2021ApJ...921..132F

Reaction schema for the formation of the recently detected ethynyl cyclopropenylidene (c-C₃HCCH) molecule are currently lacking in the literature. The present quantum chemical study shows that the reaction of the abundant ethynyl (C₂H) and cyclopropenylidene (c-C₃H₂) molecules proceeds barrierlessly through the formation of a 1 ²A c-HC₃HCCH intermediate at 71.2 kcal mol^–1 below the reactants. The uphill exit channel climb for the C-H bond dissociation will compete with a transition state 45.9 kcal mol^–1 below the reactants to form another, lower-energy intermediate (1 ²A^′ c-C₃H₂CCH) 84.3 kcal mol^–1 below the reactants. The direct dissociation from the first intermediate is likely the dominant pathway under astrophysical conditions because the hydrogen leaving group can dissipate the energy kinetically. In either case of direct dissociation or crossing the transition state, the hydrogen atom dissociation to the final products lying 27.2 kcal mol^–1 below the starting materials requires a crossover of electronic states with the singly occupied molecular orbital moving from the π cloud of the cyclopropenyl group onto the leaving H atom. As a result, this C₂H + c-C₃H₂ - c-C₃HCCH + H reaction should be as fast as theorized in astrophysical media such as TMC-1, where the titular molecule has now been observed.