The Formation of Astromolecule Ethynyl Cyclopropenylidene (c-C3HCCH) from C2H and c-C3H2

Abstract

Abstract 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 2 A 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 2 A ′ 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.