Directed gas phase preparation of ethynylallene (H2CCCHCCH; X1A′) via the crossed molecular beam reaction of the methylidyne radical (CH; X2Π) with vinylacetylene (H2CCHCCH; X1A′)

Abstract

The gas-phase bimolecular reaction of the methylidyne (CH; X²Π) radical with vinylacetylene (H₂CCHCCH; X¹A′) was conducted at a collision energy of 20.3 kJ mol⁻¹ under single collision conditions exploiting the crossed molecular beam experimental results merged with ab initio electronic structure calculations and ab initio molecular dynamics (AIMD) simulations. The laboratory data reveal that the bimolecular reaction proceeds barrierlessly via indirect scattering dynamics through long-lived C₅H₅ reaction intermediate(s) ultimately dissociating to C₅H₄ isomers along with atomic hydrogen with the latter predominantly originating from the vinylacetylene reactant as confirmed by the isotopic substitution experiments in the D1-methylidyne–vinylacetylene reaction. Combined with ab initio calculations of the potential energy surface (PES) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) calculations, the experimental determined reaction energy of −146 ± 26 kJ mol⁻¹ along with the distribution minimum of T(θ) at 90° and isotopic substitution experiments suggest ethynylallene (p1; Δ_rG = −230 ± 4 kJ mol⁻¹) as the dominant product. The ethynylallene (p1) may be formed with extensive rovibrational excitation, which would result in a lower maximum translational energy. Further, AIMD simulations reveal that the reaction dynamics leads to p1 (ethynylallene, 75%) plus atomic hydrogen with the dominant initial complex being i1 formed by methylidyne radical addition to the double CC bond in vinylacetylene. Overall, combining the crossed molecular beam experimental results with ab initio electronic structure calculations and ab initio molecular dynamics (AIMD) simulations, ethynylallene (p1) is expected to represent the dominant product in the reaction of the methylidyne (CH; X²Π) radical with vinylacetylene (H₂CCHCCH; X¹A′).