Probing relaxation models by means of Fast Field-Cycling relaxometry, NMR spectroscopy and molecular dynamics simulations: Detailed insight into the translational and rotational dynamics of a protic ionic liquid
Probing relaxation models by means of Fast Field-Cycling relaxometry, NMR spectroscopy and molecular dynamics simulations: Detailed insight into the translational and rotational dynamics of a protic ionic liquid
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DOI：
10.1016/j.molliq.2020.114207
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发表时间：
2020-12-01
影响因子：
6
通讯作者：
Ludwig, Ralf
中科院分区：
文献类型：
--
作者：
Overbeck, Viviane;Golub, Benjamin;Ludwig, Ralf
温度依赖的快速场循环（FFC）弛豫和高分辨率NMR光谱的组合提供了超过活的数量级的频率范围内的离子液体的平移和旋转动力学的研究。然而，为了利用这个宽的频率范围，必须满足某些要求：液体的粘度必须具有正确的数量级，并且材料必须表现出足够宽的液体范围。此外，NMR敏感核必须（理想地）存在于阳离子和阴离子上，并且必须提供适当描述分子动力学的适当弛豫模型。对于后者，分子动力学（MD）模拟非常适合提出有意义的弛豫模型。在这项研究中，我们采用质子离子液体三乙基铵双（三氟甲基磺酰基）-酰亚胺[TEA] NTf 2]作为模型化合物，以深入了解分子动力学过程的细节。通过寻址两种离子上的不同NMR敏感核，三乙基铵阳离子上的H-1核和NTf 2阴离子上的F-19，我们能够同时获得两种物质的平移动力学和旋转动力学。得到的温度依赖的平移扩散系数与我们的MD模拟是一致的，并发现与文献中报道的数据一致。此外，两种类型的NMR弛豫过程被用来研究分子内弛豫：（1）偶极弛豫在低频（采用FFC）寻址H-1和F-19核，和（2）四极弛豫在高频寻址H-2核。在近定量协议与MD模拟，我们表明，通过FFC获得的偶极松弛和通过高场NMR获得的四极松弛导致一致的旋转相关时间，这表明阳离子的旋转运动是占主导地位的各向同性。因此，通过FFC确定的阳离子的分子内H-1弛豫速率可以适当地由单个旋转相关时间表示，与Bloembergen珀塞尔磅（BPP）方法兼容。然而，分子动力学模拟的NTF 2阴离子，表明其重取向动力学是强烈的各向异性。因此，适当的描述的阴离子的分子内F-19弛豫速率需要一个更复杂的模型超出BPP的方法，采用至少两个相关时间：通过考虑内部旋转的CF 3基团，我们能够确定相关时间与我们的MD模拟兼容。(C)2020爱思唯尔B. V.保留所有权利。
A combination of temperature dependent Fast Field-Cycling (FFC) relaxometry and high-resolution NMR spectroscopy provides more than live orders of magnitude in frequency range for studying translational and rotational dynamics of ionic liquids. However, to make use of this broad frequency range, certain requirements have to be met: The viscosity of the liquid has to be of the right order of magnitude, and the material has to exhibit a sufficiently broad liquid range. In addition, NMR sensitive nuclei have to be present (ideally) on both, cation and anion, and appropriate relaxation models properly describing the molecular dynamics have to be available. For the latter, Molecular Dynamics (MD) simulations are ideally suited to suggest meaningful relaxation models. In this study we employ the protic ionic liquid triethylammonium bis(trifiuoromethylsulfonyl)-imide [TEA]NTf2] as model compound to gain insight into details of the molecular dynamical processes. By addressing different NMR sensitive nuclei on both ions, H-1 nudei on the triethylammonium cation, and F-19 on the NTf2 anion, we arc able to obtain translational dynamics as well as rotational dynamics for both species at the same time. The obtained temperature dependent translatoric diffusion coefficients are consistent with our MD simulations, and are found to be in agreement with data reported in the literature. In addition, two types of NMR relaxation processes are employed to investigate the intramolecular relaxation: (1) dipolar relaxation at low frequencies (employing FFC) addressing H-1 and F-19 nuclei, and (2) quadrupolar relaxation at high frequencies addressing H-2 nuclei. In near quantitative agreement with MD simulations, we show that both, dipolar relaxation obtained via FFC and quadrupolar relaxation obtained via high-field NMR are leading to consistent rotational correlation times, suggesting that the rotational motion of the cation is dominantly isotropic. Hence, the intramolecular H-1 relaxation rate of the cation determined via FFC can be appropriately expressed by a single rotational correlation time, compatible with the Bloembergen Purcell Pound (BPP) approach. MD simulations of the NTf2 anion, however, suggest that its reorientational dynamics is strongly anisotropic. Consequently, the proper description of the intramolecular F-19 relaxation rate of the anion requires a more complex model beyond the BPP approach employing at least two correlation times: By taking internal rotation of the CF3 groups into account, we are able to determine correlation times compatible with our MD simulations. (C) 2020 Elsevier B.V. All rights reserved.