| Abstract : | Over 270 molecular species have been confirmed in the interstellar medium (ISM), approximately 40% of which are more than five atoms defined as Complex Organic Molecules (COMs), reflecting interstellar molecular complexity. The chemical network connecting the formation and depletion of hundreds of these molecular species consists of thousands of possible reactive channels. Investigating the interstellar chemical network helps explore the varied characteristics of the different astrophysical environments. Additionally, the chemical network comprising deuterated species is a unique and potent way of studying the different evolutionary stages of star and planet formation, thereby understanding their chemical and dynamical history. Specifically, the D/H ratio unveils the mystery of the ‘interstellar region’ responsible for the synthesis of the complex species. In the era of ALMA and other future ground based facilities such as SKA and ngVLA, the interstellar census is expected to rise exponentially in coming years, along with the increased number of COMs and deuterated species. However, we have a handful of data for reactions responsible for formation of COMs and their deuterated forms. The rapid development in the computational tools and the powerful quantum theories can address this limitation. Here, I will present computational chemistry’s role in exploring the unknown reaction networks, particularly emphasizing the kinetic isotopic effect (KIE) and Thermodynamic isotopic effect (TIE) studies due to the difference in zero point energy of isotopologues. The expanded network is expected to provide improved astrochemical models which include deuterium fractionation and better characterize the evolution of star and planet forming regions using high sensitivity observations with ALMA and other facilities.
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