  Interstellar formamide (NH2CHO) has recently attracted significant attention
due to its potential role as a molecular building block in the formation of
precursor biomolecules relevant for the origin of life. Its formation, whether
on the surfaces of the interstellar grains or in the gas phase, is currently
debated. The present article presents new theoretical quantum chemical
computations on possible NH2CHO formation routes in water-rich amorphous ices,
simulated by a 33-H2O-molecule cluster. We have considered three possible
routes. The first one refers to a scenario used in several current
astrochemical models, that is, the radical-radical association reaction between
NH2 and HCO. Our calculations show that formamide can indeed be formed, but in
competition with formation of NH3 and CO through a direct H transfer process.
The final outcome of the NH2 + HCO reactivity depends on the relative
orientation of the two radicals on the ice surface. We then analyzed two other
possibilities, suggested here for the first time: reaction of either HCN or CN
with water molecules of the ice mantle. The reaction with HCN has been found to
be characterized by large energy barriers and, therefore, cannot occur under
the interstellar ice conditions. On the contrary, the reaction with the CN
radical can occur, possibly leading through multiple steps to the formation of
NH2CHO. For this reaction, water molecules of the ice act as catalytic active
sites since they help the H transfers involved in the process, thus reducing
the energy barriers (compared to the gas-phase analogous reaction).
Additionally, we apply a statistical model to estimate the reaction rate
coefficient when considering the cluster of 33-H2O-molecules as an isolated
moiety with respect to the surrounding environment, i.e., the rest of the ice.
