Hydrogenation of acetaldehyde on interstellar ice analogs results in limited destruction

¹ Departamento de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
² Institute of Low Temperature Science, Hokkaido University N19W8, Kita-ku, Sapporo, Hokkaido 060-0819 Japan

^★ Corresponding authors; german.molpeceres@iff.csic.es; nguyenthanh@lowtem.hokudai.ac.jp

Received: 26 August 2024
Accepted: 23 December 2024

Abstract

Context. Acetaldehyde (CH₃CHO) is one of the most abundant interstellar complex organic molecules and its hydrogenation has important implications in several fundamental processes of interstellar chemistry, such as deuterium fractionation, reactive desorption, or the relation between organic functional groups of detected molecules.

Aims. We seek to determine what the main hydrogenation paths of CH₃CHO are. As a partially unsaturated molecule, CH₃CHO can have links with more hydrogenated species, such as ethanol (C₂H₅OH), or with more unsaturated ones, such as ketene (H₂CCO).

Methods. We used highly accurate quantum chemical calculations to determine the reaction rate constants for the CH₃CHO + H/D reaction. We later studied, using more approximated methods, the fate of the majoritarian product of the reaction, the acetyl radical CH₃CO after subsequent reaction with hydrogen or deuterium atoms. Our theoretical results were tested with our experiments on the hydrogenation and deuteration of CH₃CHO ice.

Results. We find that acetaldehyde resists hydrogenation, with only a 10% of conversion to products different than CH₃CHO. This is due to a predominance of H abstraction at the HCO moiety, with reaction rate constants up to four orders of magnitude higher than the next possible reaction channel, which is hydrogenation at the aldehydic carbon. The formed CH₃CO radical experiences barrierless or nearly barrierless reactions in all possible reaction positions, reforming CH₃CHO and creating a closed loop that protects the molecule against hydrogenation. We constrained the branching ratios for the second reaction from the experiments. Our experiments agree with the calculations and from the combination of both we can explain the presence of H₂CCO, CO, CH4, C₂H₅OH, H₂CO, or CH₃OH as minor products at the end of the reaction. We provide recommendations for future modeling efforts.

Conclusions. Our results show limited destruction of acetaldehyde, reinforcing the vision of this molecule as an abundant and resilient COM. From the experiments, we are not able to observe the reactive desorption of this molecule. Our results align with other modeling works, showing that the link between CH₃CHO and C₂H₅OH is not direct. Finally, our results can explain the excess of CH₃CDO found in prestellar cores.