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; X1Σg+) 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.