Possible interstellar formation of glycine through a concerted mechanism: a computational study on the reaction of CH₂NH, 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 CH2[double bond, length as m-dash]NH, 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 CH2[double bond, length as m-dash]NH 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 CH2[double bond, length as m-dash]NH + CO + H2O and CH2[double bond, length as m-dash]NH + 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, CH2[double bond, length as m-dash]NH + 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.

Radiolysis of NH3:CO ice mixtures – implications for Solar system and interstellar ices

“…A band of the other product is seen in Fig. 2(h) at 1580 cm −1 ; it is assigned to formate ion (HCOO − ; Schutte et al 1999), an important precursor of both carbamic acid and glycine (Bossa et al 2008a;Rimola, Sodupe, & Ugliengo 2012;Nhlabatsi, Bhasi & Sitha 2016).…”

Hydrogenations of Isocyanic Acid: A Computational Study on Four Possible Concerted Paths for Formamide Formation

“…Comparing the energetics of the reactants and product, it was observed that this reaction path is exothermic in nature. Analysis of the imaginary frequency associated with the TS-2 indicates an intense vibration (2147.89i cm −1 : indicates a sharper and narrower barrier than TS-1 [89][90][91][92][93]) and the maximum contributions to the vibration were from the vibrations of the two H-atoms (of the H 2 molecule) with a very little contribution from other atoms. Comparison of the energetics of imine intermediate with that of the formamide, α-hydroxyimine was found to be less stable.…”

“…Analysis of the imaginary frequency associated with the TS-1 indicates an intense vibration (2031.64i cm −1 : indicates a very sharp and narrow barrier [89][90][91][92][93]) and the maximum contributions to the vibration are coming from the vibrations of the two H-atoms (of the H 2 molecule) with a very little contribution from other atoms. In view of this H-H bond-breaking happening in the transition state, we can say that LUMO of H 2 might be interacting with the HOMO of HNCO and partial interactions between these two orbitals can be expected.…”

Prebiotic molecules formation through the gas-phase reaction between HNO and CH2CHOH2+

“…A key aspect for the existence of glycine in the interstellar medium is to know if there are efficient routes of formation. In this regard, in recent years numerous theoretical and experimental studies have been devoted to the formation processes of glycine in the interstellar medium taking into account either gas phase or surface reactions (e.g., Andreazza et al 2006;Barrientos et al 2012;Basiuk & Kobayashi 2004;Blagojevic et al 2003;Elsila et al 2007;Largo et al 2010a,b;Maeda & Ohno 2006;Nhlabatsi et al 2016;Redondo et al 2015;Rimola et al 2012;Snow et al 2007;Woon 2002). Another aspect of the non-detection of glycine to consider is that in general, the most abundant isomer of a given generic chemical formula observed in the ISM is the most stable one (minimum energy principle or MEP; Lattelais et al 2011).…”