The ever fascinating collaboration between spectroscopists and astrophysicists has led to the successful detection of over 250 different molecular species in the Interstellar Medium (ISM). These molecular species play significant roles in diverse fields such as atmospheric chemistry, astrochemistry, prebiotic chemistry, astrophysics, astronomy, astrobiology, etc, and in our understanding of the solar system ''the world around us''. Computational chemistry tools have been employed in different studies aimed at “Understanding some of the chemistries of the different classes of interstellar molecular species”. This presentation will focus on the interstellar isomers and interstellar linear carbon chains. Beginning with the interstellar isomeric species, a unique relationship; Energy, Stability and interstellar Abundance (ESA) is found to exist among these molecular species which addresses some of the bugging questions among the interstellar molecular species. The existence of Interstellar Hydrogen Bonding which is responsible for the deviations from thermodynamically controlled processes is reported. Accurate rotational constants within experimental accuracy of a few kHz have been determined for hundreds of linear interstellar carbon chain species. Thermodynamics is utilized in accounting for the known linear interstellar carbon chains and in examining the right candidates for astronomical searches. These carbon chains molecular species are shown to also obey the ESA relationship. In this presentation, some of these results will be highlighted.