Reactivity of the Ethenium Cation (C₂H₅⁺) with Ethyne (C₂H₂): A Combined Experimental and Theoretical Study

The gas-phase reaction between the ethyl cation (C₂H₅⁺) and ethyne (C₂H₂) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C₂H₅Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C₂H₅⁺, which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C₄H₇]⁺ PES and elucidate the underlying reaction mechanisms. Two main product channels have been identified at a centre-of-mass collision energy of ∼0.1 eV: (a) C₃H₃⁺+CH₄, with BR = 0.76±0.05 and (b) C₄H₅⁺+H₂, with BR = 0.22±0.02. A third channel giving C₂H₃⁺ in association with C₂H₄ is shown to emerge at both high internal excitation of C₂H₅⁺ and high collision energies. From CS measurements, energy-dependent total rate constants in the range 4.3×10-11-5.2×10-10 cm3·molecule-1·s-1 have been obtained. Theoretical calculations indicate that both channels stem from a common covalently bound intermediate, CH₃CH₂CHCH⁺, from which barrierless and exothermic pathways exist for the production of both cyclic c-C₃H₃⁺ and linear H₂CCCH⁺ isomers of the main product channel. For the minor C₄H₅⁺ product, two isomers are energetically accessible: the three-member cyclic isomer c-C₃H₂(CH₃)⁺ and the higher energy linear structure CH₂CHCCH₂⁺, but their formation requires multiple isomerization steps and passages via transition states lying only 0.11 eV below the reagents' energy, thus explaining the smaller BR. Results have implications for the modeling of hydrocarbon chemistry in the interstellar medium and the atmospheres of planets and satellites as well as in laboratory plasmas (e.g., plasma-enhanced chemical vapor deposition of carbon nanotubes and diamond-like carbon films).