keywords: Abundance,astrochemistry,heterocycles, interstellar medium
Because of their importance in biological systems, in our understanding of the solar system and in other applications, seven heterocycles; furan, imidazole, pyridine, pyrimidine, pyrrole, quinoline and isoquinoline have been astronomically searched for in different molecular clouds with only the upper limits in the range of 4*1012 to 2.8*1021cm-2 determined for their column densities in all the cases without any successful detection. Bothered by their unsuccessful detection, the energy, stability and abundance (ESA) relationship existing among interstellar molecules has been applied to examine if these heterocycles were the best candidates for astronomical searches in terms of interstellar abundance in relation to other isomers of each group. High level quantum chemical calculations have been used to determine accurate enthalpies of formation for 67 molecules from different isomeric groups of these heterocycles. From the results, all the 7 heterocycles so far searched are the best candidates for astronomically observations as they are molecules with the least enthalpies of formation in their respective groups and by extension, the most stable and probably the most abundant species in ISM. They remain the best possible molecules among other heterocycles to be observed soon with improved sensitivity of astronomical instruments, precise rest frequencies and proper choice of astronomical sources.
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