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, (1), methylacetylene, (2), C 2 H 2 CH 3 CCH ethylene, (3), and propylene, (4), are investigated at relative collision energies between 8.8 C 2 H 4 C 3 H 6 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 (1), as well as the hitherto unobserved inter-C 3 H stellar radicals (2), (3), and (4). All reactions proceed on the triplet surface via addition C 4 H 3 C 3 H 3 C 4 H 5 of the carbon atom to the molecular n-bond. The initial collision complexes undergo hydrogen migration (1/2) or ring opening (3/4) and decompose via C-H-bond rupture to (1), (2), pro-l/c-C 3 H n-C 4 H 3 pargyl (3), and methylpropargyl (4). The explicit identication of the carbon-hydrogen exchange channel under single collision conditions identies this class of reaction as a potential pathway to carbon-bearing species in the ISM. Especially, the formation of correlates with actual astronomical observations l/c-C 3 H and explains a higher [c-ratio in the dark cloud TMC-1 as compared to the carbon star C 3 H]/[l-C 3 H] IRC ]10216. Our ndings strongly demand the incorporation of distinct structural isomers in prospective chemical models of interstellar clouds, hot cores, and circumstellar envelopes around carbon stars.