Pathways to Interstellar Amides via Carbamoyl (NH2CO) Isomers by Radical-Neutral Reactions on Ice Grain Mantles

Abstract

Explaining the formation pathways of amides on ice-grain mantels is crucial to understanding the prebiotic chemistry in an interstellar medium. In this computational study, we explore different radical-neutral formation pathways for some of the observed amides (formamide, acetamide, urea, and N-methylformamide) via intermediate carbamoyl (NH₂CO) radical precursors and their isomers. We assess the relative energy of four NH₂CO isomers in the gas phase and evaluate their binding energy on small water clusters to discern the affinity that the isomers present to an ice model. We consider three possible reaction pathways for the formation of the carbamoyl radicals, namely, the OH + HCN, CN + H₂O, and NH₂ + CO reaction channels. We computed the binding energy distribution for the HCN and CH₃CN precursors on an ice model consisting of a set of clusters of 22 water molecules each to serve as a starting point for the reactivity study on the ice surface. The computations revealed that the lowest barrier to the formation of an NH₂CO isomer corresponds to the NH₂ + CO reaction (12.6 kJ mol^–1). The OH + HCN reaction pathway results in the exothermic formation of the N-radical form of carbamoyl HN(C═O)H with a reaction barrier of 26.7 kJ mol^–1. We found that the CN + H₂O reaction displays a high energy barrier of 70.6 kJ mol^–1. Finally, we also probed the direct formation of the acetamide radical precursor via the OH + CH₃CN reaction and found that the most probable outcome on interstellar ices is the H-abstraction reaction to yield CH₂CN and H₂O. Based on these results, we believe that including alternative reaction pathways, leading to the formation of amides via the N-radical form of carbamoyl, would provide an improvement in the prediction of the amide abundances in astrochemical models, especially regarding the chemistry of star-forming regions.