Interstellar Chemistry of CN Radicals on Ices: The formation of CH3CN and CH3NC and potential connection to acetamide

TL;DR

This study investigates the reactions of CN radicals on interstellar ices, revealing their high reactivity in forming nitriles like CH3CN and CH3NC, and explores potential pathways to prebiotic molecules such as acetamide.

Contribution

It provides a detailed quantum chemical analysis of CN radical reactions on ices, highlighting their role in forming complex organic molecules relevant to prebiotic chemistry.

Findings

CN reacts with CH3 to form CH3CN and CH3NC efficiently.

Reactions on water ice depend on quantum tunnelling and ice structure.

Barrierless formation of cyanide and isocyanide on CO ices.

Abstract

Context. Among the most significant chemical functional groups of interstellar molecules are the class of nitriles, which are proposed as key prebiotic molecules due to their chemical connection to the peptide bond after hydrolysis. 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, density functional theory was employed 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, forming CH3CN and CH3NC efficiently. 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 isomerize into acetamide through a sizable 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.