Hunger Lasse, Al Sheakh Loai, Fritsch Sebastian, Villinger Alexander, Ludwig Ralf, Harville Payten, Moss Olivia, Lachowicz Anton, Johnson Mark A
Department of Chemistry, University of Rostock, 18059 Rostock, Germany.
Department Life, Light & Matter, University of Rostock, 18051 Rostock, Germany.
J Phys Chem B. 2024 Jun 6;128(22):5463-5471. doi: 10.1021/acs.jpcb.4c02094. Epub 2024 May 22.
Intermolecular interactions determine whether matter sticks together, gases condense into liquids, or liquids freeze into solids. The most prominent example is hydrogen bonding in water, responsible for the anomalous properties in the liquid phase and polymorphism in ice. The physical properties are also exceptional for ionic liquids (ILs), wherein a delicate balance of Coulomb interactions, hydrogen bonds, and dispersion interactions results in a broad liquid range and the vaporization of ILs as ion pairs. In this study, we show that strong, local, and directional hydrogen bonds govern the structures and arrangements in the solid, liquid, and gaseous phases of carboxyl-functionalized ILs. For that purpose, we explored the H-bonded motifs by X-ray diffraction and attenuated total reflection (ATR) infrared (IR) spectroscopy in the solid state, by ATR and transmission IR spectroscopy in the liquid phase, and by cryogenic ion vibrational predissociation spectroscopy (CIVPS) in the gaseous phase at low temperature. The analysis of the CO stretching bands reveals doubly hydrogen-bonded cationic dimers (c═c), resembling the archetype H-bond motif known for carboxylic acids. The like-charge doubly hydrogen-bonded ion pairs are present in the crystal structure of the IL, survive phase transition into the liquid state, and are still present in the gaseous phase even in (2,1) complexes wherein one counterion is removed and repulsive Coulomb interaction increased. The interpretation of the vibrational spectra is supported by quantum chemical methods. These observations have implications for the fundamental nature of the hydrogen bond between ions of like charge.
分子间相互作用决定了物质是否会黏附在一起、气体是否会凝结成液体或液体是否会凝固成固体。最突出的例子是水中的氢键，它导致了液相中的异常性质和冰中的多晶型现象。离子液体（ILs）的物理性质也很特殊，其中库仑相互作用、氢键和色散相互作用的微妙平衡导致了较宽的液体范围以及离子液体以离子对形式汽化。在本研究中，我们表明强的、局部的和定向的氢键支配着羧基官能化离子液体在固相、液相和气态中的结构和排列。为此，我们通过固态的X射线衍射和衰减全反射（ATR）红外（IR）光谱、液相的ATR和透射红外光谱以及低温下气态的低温离子振动预解离光谱（CIVPS）来探索氢键基序。对CO伸缩带的分析揭示了双氢键阳离子二聚体（c═c），类似于已知的羧酸原型氢键基序。相同电荷的双氢键离子对存在于离子液体的晶体结构中，在相转变为液态时仍然存在，甚至在（2,1）配合物中，其中一个抗衡离子被去除且排斥性库仑相互作用增加时，在气态中仍然存在。振动光谱的解释得到了量子化学方法的支持。这些观察结果对相同电荷离子之间氢键的基本性质具有启示意义。
