A chemical link between saturated and unsaturated aldehydes and ketenes in the interstellar medium

Laboratory experiments on the H-atom reactions of propenal (CH₂CHCHO) and propanal (CH₃CH₂CHO) at low temperatures

¹ MTA-ELTE Lendület Laboratory Astrochemistry Research Group, Institute of Chemistry, ELTE Eötvös Loránd University, PO Box 32, 1518 Budapest, Hungary
² Freie Universität Berlin, Institut für Chemie und Biochemie-Anorganische Chemie, Fabeckstrasse 34/36, 14195 Berlin, Germany
³ Laboratory of Molecular Spectroscopy, Institute of Chemistry, ELTE Eötvös Loránd University, PO Box 32, 1518 Budapest, Hungary
⁴ Hevesy György PhD School of Chemistry, Institute of Chemistry, ELTE Eötvös Loránd University, PO Box 32, 1518 Budapest, Hungary
⁵ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, PR China
⁶ Centre for Astrophysics and Space Science, ELTE Eötvös Loránd University, PO Box 32, 1518 Budapest, Hungary

^★ Corresponding author; gyorgy.tarczay@ttk.elte.hu

Received: 20 August 2024
Accepted: 8 November 2024

Abstract

Context. Propenal (CH₂CHCHO) and propanal (CH₃CH₂CHO) have been detected in various regions of the interstellar medium (ISM), from star-forming regions to a comet’s dusty coma. These molecules attract considerable attention due to their structural similarity to aldose sugars and their potential role in prebiotic astrochemistry. Their reactions with H atoms may significantly contribute to the chemical diversity in the ISM and link these molecules with each other and other isomers.

Aims. In this study, we aimed to investigate the astrophysically relevant low-temperature reactions of propenal and propanal molecules with H atoms to explore possible reaction pathways between these molecules and their isomers.

Methods. Propenal and propanal were isolated in solid para-H₂ at 3.1 K. This medium, with its weak interactions, provides spec-troscopic data close to gas-phase values and allows for studying highly reactive short-lived species. Additionally, H atoms can be conveniently generated, they diffuse, and they react with the isolated molecules. The reactions were monitored using infrared (IR) spectroscopy. Quantum-chemical computations were employed to determine possible reaction paths and aid in spectral assignments.

Results. The reaction of CH₂CHCHO and CH₃CH₂CHO with H atoms in the first step results in the production of CH₂CH ^•CO/^•CH₂CHCO, CH₃CH₂^•CO, and CH₃^•CHCHO radicals. Further H-atom reactions of CH₃^•CHCHO and R^•CO radicals lead to the formation of methylketene (CH₃CHCO) as the product of both the reaction of propenal and propanal. The two-step addition of H atoms to CH₂CHCHO was found tentatively to produce CH₃CH₂CHO.

Conclusions. The radicals observed in the experiments are likely produced in dark molecular clouds on icy grains, increasing interstellar chemical complexity. The experiments suggest that H-atom reactions with propanal and propenal are important channels for methylketene production. The observed reactions imply that consecutive H-atom addition and H-atom abstraction reactions of propenal and propanal can catalyze interstellar H₂ formation.