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

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.