Infrared spectroscopy of isolated acetylacetone ion complexes with H+, Li+, and NH4+ : a tale of two tautomers

Abstract

Acetylacetone (ACAC) is a prototypical 𝛽-diketone whose delicate keto-enol tautomeric equilibrium is highly sensitive to the interaction with external agents. In this work, we employ infrared ion spectroscopy to characterize cationic complexes of acetylacetone with H+, Li+, and NH4+, generated via electrospray ionization of methanol solution and isolated in an ion trap at room temperature. The recorded spectral signatures reveal that protonated acetylacetone is stabilized in both the enol and keto tautomeric forms of ACAC. In contrast, the complexes with Li+, and NH4+ are observed in the keto form exclusively, highlighting the decisive role of the cation in steering tautomeric preference. 𝐴𝑏 𝑖𝑛𝑖𝑡𝑖𝑜 molecular dynamics simulations expose the fluxional behavior of the protonated and ammonium-complexed species, capturing dynamical effects that static calculations fail to reproduce. These findings provide insights into cation-dependent tautomer stabilization of acetylacetone, with implications for the rationalization of the coordination chemistry of diketones in complex environments.