Is ESA Relationship The Tool In Searching For Interstellar Heterocycles?
  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 the 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.
