Formation of Formamide from HCN + H₂O: A Computational Study on the Roles of a Second H₂O as a Catalyst, as a Spectator, and as a Reactant

Formamide (NH₂CHO), 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 H₂O, 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 H₂O) was found to be acting as a reactant, in the second step of the reaction path. Effects of second H₂O 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 H₂O, 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.