Elucidating the chemical dynamics of the elementary reactions of the 1-propynyl radical (CH3CC; X2A1) with 2-methylpropene ((CH3)2CCH2; X1A1)

Abstract

Exploiting the crossed molecular beam technique, we studied the reaction of the 1-propynyl radical (CH₃CC; X²A₁) with 2-methylpropene (isobutylene; (CH₃)₂CCH₂; X¹A₁) at a collision energy of 38 ± 3 kJ mol⁻¹. The experimental results along with ab initio and statistical calculations revealed that the reaction has no entrance barrier and proceeds via indirect scattering dynamics involving C₇H₁₁ 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 C₇H₁₁ intermediate formed from the C1 addition either emits atomic hydrogen or undergoes isomerization via [1,2-H] shift from the CH₃ or CH₂ 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) via 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 via the HAVA mechanism.