Atomic carbon gets chemisorbed with a water molecule when lands on water ice surfaces. The formed species can easily react with open-shell species, opening new synthetic paths towards forming interstellar complex organic molecules.

The reactivity of interstellar carbon atoms (C) on the water-dominated ices is one of the possible ways to form interstellar complex organic molecules (iCOMs). In this work, we report a quantum chemical study of the coupling reaction of C (3P) with an icy water molecule, alongside possible subsequent reactions with the most abundant closed shell frozen species (NH3, CO, CO2 and H2), atoms (H, N and O), and molecular radicals (OH, NH2 and CH3). We found that C spontaneously reacts with the water molecule, resulting in the formation of 3C-OH2, a highly reactive species due to its triplet electronic state. While reactions with the closed-shell species do not show any reactivity, reactions with N and O  form CN and CO, respectively, the latter ending up into methanol upon subsequent hydrogenation. The reactions with OH, CH3 and NH2 form methanediol, ethanol and methanimine, respectively, upon subsequent hydrogenation. We also propose an explanation for methane formation, observed in experiments through H additions to C in the presence of ices. The astrochemical implications of this work are: i) atomic C on water ice is locked into 3C-OH2, making difficult the reactivity of bare C atoms on the icy surfaces, contrary to what is assumed in astrochemical current models; and  ii) the extraordinary reactivity of 3C-OH2 provides new routes towards the formation of iCOMs in a non-energetic way, in particular ethanol, mother of other iCOMs once in the gas-phase.

This work has been published in The Astrophysical Journal.

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