Neutral-Neutral Reactions in the Interstellar Medium I: Formation of Carbon Hydride Radicals Via Reaction of Carbon Atoms with Unsaturated Hydrocarbons

The reactions of ground-state atomic carbon with acetylene, C (1), methylacetylene, (2), 2H2 CH3CCH ethylene, C (3), and propylene, (4), are investigated at relative collision energies between 8.8 2H4 C3Hand 45 kJ mol~1 in crossed-beam experim6 ents 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 C (1), as well as the hitherto unobserved inter- 3H stellar radicals C (2), (3), and (4). All reactions proceed on the triplet surface via addition 4H3 C3H3 C4Hof the carbon atom to the molecular n-bon5d. The initial collision complexes undergo hydrogen migration (1/2) or ring opening (3/4) and decompose via C-H-bond rupture to l/c-C (1), (2), pro- 3H n-C4Hpargyl (3), and methylpropargyl (4). The explicit identiÐcation of the carbon-hydrogen exchang3e channel under single collision conditions identiÐes this class of reaction as a potential pathway to carbon-bearing species in the ISM. Especially, the formation of l/c-C correlates with actual astronomical observations 3H and explains a higher [c-C ratio in the dark cloud TMC-1 as compared to the carbon star 3H]/[l-C3H] IRC ]10216. Our Ðndings strongly demand the incorporation of distinct structural isomers in rospective chemical models of interstellar clouds, hot cores, and circumstellar envelopes around carbon stars.