Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Iran.
Phys Chem Chem Phys. 2009 Oct 21;11(39):8821-8. doi: 10.1039/b905233j. Epub 2009 Jul 31.
Guest molecules in nonspherical cages of inclusion compounds can possess non-uniform spatial distributions and motion. This can lead to anisotropic lineshapes in the solid-state NMR spectra of the guest species. In this work, we use our previously developed molecular dynamics-based methodology to calculate the lineshape anisotropy of guest CO species in cages of the structure I (sI) clathrate hydrate as an example of the above phenomenon. The linear CO molecules in the oblate large sI clathrate hydrate cages have a preferential alignment in the plane parallel to the two hexagonal faces of the cages. Molecular dynamics simulations are performed at three temperatures in the stability range of the CO sI clathrate to determine the angular distribution of the CO guests in the large and small cages. The experimental (13)C NMR powder lineshapes of CO guests in the large cages become narrower as the temperature is increased from 77 K to 220 K (the limit of the stability of the clathrate). Good agreements between the calculated and experimental powder lineshapes are obtained. No assumptions regarding the nature of the guest motions in the cages are required. The dynamics of guest rotation are characterized by studying the orientational autocorrelation function (OACF) for the CO molecules in the large and small cages at different temperatures.
客体分子在包合物的非球笼中可能具有不均匀的空间分布和运动。这可能导致客体物种在固态 NMR 谱中的各向异性谱线形状。在这项工作中，我们使用先前开发的基于分子动力学的方法，以结构 I（sI）笼状水合物笼中客体 CO 物种的线宽各向异性为例，研究了上述现象。在扁长的大 sI 笼状水合物笼中的线性 CO 分子在平行于笼的两个六方面的平面中具有优先取向。在 CO sI 笼状水合物的稳定范围内的三个温度下进行分子动力学模拟，以确定大笼和小笼中 CO 客体的角分布。随着温度从 77 K 升高到 220 K（笼状水合物的稳定极限），CO 客体在大笼中的实验（13）C NMR 粉末谱线形状变窄。计算和实验粉末谱线形状之间得到了很好的一致性。不需要对笼中客体运动的性质做出任何假设。通过研究不同温度下大、小笼中 CO 分子的取向自相关函数（OACF）来表征客体旋转的动力学。
