus atomic hydrogen (H) via an indirect reaction mechanism with a reaction energy of -123 ± 18 kJ mol^-1. Using the DCCD isotopologue, we confirmed that the hydrogen atom is lost from the acetylene reactant. Our computational analysis suggests the reaction proceeds by the barrierless addition of the 1-propynyl radical to acetylene, resulting in C₅H₅ intermediate(s) that dissociate preferentially to methyldiacetylene (CH₃CCCCH; X¹A₁) via hydrogen atom emission with a computed reaction energy of -123 ± 4 kJ mol^-1. The barrierless nature of this reaction scheme suggests the 1-propynyl radical may be a key intermediate in hydrocarbon chain growth in cold molecular clouds like TMC-1, where methyl-substituted (poly)acetylenes are known to exist.