Gas-Phase Study of the Elementary Reaction of the D1-Ethynyl Radical (CD; XΣ) with Propylene (CH; XA') under Single-Collision Conditions.

The bimolecular gas-phase reactions of the D1-ethynyl radical (CD; XΣ) with propylene (CH; XA') and partially substituted D3-3,3,3-propylene (CHCD; XA') were studied under single collision conditions utilizing the crossed molecular beams technique. Combining our laboratory data with electronic structure and statistical calculations, the D1-ethynyl radical is found to add without barrier to the C1 and C2 carbons of the propylene reactant, resulting in doublet CHD intermediate(s) with lifetime(s) longer than their rotational period(s). These intermediates undergo isomerization and unimolecular decomposition via atomic hydrogen loss through tight exit transition states forming predominantly /-3-penten-1-yne ((HCC)CH═CH(CH)) and, to a minor amount, 3-methyl-3-buten-1-yne ((HCC)C(CH)═CH) via overall exoergic reactions. Although the title reaction does not lead to the cyclopentadiene molecule (c-CH, XA), high-temperature environments can convert the identified acyclic CH isomers through hydrogen atom assisted isomerization to cyclopentadiene (c-CH, XA). Since both the ethynyl radical and propylene reactants have been observed in cold interstellar environments such as TMC-1 and the reaction is exoergic and all barriers lie below the energy of the separated reactants, these CH product isomers are predicted to form in those low-temperature regions.