Two-State Spin-Forbidden Formation of Amide Molecules in the Interstellar Medium

The C₂H₅NO isomers are the simplest family of molecules containing a peptide bond and therefore highly relevant to astrochemistry and astrobiology. We investigate the possible formation mechanisms of the C₂H₅NO amide isomers from the precursors detected in the interstellar medium, focusing on the reaction pathways involving two electronic states with different spin multiplicities. To identify the barrierless reaction pathways, we performed density functional theory and high-level coupled cluster calculations on the reactants, products, intermediates, transition states, and minimum-energy crossing points between singlet and triplet electronic states. Our calculations demonstrate the significance of two-state spin-forbidden pathways in the formation of acetamide, N-methylformamide, and acetimidic acid from acetaldehyde, imidogen, formamide, and methylene. The proposed spin-forbidden pathways provide simple barrierless mechanisms for the formation of the amide isomers in the gas phase, in contrast to the previously proposed reactions catalyzed by surfaces of interstellar icy grains. The proposed mechanisms are consistent with the abundances of acetamide and N-methylformamide observed in Sagittarius B2 North and predict the presence of acetimidic acid in the same region, motivating future observational efforts to identify this molecule in the interstellar medium.