Formation of interstellar complex organic molecules on water-rich ices triggered by atomic carbon freezing

Stefano Ferrero; Cecilia Ceccarelli; Piero Ugliengo; Mariona Sodupe; Albert Rimola

paper

Formation of interstellar complex organic molecules on water-rich ices triggered by atomic carbon freezing

arXiv:2310.14831

Abstract

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 ( $^{3}$ P) with an icy water molecule, alongside possible subsequent reactions with the most abundant closed shell frozen species (NH $_{3}$ , CO, CO $_{2}$ and H $_{2}$ ), atoms (H, N and O), and molecular radicals (OH, NH $_{2}$ and CH $_{3}$ ). We found that C spontaneously reacts with the water molecule, resulting in the formation of $^{3}$ C-OH $_{2}$ , 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, CH $_{3}$ and NH $_{2}$ 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 $^{3}$ C-OH $_{2}$ , 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 $^{3}$ C-OH $_{2}$ 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.