Radiation-driven Destruction of N-heterocycles in the Presence and Absence of Water Ice

Simple N-heterocycles are expected to be an abundant class of molecules within the interstellar medium, and increasingly complex heterocycles (e.g., nucleobases) have been detected within meteoritic organic material and in samples returned from the carbonaceous asteroid Bennu. Despite this, these molecules have not been detected in any interstellar environment or in the outer solar system. One possible reason for the nondetection of N-heterocycles could be that they are less stable to radiation than the aromatic compounds that have been identified in space (e.g., benzene). Here, we present the radiolytic destruction kinetics of benzene and several N-heterocycles, both as single-component ices and as dilute water-ice mixtures at 15 K, where we have quantified the radiolytic destruction rate constants and radiolytic half-lives of these aromatic molecules using IR spectroscopy. We found that the destruction rate constants for single-component ices, and to a lesser extent for water-ice mixtures, depended on the number of nitrogen atoms in the aromatic ring. Our radiolytic half-lives indicate that these molecules should persist in extraterrestrial radiation environments, and radiolytic destruction cannot fully explain the nondetections of N-heterocycles.