Department of Natural Sciences, Northeastern State University , 611 N Grand Avenue, Tahlequah, Oklahoma 74464, United States.
J Phys Chem B. 2017 Apr 13;121(14):3099-3110. doi: 10.1021/acs.jpcb.7b00620. Epub 2017 Mar 29.
Ionic liquids are a fertile and active area of research, in part, due to the unique properties these solvents offer over traditional molecular solvents. Because these properties are rooted in the fundamental ion-ion interactions that govern their liquid structure, there is a strong motivation to characterize the liquid structure of ionic liquids. Infrared spectroscopy is a standard analytical tool for assessing liquid structures, for the intramolecular vibrational modes of the ions composing the materials are often quite sensitive to their local potential energy environment. In this work, we demonstrate that the band asymmetry for the ν(SNS) anion mode of N(Tf)-based ionic liquids originates from the dynamic coupling of vibrationally induced dipole moments of anions across a quasilattice. The magnitude of TO-LO splitting is linearly correlated with the number densities of the ionic liquids; an observation that is in accord with the predictions of dipolar coupling theory. Dipole moment derivatives of ν(SNS) calculated from dipolar coupling theory, (∂μ/∂q), are lower than those obtained from independent measurements of (∂μ/∂q). The most likely explanation for this disparity is that although ionic liquids possess sufficient long-range structure to support TO-LO splitting of infrared-active modes, there is enough orientational and translational disorder in the quasilattice to partially disrupt the coupling of vibrationally induced dipole moments across the quasilattice. This will result in diminished amounts of TO-LO splitting than would be expected if the ionic liquid were a perfect crystal at 0 K. Impacts of cation molecular structure and the formation of a binary solution on the liquid structure are also explored.
离子液体是一个充满活力和活跃的研究领域，部分原因是这些溶剂具有传统分子溶剂所不具备的独特性质。由于这些性质源于控制其液体结构的基本离子-离子相互作用，因此强烈需要表征离子液体的液体结构。红外光谱是评估液体结构的标准分析工具，因为构成材料的离子的分子内振动模式通常对其局部势能环境非常敏感。在这项工作中，我们证明了基于 N(Tf)的离子液体的 ν(SNS)阴离子模式的带不对称性源于阴离子振动诱导偶极矩在准晶格中的动态耦合。TO-LO 分裂的大小与离子液体的数密度线性相关；这一观察结果与偶极耦合理论的预测一致。从偶极耦合理论计算的 ν(SNS)的偶极矩导数（∂μ/∂q）低于从独立测量得到的（∂μ/∂q）。造成这种差异的最可能原因是，尽管离子液体具有足够的远程结构来支持红外活性模式的 TO-LO 分裂，但准晶格中存在足够的取向和平移无序，部分破坏了准晶格中振动诱导偶极矩的耦合。如果离子液体在 0 K 时是完美的晶体，那么这将导致 TO-LO 分裂的量减少。还探讨了阳离子分子结构和二元溶液的形成对液体结构的影响。
