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)(H₂O)_1,2, 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)(H₂O)₃ ion causes a significant change in the spectroscopy, attributed to proton transfer from the H⁺(Hacac) ion into the water solvent.

中文翻译：

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质子化乙酰丙酮及其水团的红外光谱：烯醇-酮互变异构体和离子→溶剂质子转移

乙酰丙酮，H ⁺（Hacac）的质子化离子及其氩标记的类似物是通过脉冲放电产生的，并在超音速膨胀中冷却。对这些离子进行质量分析，在飞行时间光谱仪中进行选择，并使用稀有气体原子标记方法通过红外激光光解离光谱进行研究。在DFT / B3LYP水平上的计算研究被用来阐明这些离子的结构和光谱，这些离子预期以基于烯醇或酮基的互变异构体的形式存在。发现质子化的乙酰丙酮离子形成单一的基于烯醇的异构体。向该离子添加一个或两个水分子，例如H ⁺（Hacac）（H ₂ O）_1,2，虽然主要是少量的酮结构也有助于光谱，但主要产生基于烯醇的结构。在这些系统中，由氢键形成的振动模式在理论上没有得到很好的描述。添加第三种水分子以形成H ⁺（Hacac）（H ₂ O）₃离子会导致光谱发生重大变化，这归因于质子从H ⁺（Hacac）离子转移到水溶剂中。