2025ApJ...990..189B

In cold molecular clouds, UV photolysis of icy grain mantles generates radicals that lead to new molecule formation. When radical diffusion is limited by low temperatures, oxygen atom addition and insertion reactions, enabled by photolysis of common ice components such as H₂O, CO₂, CO, and O₃, offer an alternative route to chemical complexity through the production of metastable, highly reactive O(¹D) atoms. We examine the reactivity of these oxygen atoms generated by UV photolysis of O₃ with methyl cyanide (CH₃CN). These studies are conducted in an ultra-high vacuum chamber at cryogenic and low-pressure conditions equipped with in situ infrared spectroscopy to monitor destruction and product formation in real time. We conclude that oxygen atoms rapidly insert into CH₃CN to produce primarily methyl isocyanate (CH₃NCO) in matrix-free ices. Over the range from 10 to 40 K, we observe no temperature dependence to either CH₃CN destruction or CH₃NCO production. When placing CH₃CN:O₃ in H₂O and CO₂ ice matrices, we find that CH₃NCO formation remains robust, but that the yield likely decreases due to competing reaction pathways. In the case of the H₂O ice, we also observe a shift in product branching ratios toward alternative pathways such as the formation of hydroxyacetonitrile (HOCH₂CN). Overall, our results demonstrate that oxygen atom reactivity provides an important channel for generating chemical complexity from nitriles on cold grains where radical mobility is limited.