Formation of Carbonisocyanatidic Amide in Interstellar Ice Analogs as a Candidate Precursor to Prebiotic Pyrimidines

Understanding how pyrimidine nucleobases form in space is central to prebiotic chemistry, yet the pathways assembling these heterocycles remain unresolved. Carbonisocyanatidic amide (H2NC(O)NCO), an acyclic intermediate with heteroatom connectivity relevant to uracil and thymine, has not been detected in interstellar ice analogs. Here, we show that carbonisocyanatidic amide is formed in low-temperature isocyanic acid (HNCO) ices exposed to energetic electrons and is identified during the desorption phase by synchrotron vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry combined with high-level quantum chemical calculations. A plausible, product-constrained network involves hydrogen abstraction and addition steps that generate radical intermediates, followed by radical–radical recombination and carbon–nitrogen bond formation. These results establish an experimental link between simple interstellar volatiles and nitrogen-rich molecular architectures relevant to pyrimidine nucleobases, supporting an astrochemical route to prebiotic intermediates in icy grains and their possible delivery to early Earth.