A combined experimental and computational study on the reaction dynamics of the 1-propynyl (CH₃CC, X²A₁)–propylene (CH₃CHCH₂, X¹A′) system: formation of 1,3-dimethylvinylacetylene (CH₃CCCHCHCH₃, X¹A′) under single collision conditions

The reaction of the 1-propynyl radical (CH₃CC; X²A₁) with propylene (CH₃CHCH₂; X¹A′) was studied in a crossed molecular beam machine at a collision energy of 37 ± 1 kJ mol⁻¹. Experimental data combined with high-level electronic structure (CCSD(T)-F12/cc-pVTZ-F12//ωB97X-D/6-311G(d,p)) and RRKM calculations reveal the reaction mechanism. The overall barrierless and exoergic reaction involves indirect reaction dynamics and commences preferentially with addition of 1-propynyl with its radical centre to the carbon–carbon double bond at the terminal carbon atom of propylene. This work focuses on molecular mass growth process (hydrogen loss channels) although theory suggests methyl loss as a prevalent channel. In these processes, the C₆H₉ collision complexes either emit atomic hydrogen or undergo isomerisation followed by atomic hydrogen loss to preferentially yield the cis/trans isomers of 1,3-dimethylvinylacetylene (2-hexen-4-yne) as the primary product. Analysis of reaction dynamics of 1-propynyl and ethynyl radicals with propylene along with their fractional abundance in deep space suggests formation of methyl- and dimethyl derivatives of vinylacetylene in cold molecular clouds. Once formed they may engage in fundamental molecular mass growth processes via the barrierless Hydrogen Abstraction Vinylacetylene Addition mechanism that leads to the formation of methyl- and dimethylnaphthalenes thus providing a versatile route to methyl-substituted PAHs in interstellar medium.