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.