Interstellar chemistry of CN radicals on ices: The formation of CH₃CN and CH₃NC and potential connection to acetamide

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 • CH 3 radicals can readily react to form methyl cyanide (CH 3 CN) and its isomer methyl isocyanide (CH 3 NC) and (b) the feasibility of the reaction (CN···H 2 O) 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 • CH 3 radicals on water ices, efficiently forming CH 3 CN and CH 3 NC. However, these reactions are driven by diffusion of • CH 3 towards the reactive site and subsequently compete with back-diffusion of • CH 3 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 CH 3 + • C(OH) = NH → CH 3 C(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.