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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). 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. We found that the CN + H2O reaction displays a high energy barrier of 70.6 kJ/mol. 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., Comment: Journal is ACS Earth Space Chem, 2024. ASAP (As Soon As Publishable)

Chemical processes taking place on ice-grain mantles are pivotal to the complex chemistry of interstellar environments. In this study, we conducted a comprehensive analysis of the catalytic effects of an amorphous solid water (ASW) surface on the reaction between ammonia (NH$_3$) and formaldehyde (H$_2$CO) to form aminomethanol (NH$_2$CH$_2$OH) using density functional theory. We identified potential catalytic sites based on the binding energy distribution of NH$_3$ and H$_2$CO reactants, on a set-of-clusters surface model composed of 22 water molecules and found a total of 14 reaction paths. Our results indicate that the catalytic sites can be categorized into four groups, depending on the interactions of the carbonyl oxygen and the amino group with the ice surface in the reactant complex. A detailed analysis of the reaction mechanism using Intrinsic Reaction Coordinate and reaction force analysis revealed three distinct chemical events for this reaction: formation of the C--N bond, breaking of the N--H bond, and formation of the O--H hydroxyl bond. Depending on the type of catalytic site, these events can occur within a single, concerted, albeit asynchronous, step, or can be isolated in a step-wise mechanism, with the lowest overall transition state energy observed at 1.3 kcal mol$^{-1}$. A key requirement for the low-energy mechanism is the presence of a pair of dangling OH bonds on the surface, found at 5\% of the potential catalytic sites on an ASW porous surface.