IR Spectroscopy of Protonated Acetylacetone and Its Water Clusters: Enol-Keto Tautomers and Ion→Solvent Proton Transfer.

Protonated ions of acetylacetone, H(Hacac), and their argon-tagged analogues are produced via a pulsed discharge and cooled in a supersonic expansion. These ions are mass analyzed, selected in a time-of-flight spectrometer, and studied with infrared laser photodissociation spectroscopy using the method of rare-gas atom tagging. Computational studies at the DFT/B3LYP level are employed to elucidate the structures and spectra of these ions, which are expected to exist as either enol- or keto-based tautomers. The protonated acetylacetone ion is found to form a single enol-based isomer. Adding one or two water molecules to this ion, for example, H(Hacac)(HO), produces primarily enol-based structures, although a small concentration of keto structures also contribute to the spectra. The vibrational patterns resulting from hydrogen bonding in these systems are not well-described by theory. Addition of a third water molecule to form the H(Hacac)(HO) ion causes a significant change in the spectroscopy, attributed to proton transfer from the H(Hacac) ion into the water solvent.