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

• Published in: Physical chemistry chemical physics : PCCP Vol. 26; no. 7; pp. 6448 - 6457
• Main Authors: Medvedkov, Iakov A, Nikolayev, Anatoliy A, Yang, Zhenghai, Goettl, Shane J, Mebel, Alexander M, Kaiser, Ralf I
• Format: Journal Article
• Published: 14.02.2024
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d3cp05872g

Exploiting the crossed molecular beam technique, we studied the reaction of the 1-propynyl radical (CH 3 CC; X 2 A 1 ) with 2-methylpropene (isobutylene; (CH 3 ) 2 CCH 2 ; X 1 A 1 ) 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 C 7 H 11 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 -electron density at the C1 and/or C2 position in 2-methylpropene. Further, the C 7 H 11 intermediate formed from the C1 addition either emits atomic hydrogen or undergoes isomerization via [1,2-H] shift from the CH 3 or CH 2 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. Exploiting the crossed molecular beam technique, we studied the reaction of the 1-propynyl radical with 2-methylpropene (isobutylene) preferentially leading to trimethylvinylacetylene. This reaction is feasible in cold molecular clouds.