Catalysis of Acetone Enolization by Nitrogen-Containing Ligands: An Infrared Spectroscopic Study of (CH ₃ COCH ₃ –X) ⁺ Complexes (X = CH ₃ CN, CH ₂ CHCN, NH ₃ )

Enols, the tautomeric forms of carbonyl compounds, are considered to play key roles in the formation of prebiotic molecules in the interstellar medium. Herein, we investigated the possibility of nitrogen-containing molecules in cationic acetone enolization by measuring the infrared spectra of acetone-acetonitrile, acetone-acrylonitrile, and acetone-ammonia binary cationic complexes in the range of 2400-3800 cm^-1 using a vacuum ultraviolet photoionization and infrared dissociation (VUV-IR) scheme. To determine the structures of the complex isomers corresponding to the observed spectra, <i>ab initio</i> electronic structure calculations and infrared spectral simulations were employed. The results suggest that ammonia effectively catalyzes cationic acetone enolization, whereas acetonitrile and acrylonitrile do not exhibit observable catalytic activity in this process. Through reaction pathway searching and microcanonical rate constant calculations, the lack of catalytic activity in the nitrile-containing complexes stems from their low-energy lone-pair electrons and insufficient excess energy acquired during photoionization. These factors collectively hinder the key proton-transfer reaction underlying cationic acetone enolization. These findings provide insights into the multiscale regulatory mechanisms that control enolization pathways in interstellar chemistry.