Formation of Complex Organic Molecules on Interstellar CO Ices? Insights from Computational Chemistry Simulations

Abstract

The carbon ( ^3 P) 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 C=C=O ( ^3 Σ ^− ) species, which can be easily hydrogenated twice to form ketene (H _2 CCO). Ketene is very reactive in terrestrial conditions, usually found as an intermediate that is difficult to isolate in chemical synthesis laboratories. These characteristics suggest that ketene can be a good candidate to form interstellar complex organic molecules 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 NH _2 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 favorable in the ISM. Results indicate that the addition of H to ketene (helped by tunneling) to form the acetyl radical (CH _3 CO) is the most preferred path as the reactions with OH and NH _2 possess activation energies (≥9 kJ mol ^−1 ) hard to surmount in the ISM conditions unless external processes provide energy to the system. Thus, acetaldehyde (CH _3 CHO) and, probably, ethanol (CH _3 CH _2 OH) formation via further hydrogenations, are the possible unique operating synthetic routes. Moreover, from the computed, relatively large binding energies of OH and NH _2 on CO ice, slow diffusion is expected, hampering possible radical–radical couplings with CH _3 CO. The astrophysical implications of these findings are discussed considering the incoming James Webb Space Telescope observations.

Keywords

• Astrochemistry
• Interstellar medium
• Interstellar molecules
• Interstellar dust
• Surface ices
• Complex organic molecules