Activation and conversion of unsaturated aldehydes by H atoms in the interstellar medium: A laboratory study on crotonaldehyde

Abstract

Crotonaldehyde (CH3CHCHCHO) belongs to the alkenal family, a class of organic compounds thought to play a key role in the prebiotic synthesis of amino acids and sugars. In extraterrestrial environments, it may plausibly form from the abundant interstellar propenal. The reactivity of crotonaldehyde with H atoms provides insight into the chemical pathways linking key classes of interstellar complex organic molecules, particularly aldehydes and ketenes. This work aims to investigate the astrophysically relevant low-temperature H-atom reactions of crotonaldehyde in a solid medium to explore its potential chemical evolution in the interstellar medium (ISM). Crotonaldehyde was isolated in solid para-H2 matrices at 3.1 K. The reactions were monitored via infrared (IR) spectroscopy. Quantum-chemical computations were performed to estimate the energetics and barriers of the reactions and verify the spectral assignments. The main product of the reactions of crotonaldehyde with H atoms is the CH3CHCH•CO radical, for which seven fundamental modes were assigned in the IR spectra. Absorption features of CH3•CHCHCO were also detected. Subsequent H-atom reactions with these radicals were found to yield ethylketene (CH3CH2CHCO). The H-assisted isomerization to ketenes through the formation of R•CO radicals may be a common feature in the H-atom reactions of α,β-alkenals. Thus, corresponding ketenes may be expected in regions of the ISM where α,β-alkenals have been detected. The alkenal–ketene isomerization reaction may also contribute to the catalytic formation of interstellar H2. These findings confirm the H-atom activation of aldehydes in the ISM, revealing the formation of intermediates that are essential to increase the interstellar chemical complexity.