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

Context . Propenal (CH 2 CHCHO) and propanal (CH 3 CH 2 CHO) 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-H 2 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 CH 2 CHCHO and CH 3 CH 2 CHO with H atoms in the first step results in the production of CH 2 CH • CO/ • CH 2 CHCO, CH 3 CH 2 • CO, and CH 3 • CHCHO radicals. Further H-atom reactions of CH 3 • CHCHO and R • CO radicals lead to the formation of methylketene (CH 3 CHCO) as the product of both the reaction of propenal and propanal. The two-step addition of H atoms to CH 2 CHCHO was found tentatively to produce CH 3 CH 2 CHO. 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 H 2 formation.