Optimizing the searches for interstellar heterocycles

It is a fact that interstellar formation processes are thermodynamically affected. Based on this, the seven heterocycles; imidazole, pyridine, pyrimidine, pyrrole, quinoline, isoquinoline and furan that have been searched for from different astronomical sources with only upper limits of their column density determined without any successful detection remain the best candidates for astronomical observation with respect to their isomers. These molecules are believed to be formed on the surface of the interstellar dust grains and as such, they are susceptible to interstellar hydrogen bonding. In this study, a two way approach using ab initio quantum chemical simulations is considered in optimizing the searches for these molecules in interstellar medium. Firstly, these molecules and their isomers are subjected to the effect of interstellar hydrogen bonding. Secondly, the deuterated analogues of these heterocycles are examined for their possible detectability. From the results, all the heterocycles except furan are found to be strongly bonded to the surfaces of the interstellar dust grains thereby reducing their abundances, thus contributing to their unsuccessful detection. Successful detection of furan remains highly feasible. With respect to their D-analogues, the computed Boltzmann factor indicates that they are formed under the dense molecular cloud conditions where major deuterium fractionation dominates implying very high D/H ratio above the cosmic D/H ratio which suggests the detectability of these deuterated species. © 2021 COSPAR