Anorganische Chemie III and Nordbayerisches NMR Zentrum, Universität Bayreuth, 95440 Bayreuth, Germany.
Department Chemie, Johannes Gutenberg-Universität, 55128 Mainz, Germany.
J Phys Chem B. 2021 Dec 16;125(49):13519-13532. doi: 10.1021/acs.jpcb.1c06722. Epub 2021 Dec 3.
Spin-lattice relaxation rates (ω,), probed via high-field and field-cycling nuclear magnetic resonance (NMR), are used to test the validity of frequency-temperature superposition (FTS) for the reorientation dynamics in viscous liquids. For several liquids, FTS is found to apply so that master curves can be generated. The susceptibility spectra are highly similar to those obtained from depolarized light scattering (DLS) and reveal an excess wing. Where FTS works, two approaches are suggested to access the susceptibility: (i) a plot of deuteron () vs the spin-spin relaxation rate () and (ii) a plot of () vs an independently measured reference time τ(). Using single-frequency scans, (i) allows one to extract the relaxation stretching as well as the NMR coupling constant. Surveying 26 data sets, we find Kohlrausch functions with exponents 0.39 < β ≤ 0.67. Plots of the spin-spin relaxation rate ─rescaled by the NMR coupling constant─as a function of temperature allow one to test how well site-specific NMR relaxations couple to a given reference process. Upon cooling of flexible molecule liquids, the site-specific dynamics is found to merge, suggesting that near the molecules reorient essentially as a rigid entity. This presents a possible resolution for the much lower stretching parameters reported here at high temperatures that contrast with the ones that were reported to be universal in a recent DLS study close to . Our analysis underlines that deuteron relaxation is a uniquely powerful tool to probe single-particle reorientation.
自旋晶格弛豫率（ω,），通过高场和场循环核磁共振（NMR）探测，用于测试重定向动力学在粘性液体中的频率-温度叠加（FTS）的有效性。对于几种液体，发现 FTS 适用，因此可以生成主曲线。磁化率谱与从去极化光散射（DLS）获得的谱非常相似，并显示出过剩翼。在 FTS 适用的情况下，建议使用两种方法来获取磁化率：（i）氘核（）与自旋-自旋弛豫率（）的图，以及（ii）（）与独立测量的参考时间 τ（）的图。使用单频扫描，（i）可以提取弛豫拉伸以及 NMR 耦合常数。调查了 26 个数据集，我们发现 Kohlrausch 函数的指数为 0.39 <β≤ 0.67。将自旋-自旋弛豫率 ─通过 NMR 耦合常数进行缩放─作为温度的函数绘制，可以测试局域 NMR 弛豫与给定参考过程的耦合程度如何。在柔性分子液体冷却时，发现局域动力学合并，表明在接近时，分子基本上作为刚性实体进行重定向。这为在最近的 DLS 研究中接近时报道的普遍存在的参数提供了一个可能的解决方案，该研究报道了在高温下报告的拉伸参数要低得多。我们的分析强调了氘核弛豫是探测单粒子重定向的独特有力工具。
