Combined Experimental and Computational Study on the Reaction Dynamics of the 1‑Propynyl (CH₃CC)–1,3-Butadiene (CH₂CHCHCH₂) System and the Formation of Toluene under Single Collision Conditions

The crossed beams reactions of the 1-propynyl radical (CH3CC; X2A1) with 1,3-butadiene (CH2CHCHCH2; X1Ag), 1,3-butadiene-d6 (CD2CDCDCD2; X1Ag), 1,3-butadiene-d4 (CD2CHCHCD2; X1Ag), and 1,3-butadiene-d2 (CH2CDCDCH2; X1Ag) were performed under single collision conditions at collision energies of about 40 kJ mol–1. The underlying reaction mechanisms were unraveled through the combination of the experimental data with electronic structure calculations at the CCSD­(T)-F12/cc-pVTZ-f12//B3LYP/6-311G­(d,p) + ZPE­(B3LYP/6-311G­(d,p) level of theory along with statistical Rice–Ramsperger–Kassel–Marcus (RRKM) calculations. Together, these data suggest the formation of the thermodynamically most stable C7H8 isomertoluene (C6H5CH3)via the barrierless addition of 1-propynyl to the 1,3-butadiene terminal carbon atom, forming a low-lying C7H9 intermediate that undergoes multiple isomerization steps resulting in cyclization and ultimately aromatization following hydrogen atom elimination. RRKM calculations predict that the thermodynamically less stable isomers 1,3-heptadien-5-yne, 5-methylene-1,3-cyclohexadiene, and 3-methylene-1-hexen-4-yne are also synthesized. Since the 1-propynyl radical may be present in cold molecular clouds such as TMC-1, this pathway could potentially serve as a carrier of the methyl group incorporating itself into methyl-substituted (poly)­acetylenes or aromatic systems such as toluene via overall exoergic reaction mechanisms that are uninhibited by an entrance barrier. Such pathways are a necessary alternative to existing high energy reactions leading to toluene that are formally closed in the cold regions of space and are an important step toward understanding the synthesis of polycyclic aromatic hydrocarbons (PAHs) in space’s harsh extremes