Probing molecular interaction in ionic liquids by low frequency spectroscopy: Coulomb energy, hydrogen bonding and dispersion forces
Probing molecular interaction in ionic liquids by low frequency spectroscopy: Coulomb energy, hydrogen bonding and dispersion forces
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DOI：
10.1039/c4cp01476f
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发表时间：
2014-10-28
影响因子：
3.3
通讯作者：
Ludwig, Ralf
中科院分区：
文献类型：
--
作者：
Fumino, Koichi;Reimann, Sebastian;Ludwig, Ralf
离子液体被定义为仅由离子组成、熔点低于 100°C 的盐。这些非凡的液体具有独特而迷人的特性，为科学技术提供了新的机遇。新的离子组合提供了不断变化的物理特性，从而为此类液体材料提供了新的潜在应用。离子液体的结构和性质在很大程度上是由阴离子和阳离子之间的分子间相互作用决定的。从这个角度来看，我们表明远红外和太赫兹光谱是研究这些库仑流体中阳离子阴离子相互作用的合适方法。密度泛函理论计算和分子动力学模拟支持对测量的低频光谱的解释。我们展示了选定的非质子和质子离子液体及其与分子溶剂的混合物的结果。我们特别关注分子间相互作用的强度和类型，以及离子及其组合的特性如何影响这两个参数。我们表明，阳离子和阴离子之间的总相互作用是库仑力、氢键和色散力之间微妙平衡的结果。对于质子离子液体，我们可以测量低频光谱中不同的振动模式，清楚地表明以线性和中强氢键为特征的阳离子阴离子相互作用。使用同位素替代，我们已经能够剖析与阳离子和阴离子之间的纯相互作用强度以及仅与不同的约简质量相关的频移。在这种情况下，我们还展示了这些不同类型的相互作用如何影响离子液体的物理性质，例如熔点、粘度或汽化焓。此外，我们证明低频光谱也可用于研究离子形态。低振动特征可以分配给接触离子对和溶剂分离离子对。总之，我们展示了如何利用低频光谱的详细知识来理解离子液体及其与分子溶剂的混合物中温度、溶剂极性和溶剂浓度的变化引起的相互作用强度和结构的变化。原则上，所使用的方法组合适合研究纯分子液体及其溶液（包括纳米颗粒等添加剂材料）中的分子间相互作用。
Ionic liquids are defined as salts composed solely of ions with melting points below 100 C. These remarkable liquids have unique and fascinating properties and offer new opportunities for science and technology. New combinations of ions provide changing physical properties and thus novel potential applications for this class of liquid materials. To a large extent, the structure and properties of ionic liquids are determined by the intermolecular interaction between anions and cations. In this perspective we show that far infrared and terahertz spectroscopy are suitable methods for studying the cation anion interaction in these Coulomb fluids. The interpretation of the measured low frequency spectra is supported by density functional theory calculations and molecular dynamics simulations. We present results for selected aprotic and protic ionic liquids and their mixtures with molecular solvents. In particular, we focus on the strength and type of intermolecular interaction and how both parameters are influenced by the character of the ions and their combinations. We show that the total interaction between cations and anions is a result of a subtle balance between Coulomb forces, hydrogen bonds and dispersion forces. For protic ionic liquids we could measure distinct vibrational modes in the low frequency spectra indicating clearly the cation anion interaction characterized by linear and medium to strong hydrogen bonds. Using isotopic substitution we have been able to dissect frequency shifts related to pure interaction strength between cations and anions and to different reduced masses only. In this context we also show how these different types of interaction may influence the physical properties of ionic liquids such as the melting point, viscosity or enthalpy of vaporization. Furthermore we demonstrate that low frequency spectroscopy can also be used for studying ion speciation. Low vibrational features can be assigned to contact ion pairs and solvent separated ion pairs. In conclusion we showed how detailed knowledge of the low frequency spectra can be used to understand the change in interaction strength and structure by variation of temperature, solvent polarity and solvent concentration in ionic liquids and their mixtures with molecular solvents. In principle the used combination of methods is suitable for studying intermolecular interaction in pure molecular liquids and their solutions including additive materials such as nanoparticles.