Neutral‐Neutral Reactions in the Interstellar Medium. II. Isotope Effects in the Formation of Linear and Cyclic C 3 H and C 3 D Radicals in Interstellar Environments

Abstract

The energetics of the atom neutral reaction C(3Pj) + 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.