Amides, such as urea, are key molecules in prebiotic chemistry because they possess the C-N bond, which is also found in amino acids and nucleobases. Urea has been detected in a few sources in the interstellar medium, raising questions about its synthesis. We characterized various formation mechanisms on the icy mantles of dust grains. We found that presence of icy surfaces is crucial, as they act as reactant concentrators and likely play the role of third body.

Urea is a prebiotic molecule that has been detected in few sources of the interstellar medium (ISM) and in Murchison meteorite. Being stable against ultraviolet radiation and high-energy electron bombardment, urea is expected to be present in interstellar ices. Theoretical and experimental studies suggest that isocyanic acid (HNCO) and formamide (NH2CHO) are possible precursors of urea. However, uncertainties still exist regarding its formation routes. Previous computational works characterised urea formation in the gas phase or in presence of few water molecules by reaction of formamide with nitrogen-bearing species. In this work, we investigated the reaction of HNCO + NH3 on an 18 water molecules ice cluster model mimicking interstellar ice mantles by means of quantum chemical computations. We characterised different mechanisms involving both closed-shell and open-shell species at B3LYP-D3(BJ)/ma-def2-TZVP level of theory, in which the radical-radical H2NCO + NH2 coupling has been found to be the most favourable one due to being almost barrierless. In this path, the presence of the icy surfaces is crucial for acting as reactant concentrators/suppliers, as well as third bodies able to dissipate the energy liberated during the urea formation.

This work has been published in Icarus.

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