NEUTRAL-NEUTRAL REACTIONS IN THE INTERSTELLAR MEDIUM. I. FORMATION OF CARBON HYDRIDE RADICALS VIA REACTION OF CARBON ATOMS WITH UNSATURATED HYDROCARBONS

摘要

The reactions of ground-state atomic carbon with acetylene, C_2H_2, methylacetylene, CH_3CCH, ethylene, C_2H_4, and propylene, C_3H_6, 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 C_3H, as well as the hitherto unobserved interstellar radicals C_4H_3, C_3H_3, and C_4H_5. 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 or ring opening and decompose via C-H-bond rupture to l/c-C_3H, n-C_4H_3, pro-pargyl, and methylpropargyl. 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-C_3H correlates with actual astronomical observations and explains a higher [c-C_3H]/[l-C_3H] 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.