Towards reliable description of rotational and translational motion in ionic liquids by means of field cycling and high field NMR relaxometry as well as molecular dynamics simulations

在这个项目中，我们希望通过高场（HF）和场循环（FC）核磁共振弛豫测量以及分子动力学（MD）模拟来研究离子液体（ILs）的旋转和平移运动。结合特殊合成的ILs，我们能够在宽频率和温度范围内进行测量和模拟。我们的方法允许测试常用的核磁共振弛豫模型的适用性，并为更复杂的现象提出模型。通过这种方式，我们克服了早期的问题，即过于狭窄的温度范围和小数据集限制了科学家们应用简单的Bloembergen-Purcell-Pound （BBP）关系。在这里，我们首先研究了纯分子内四极性（2H）弛豫的分子载体ND和OD的离子，它们都参与氢键。广泛的液体范围低至玻璃化转变温度，提供频率相关信息，并允许考虑更复杂的模型，包括各向异性或内部运动。我们还确定了以氢键与不同强相互作用阴离子为特征的键的可靠的氘核四极耦合常数和旋转相关时间。其次，我们通过FC弛豫仪在宽温度和频率范围内测量的偶极弛豫率（1H和19F）来确定旋转和平动动力学，以揭示谱密度的细节。在这里，挑战在于以可靠的方式将核磁弛豫色散（NMRD）谱的总弛豫率分解为分子内和分子间的贡献。这个过程将通过使用部分氘化的il来抑制1H弛豫来支持。在阳离子和阴离子中寻址不同的分子向量允许验证各向异性或内部旋转。我们将得到的平动扩散系数与低频色散定律和脉冲场梯度核磁共振得到的平动扩散系数进行了比较。第三，我们从经典MD模拟中确定了最相关il的相关函数、耦合参数和弛豫率。这里的挑战是通过实验来解决重叠的温度范围。这允许证明我们用于评估测量松弛数据的潜在松弛模型。除了验证弛豫模型之外，实验和模拟方法的结合还提供了在分子水平上对ILs结构和动力学的深入了解。

In this project we want to study the rotational and translational motion in ionic liquids (ILs) by means of high field (HF) and field cycling (FC) NMR relaxometry as well as molecular dynamics (MD) simulations. In combination with specially synthesized ILs, we are able to measure and simulate over broad frequency and temperature ranges. Our approach allows testing the applicability of commonly used NMR relaxation models and suggesting models for more complex phenomena. This way, we overcome earlier problems, where too narrow temperature ranges and small data sets constrained scientists to the application of the simple Bloembergen-Purcell-Pound (BBP) relation.Here firstly, we study purely intramolecular quadrupolar (2H) relaxation for molecular vectors ND and OD of the cations in the ILs, which are all involved in hydrogen bonding. The broad liquid ranges down to glass transition temperature, provide frequency dependent information and allow for considering more sophisticated models including anisotropic or internal motion. We also determine reliable deuteron quadrupole coupling constants and rotational correlation times for bonds characterized by hydrogen bonds with differently strong interacting anions. Secondly, we determine rotational and translational dynamics from dipolar relaxation rates (1H and 19F) measured by FC relaxometry for broad temperature and frequency ranges to unravel details of the spectral densities. Here, the challenge is to dissect the total relaxation rates of the nuclear magnetic relaxation dispersion (NMRD) profiles into intra- and intermolecular contributions in a reliable way. This procedure will be supported by using partially deuterated ILs for suppressing 1H relaxation. Addressing different molecular vectors within the cations and anions allow validating anisotropic or internal rotation. The resulting translational diffusion coefficients we compare with those obtained from the low frequency dispersion law and pulsed field gradient NMR. Thirdly, for the most relevant ILs we determine the correlation functions, coupling parameters and relaxation rates from classical MD simulations. The challenge here is to address overlapping temperature ranges with experiment. That allows justifying the underlying relaxation models we used for evaluating the measured relaxation data. Beyond validating the relaxation models this combination of experimental and simulation methods provides insight into structure and dynamics of ILs at the molecular level.