Can Formamide Be Formed on Interstellar Ice? An Atomistic Perspective

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 computations based on quantum chemical and kinetic calculations 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 also 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). Our conclusion is that CN quickly reacts with a molecule of amorphous ice and that it can synthesize formamide, even though the efficiency of the NH2CHO formation is difficult to estimate as it depends on the unknown number of ice water active sites and the fine details of energy transfer through the ice body itself. Our results have two important consequences on the modeling of interstellar surface-chemistry. First, the H2O molecules of the ice, usually considered as an inert support in astrochemical models, can instead react with active radicals, like CN, forming more complex species, and can also act as catalysts by helping H transfer processes. Second, most of the involved intermediate steps toward formamide formation on the 33-H2O molecule cluster are so fast that it is unlikely that the energy released in each of them can be dispersed in the entire ice body of the grain. In other words, the system cannot be fully equilibrated at the grain temperature in each intermediate step, as assumed in all current models, because the localized energy can promote endothermic or high barrier processes in small portions of the ice before complete equilibration. The time scale of energy redistribution within the ice molecules, a poorly characterized process, should be explicitly accounted for if a realistic model of grain surface chemistry is pursued.