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) NMR 弛豫测量以及分子动力学 (MD) 模拟来研究离子液体 (IL) 中的旋转和平移运动。结合专门合成的 IL，我们能够在广泛的频率和温度范围内进行测量和模拟。我们的方法可以测试常用的 NMR 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，即温度范围太窄和数据集太小，限制了科学家应用简单的 Bloembergen-Purcell-Pound (BBP) 关系。首先，我们研究了 IL 中阳离子的分子矢量 ND 和 OD 的纯分子内四极 (2H) 弛豫，这些都与氢键有关。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氘核四极耦合常数和旋转相关时间。其次，我们根据 FC 弛豫测量法在宽温度和频率范围内测量的偶极弛豫率（1H 和 19F）确定旋转和平移动力学，以揭示光谱密度的细节。这里的挑战是以可靠的方式将核磁弛豫色散（NMRD）曲线的总弛豫率剖析为分子内和分子间的贡献。该过程将通过使用部分氘化 IL 来抑制 1H 弛豫来支持。解决阳离子和阴离子内的不同分子载体可以验证各向异性或内旋转。我们将所得的平移扩散系数与从低频色散定律和脉冲场梯度核磁共振获得的平移扩散系数进行比较。第三，对于最相关的 IL，我们从经典 MD 模拟中确定相关函数、耦合参数和弛豫率。这里的挑战是通过实验解决重叠的温度范围。这可以证明我们用于评估测量的松弛数据的基础松弛模型的合理性。除了验证弛豫模型之外，实验和模拟方法的结合还可以在分子水平上深入了解 IL 的结构和动力学。

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.