Atomic carbon gets chemisorbed with a CO molecule when lands on water ice surfaces. The formed species can easily be hydrogenated forming ketene (H2CCO), a key species towards molecular complexity as shown in this work when reacting with open-shell species (atoms and radicals)
Carbon (3P) atom is a reactive species that, according to laboratory experiments and theoretical calculations, condensates with interstellar ice components. This fact is of uttermost importance for the chemistry in the interstellar medium (ISM) because the condensation reaction is barrierless and the subsequent species formed are still reactive given their open-shell character. Carbon condensation on CO-rich ices forms the \ch{C=C=O} (3Σ−) species, which can be easily hydrogenated twice to form ketene (H2CCO). Ketene is very reactive in terrestrial conditions, usually found as an intermediate hard to be isolated in chemical synthesis laboratories. These characteristics suggest that ketene can be a good candidate to form interstellar complex organic molecules (iCOMs) via a two-step process, i.e., its activation followed by a radical-radical coupling. In this work, reactions between ketene and atomic H, and the OH and NH2 radicals on a CO-rich ice model have been explored by means of quantum chemical calculations complemented by kinetic calculations to evaluate if they are favourable in the ISM. Results indicate that H addition to ketene (helped by tunneling) to form the acetyl radical (CH3CO) is the most preferred path, as the reactions with OH and NH2 possess activation energies (≥ 9kJ/mol) hard to surmount in the ISM conditions, unless external processes provide energy to the system. Thus, acetaldehyde (CH3CHO) and, probably, ethanol (CH3CH2OH) formation via further hydrogenations are the possible unique operating synthetic routes. Moreover, from the computed relatively large binding energies of OH and NH2 on CO ice, slow diffusion is expected, hampering possible radical-radical couplings with CH3CO. The astrophysical implications of these findings are discussed considering the incoming James Webb Space Telescope observations.
This work has been published in The Astrophysical Journal.
Link to the article as open access in ApJ: https://iopscience.iop.org/article/10.3847/1538-4357/acd192
Link to the article as green open access: https://arxiv.org/abs/2305.16116