Elucidating the chemical dynamics of the elementary reactions of the 1-propynyl radical (CHCC; XA) with 2-methylpropene ((CH)CCH; XA).

Exploiting the crossed molecular beam technique, we studied the reaction of the 1-propynyl radical (CHCC; XA) with 2-methylpropene (isobutylene; (CH)CCH; XA) at a collision energy of 38 ± 3 kJ mol. The experimental results along with and statistical calculations revealed that the reaction has no entrance barrier and proceeds indirect scattering dynamics involving CH intermediates with lifetimes longer than their rotation period(s). The reaction is initiated by the addition of the 1-propynyl radical with its radical center to the π-electron density at the C1 and/or C2 position in 2-methylpropene. Further, the CH intermediate formed from the C1 addition either emits atomic hydrogen or undergoes isomerization [1,2-H] shift from the CH or CH group prior to atomic hydrogen loss preferentially leading to 1,2,4-trimethylvinylacetylene (2-methylhex-2-en-4-yne) as the dominant product. The molecular structures of the collisional complexes promote hydrogen atom loss channels. RRKM results show that hydrogen elimination channels dominate in this reaction, with a branching ratio exceeding 70%. Since the reaction of the 1-propynyl radical with 2-methylpropene has no entrance barrier, is exoergic, and all transition states involved are located below the energy of the separated reactants, bimolecular collisions are feasible to form trimethylsubstituted 1,3-enyne (p1) a single collision event even at temperatures as low as 10 K prevailing in cold molecular clouds such as G+0.693. The formation of trimethylsubstituted vinylacetylene could serve as the starting point of fundamental molecular mass growth processes leading to di- and trimethylsubstituted naphthalenes the HAVA mechanism.