University of Ulm, 89069 Ulm, Germany.
Institute of Physics, Kazan Federal University, Kazan 420008 Tatarstan, Russia.
Prog Nucl Magn Reson Spectrosc. 2017 Aug;101:18-50. doi: 10.1016/j.pnmrs.2017.04.001. Epub 2017 Apr 9.
Field-cycling NMR relaxometry is a well-established technique for probing molecular dynamics in a frequency range from typically a few kHz up to several tens of MHz. For the interpretation of relaxometry data, it is quite often assumed that the spin-lattice relaxation process is of an intra-molecular nature so that rotational fluctuations dominate. However, dipolar interactions as the main type of couplings between protons and other dipolar species without quadrupole moments can imply appreciable inter-molecular contributions. These fluctuate due to translational displacements and to a lesser degree also by rotational reorientations in the short-range limit. The analysis of the inter-molecular proton spin-lattice relaxation rate thus permits one to evaluate self-diffusion variables such as the diffusion coefficient or the mean square displacement on a time scale from nanoseconds to several hundreds of microseconds. Numerous applications to solvents, plastic crystals and polymers will be reviewed. The technique is of particular interest for polymer dynamics since inter-molecular spin-lattice relaxation diffusometry bridges the time scales of quasi-elastic neutron scattering and field-gradient NMR diffusometry. This is just the range where model-specific intra-coil mechanisms are assumed to occur. They are expected to reveal themselves by characteristic power laws for the time-dependence of the mean-square segment displacement. These can be favorably tested on this basis. Results reported in the literature will be compared with theoretical predictions. On the other hand, there is a second way for translational diffusion phenomena to affect the spin-lattice relaxation dispersion. If rotational diffusion of molecules is restricted, translational diffusion properties can be deduced even from molecular reorientation dynamics detected by intra-molecular spin-lattice relaxation. This sort of scenario will be relevant for adsorbates on surfaces or polymer segments under entanglement and chain connectivity constraints. Under such conditions, reorientations will be correlated with translational displacements leading to the so-called RMTD relaxation process (reorientation mediated by translational displacements). Applications to porous glasses, protein solutions, lipid bilayers, and clays will be discussed. Finally, we will address the intriguing fact that the various time limits of the segment mean-square displacement of polymers in some cases perfectly reproduce predictions of the tube/reptation model whereas the reorientation dynamics suggests strongly deviating power laws.
场频变 NMR 弛豫谱是一种成熟的技术，可用于探测分子动力学，其频率范围通常从几 kHz 到几十 MHz。为了解释弛豫谱数据，人们经常假设自旋晶格弛豫过程是分子内的，因此旋转波动占主导地位。然而，偶极相互作用作为质子与其他没有四极矩的偶极子之间的主要耦合类型，可以暗示相当大的分子间贡献。这些波动是由于平移位移引起的，在短程极限下，也有较小程度的旋转重取向引起的。分析分子间质子自旋晶格弛豫率可以评估自扩散变量，如扩散系数或在纳秒到几百微秒的时间尺度上的均方位移。将对溶剂、塑料晶体和聚合物进行大量应用综述。该技术对于聚合物动力学特别感兴趣，因为分子间自旋晶格弛豫扩散谱学在准弹性中子散射和场梯度 NMR 扩散谱学的时间尺度上架起了桥梁。这正是假设特定于模型的内线圈机制发生的范围。预计它们会通过均方段位移时间依赖性的特征幂律来显现自己。可以在此基础上进行有利的测试。文献中报道的结果将与理论预测进行比较。另一方面，平移扩散现象影响自旋晶格弛豫色散的第二种方式。如果分子的旋转扩散受到限制，即使从通过分子内自旋晶格弛豫检测到的分子重取向动力学中，也可以推导出平移扩散性质。这种情况将与表面上的吸附物或缠结和链连接性约束下的聚合物段有关。在这种情况下，重取向将与平移位移相关，导致所谓的 RMTD 弛豫过程（通过平移位移介导的重取向）。将讨论多孔玻璃、蛋白质溶液、脂质双层和粘土的应用。最后，我们将探讨一个有趣的事实，即聚合物段的均方位移的各种时间限制在某些情况下完美地再现了管/蠕动模型的预测，而重取向动力学则表明强烈偏离幂律。
