A Combined Crossed Molecular Beam and Theoretical Investigation of the Elementary Reaction of Tricarbon (C₃(X¹Σ_g⁺)) with Diacetylene (C₄H₂(X¹Σ_g⁺)): Gas Phase Formation of the Heptatriynylidyne Radical (l-C₇H(X²Π))

An elucidation of the underlying formation pathways to acyclic hydrocarbons such as polyynes (C_nH₂), cumulenes (C_nH₂), and linear resonantly stabilized linear radicals (l-C_nH) is indispensable to understand the hydrocarbon chemistry in extreme low- and high-temperature environments. In this study, we exploited the crossed molecular beam technique to investigate the reaction of tricarbon C₃(X¹Σ_g⁺) with diacetylene (butadiyne; HCCCCH; X¹Σ_g⁺) at a collision energy of 47 ± 1 kJ mol^-1. The experimental data were merged with ab initio calculations of the singlet C₇H₂ potential energy surface (PES) revealing that the reaction is initiated via the formation of an initial van der Waals reactant complex in the entrance channel. Subsequent rearrangements lead to various carbene-type and cyclic intermediates via ring-opening, ring-closure, and hydrogen migration processes, eventually forming acyclic C₇H₂ isomers prior to their barrierless unimolecular decomposition to the most stable linear isomer, heptatriynylidyne (C₇H, X²Π) in an overall endoergic reaction (+57 kJ mol^-1). The reaction exhibits strong similarities to the tricarbon-acetylene (C₃-C₂H₂). The significant energy threshold suggests that the tricarbon reaction with (poly)acetylenes forming resonantly stabilized linear radicals is open in high-temperature environments such as combustion flames and circumstellar envelopes of carbon stars and planetary nebulae as their descendants; however, these reactions are closed in low-temperature environments as in cold molecular clouds and hydrocarbon-rich atmospheres of planets and their moons such as in Titan.