1999ApJ...510..784K

The energetics of the atom neutral reaction C(³P_j_) + C₂HD to form C₃D/C₃H isomers are investigated by ab initio calculations. In cold molecular clouds, the computed reaction endothermicity of 5.8 kJ.mol^–1 to yield the linear isomer l-C₃H indicates that the reaction energy cannot be covered by the reactants' averaged translational energy of about 0.08 kJ.mol^–1. This suggests that only the deuterated linear isomer, l-C₃D, could be formed in these environments. In strong contrast, reaction pathways to form the cyclic isomers c-C₃D and c-C₃H are both exothermic and hence could take place. These findings should be considered in prospective astronomical surveys of the fractional abundances of l-C₃H versus l-C₃D observed toward molecular clouds, since our results suggest an enhanced deuterium enrichment of the linear isomer versus the cyclic one. In a warmer outflow of carbon stars, the reaction endothermicity to form l-C₃H could be provided by the enhanced averaged translational temperature of the reactants, and the isotopic enrichment is expected to be less pronounced compared to colder interstellar environments.