Neutral-neutral Reactions in the Interstellar Medium. Ii. Isotope Effects in the Formation of Linear and Cyclic and Radicals

The energetics of the atom neutral reaction to form isomers are investiC(3P j ) ] 2 33 gated by ab initio calculations. In cold molecular clouds, the computed reaction endothermicity of 5.8 kJ mol~1 to yield the linear isomer indicates that the reaction energy cannot be covered by the reacl-C3H tantsÏ averaged translational energy of about 0.08 kJ mol~1. This suggests that only the deuterated linear isomer, could be formed in these environments. In strong contrast, reaction pathways to form the l-C3D, cyclic isomers and are both exothermic and hence could take place. These Ðndings should c-C3D c-C3H be considered in prospective astronomical surveys of the fractional abundances of versus l-C3H 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 outÑow of carbon stars, the reaction endothermicity to form could be provided by the enhanced averaged translational temperature of the reactants, and l-C3H the isotopic enrichment is expected to be less pronounced compared to colder interstellar environments. Subject headings : ISM: molecules È molecular processes