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Formamide (NH 2 CHO), being the smallest and fundamental building block of life (with a peptide linkage), has recently been able to attract much interests, in the field of astrochemistry, astrophysics, and astrobiology. In this work, using quantum mechanical computations, reactions between HCN and H 2 O, leading to the formation of formamide, have been analyzed. For the first time, an alternative and competing reaction channel, which proceeds via a geminal diol intermediate, for the formation of formamide, has been proposed. In this alternative channel, an extra water molecule (second H 2 O) was found to be acting as a reactant, in the second step of the reaction path. Effects of second H 2 O molecule in the reaction paths, providing catalytic assistance to the reaction or behaving like a spectator (concept is introduced for the first time for this reaction), have also been analyzed. Usefulness of spectator behavior is highlighted for the reactions happening on the rigid water-ice surfaces, where the water-ice may not be getting involved for any catalytic assistance. In light of catalytic assistances provided by the second H 2 O, prominent effects in reducing the barrier heights drastically (even for the second step of the reaction, the barrier height was found to be below the reactants), through a hydrogen relay transport mechanism, were observed. In addition to the mechanism studies, interstellar feasibilities of all the reaction channels and their significances are discussed in detail.

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Formamide (NH2CHO) is the smallest molecular unit that contains the basic peptide linkage and thus has recently attracted a great amount of interest in the field of astrochemistry. In this work using computational calculations, we have analyzed the three possible reaction paths for the reaction between CO and NH3 to form formamide in both neutral–neutral and cation–neutral reaction surfaces. All of these three paths strongly favor the path of 1,2-hydrogen migration, which was discounted by previous studies in view of the constraints from steric factor. We have also analyzed the significant role played by prereaction complexes in these three reaction paths. We have proposed that for the neutral–neutral reaction path, formation of formamide in the low temperature interstellar clouds was hypothesized to proceed via hydrogen tunneling assisted by a tunneling ready like state as prereaction complex. On the other hand, for the two cation–neutral reactions, any tunneling cannot facilitate formation of formamide in the interstellar clouds. Rather in one case as all the stationary points are below the reactants, it can facilitate the reaction, whereas in the second case the reaction is only possible if it can get some catalytic assistance.

Interstellar formations and significances of formamide as a possible prebiotic precursor, for many complex organic molecules of life are well studied in recent times. In this work, computational studies (B3LYP, wB97xD, MP2, and CCSD(T)) have been carried out to study the formation mechanism of formamide, from HNCO + H 2 , via concerted paths. Beside the well-studied, Direct Route, three new routes, imine, carbene and Oxime intermediate routes have been investigated. Carbene and oxime routes were found to be endothermic and hence may not be useful in interstellar circumstances. On the other hand, imine route, being slightly exothermic, may be considered as an auxiliary channel to the direct route. Detailed studies for these two routes have been carried out. Also, based only on a qualitative analysis of possibility of tunnelling, we have suggested the possible usefulness of these two reaction paths in cryogenic conditions of interstellar molecular clouds. We hope this study will be useful to our future understanding about the astrochemistry of formamide and origin of life.

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