Issue |
A&A
Volume 699, July 2025
|
|
---|---|---|
Article Number | A235 | |
Number of page(s) | 10 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/202555360 | |
Published online | 11 July 2025 |
Interstellar chemistry of CN radicals on ices: The formation of CH3CN and CH3NC and potential connection to acetamide
1 Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University,
PO Box 9502,
2300 RA Leiden, The Netherlands
2 Leiden Observatory, Leiden University,
PO Box 9513,
2300 RA Leiden, The Netherlands
⋆ Corresponding author: j.enrique.romero@lic.leidenuniv.nl
Received:
1
May
2025
Accepted:
2
June
2025
Context. Among the most significant chemical functional groups of interstellar molecules are the class of nitriles, which are suggested to be key prebiotic molecules due to their chemical connection to the peptide bond after hydrolysis. The •CN radicals, the simplest representative of this group, have been shown to exhibit strong interactions with interstellar water ices, potentially impacting their reactivity with other radicals nearby.
Aims. This study explores (a) whether CN and •CH3 radicals can readily react to form methyl cyanide (CH3CN) and its isomer methyl isocyanide (CH3NC) and (b) the feasibility of the reaction (CN···H2O)hemi → •C(OH) = NH and its potential role in the formation of acetamide.
Methods. Following a benchmark, we employed density functional theory to map the potential energy surfaces of these chemical processes, focusing on their reactivity on water and carbon monoxide ices.
Results. The results show that CN reacts with •CH3 radicals on water ices, efficiently forming CH3CN and CH3NC. However, these reactions are driven by diffusion of •CH3 towards the reactive site and subsequently compete with back-diffusion of •CH3 from that site. The formation of the radical intermediate •C(OH) = NH on water ice requires quantum tunnelling, and assuming that acetimidic acid forms via CH3 + •C(OH) = NH → CH3C(OH) = NH, it can also only isomerise into acetamide through a sizeable barrier thanks to quantum tunnelling. Both quantum tunnelling-driven reactions are highly dependent on the local structure of the water ice. Finally, radical coupling reactions on carbon monoxide ices are found to be barrierless for all cases, and again both the cyanide and the isocyanide are formed.
Conclusions. This work reinforces the conclusion that •CN radicals on interstellar grain surfaces are highly reactive and unlikely to persist unaltered.
Key words: astrochemistry / molecular processes / solid state: volatile / ISM: molecules
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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