Possible interstellar formation of glycine through a concerted mechanism: a computational study on the reaction of CH2NH, CO2 and H2

Abstract

Glycine being the simplest amino acid and also having significant astrobiological implications, has meant that intensive investigations have been carried out in the past, starting from its detection in the interstellar medium (ISM) to analysis of meteorites and cometary samples and laboratory synthesis, as well as computational studies on the possible reaction paths. In this present work quantum chemical calculations have been performed to investigate the possible interstellar formation of glycine via two different paths; (1) in a two-step process via a dihydroxy carbene intermediate and (2) through a one-step concerted mechanism, starting from reactants like CH₂NH, CO, CO₂, H₂O and H₂. For the two reactions representing the carbene route, it was observed that the formation of dihydroxy carbene from either CO + H₂O or CO₂ + H₂ is highly endothermic with large barrier heights, whereas the subsequent step of interaction of this carbene with CH₂NH to give glycine is exothermic and the barrier is below the reactants. Based on this observation it is suggested that the formation of glycine via the carbene route is a least favourable or even unfavourable path. On the other hand, the two reactions CH₂NH + CO + H₂O and CH₂NH + CO₂ + H₂ representing the concerted paths were found to be favourable in leading to the formation of glycine. After an extensive study on the first concerted reaction in our previous work (Phys. Chem. Chem. Phys., 2016, 18, 375–381), in this work a detailed investigation has been carried out for the second concerted reaction, CH₂NH + CO₂ + H₂, which can possibly lead to the interstellar formation of glycine. It was observed that this reaction proceeds through a large barrier and at the same time the transition state shows prominent hydrogen dynamics, indicating a tunnelling possibility for this reaction. Based on these observations the possible formation of glycine via this reaction in hot-cores and in cold interstellar clouds has been proposed. The cold-core possibility of this reaction is argued on the basis of the phenomenon of tunnelling assisted by a van der Waals' complex.