Neutral‐Neutral Reactions in the Interstellar Medium. I. Formation of Carbon Hydride Radicals via Reaction of Carbon Atoms with Unsaturated Hydrocarbons

Abstract

The reactions of ground-state atomic carbon with acetylene, C₂H₂ (1), methylacetylene, CH₃CCH (2), ethylene, C₂H₄ (3), and propylene, C₃H₆ (4), are investigated at relative collision energies between 8.8 and 45 kJ mol^-1 in crossed-beam experiments to elucidate the reaction products and chemical dynamics of atom-neutral encounters relevant to the formation of carbon-bearing molecules in the interstellar medium (ISM). Reactive scattering signal is found for C₃H (1), as well as the hitherto unobserved interstellar radicals C₄H₃ (2), C₃H₃ (3), and C₄H₅ (4). All reactions proceed on the triplet surface via addition of the carbon atom to the molecular π-bond. The initial collision complexes undergo hydrogen migration (1/2) or ring opening (3/4) and decompose via C-H-bond rupture to l/c-C₃H (1), n-C₄H₃ (2), propargyl (3), and methylpropargyl (4). The explicit identification of the carbon-hydrogen exchange channel under single collision conditions identifies this class of reaction as a potential pathway to carbon-bearing species in the ISM. Especially, the formation of l/c-C₃H correlates with actual astronomical observations and explains a higher [c-C₃H]/[l-C₃H] ratio in the dark cloud TMC-1 as compared to the carbon star IRC +10216. Our findings strongly demand the incorporation of distinct structural isomers in prospective chemical models of interstellar clouds, hot cores, and circumstellar envelopes around carbon stars.