A combined experimental and computational study on the reaction dynamics of the 1-propynyl radical (CHCC; XA) with ethylene (HCCH; XA) and the formation of 1-penten-3-yne (CHCHCCCH; XA').

The crossed molecular beam reactions of the 1-propynyl radical (CHCC; XA) with ethylene (HCCH; XA) and ethylene-d (DCCD; XA) were performed at collision energies of 31 kJ mol under single collision conditions. Combining our laboratory data with ab initio electronic structure and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations, we reveal that the reaction is initiated by the barrierless addition of the 1-propynyl radical to the π-electron density of the unsaturated hydrocarbon of ethylene leading to a doublet CH intermediate(s) with a life time(s) longer than the rotation period(s). The reaction eventually produces 1-penten-3-yne (p1) plus a hydrogen atom with an overall reaction exoergicity of 111 ± 16 kJ mol. About 35% of p1 originates from the initial collision complex followed by C-H bond rupture via a tight exit transition state located 22 kJ mol above the separated products. The collision complex (i1) can also undergo a [1,2] hydrogen atom shift to the CHCHCCCH intermediate (i2) prior to a hydrogen atom release; RRKM calculations suggest that this pathway contributes to about 65% of p1. In higher density environments such as in combustion flames and circumstellar envelopes of carbon stars close to the central star, 1-penten-3-yne (p1) may eventually form the cyclopentadiene (c-CH) isomer via hydrogen atom assisted isomerization followed by hydrogen abstraction to the cyclopentadienyl radical (c-CH) as an important pathway to key precursors to polycyclic aromatic hydrocarbons (PAHs) and to carbonaceous nanoparticles.