Karjalainen Jouni, Vaara Juha, Straka Michal, Lantto Perttu
NMR Research Group, University of Oulu, Oulu, Finland.
Phys Chem Chem Phys. 2015 Mar 21;17(11):7158-71. doi: 10.1039/c4cp04868g.
Applications of liquid crystals (LCs), such as smart windows and the ubiquitous display devices, are based on controlling the orientational and translational order in a small volume of LC medium. Hence, understanding the effects of confinement to the liquid crystal phase behaviour is essential. The NMR shielding of (129)Xe atoms dissolved in LCs constitutes a very sensitive probe to the details of LC environment. Linking the experimental results to microscopic phenomena calls for molecular simulations. In this work, the NMR shielding of atomic (129)Xe dissolved in a uniaxial thermotropic LC confined to nanosized cylindrical cavities is computed from coarse-grained (CG) isobaric Monte Carlo (MC) simulations with a quantum-chemically (QC) pre-parameterised pairwise-additive model for the Xe nuclear shielding tensor. We report the results for the (129)Xe nuclear shielding and its connection to the structure and order of the LC appropriate to two different cavity sizes, as well as a comparison to the results of bulk (non-confined) simulations. We find that the confinement changes the LC phase structure dramatically and gives rise to the coexistence of varying degrees of LC order, which is reflected in the Xe shielding. Furthermore, we qualitatively reproduce the behaviour of the mean (129)Xe chemical shift with respect to temperature for atomic Xe dissolved in LC confined to controlled-pore glass materials. In the small-radius cavity the nematic - paranematic phase transition is revealed only by the anisotropic component of the (129)Xe nuclear shielding. In the larger cavity, the nematic - paranematic - isotropic transition is clearly seen in the Xe shielding. The simulated (129)Xe NMR shielding is insensitive to the smectic-A - nematic transition, since in the smectic-A phase, the Xe atoms largely occupy the imperfect layer structure near the cavity walls. The direct contribution of the cavity wall to (129)Xe nuclear shielding is dependent on the cavity size but independent of temperature. Our results show that the combination of CG simulations and a QC pre-parameterised (129)Xe NMR shielding allows efficient studies of the phase behaviour and structure of complex systems containing thousands of molecules, and brings us closer to the simulation of NMR experiments.
液晶（LC）的应用，如智能窗户和无处不在的显示设备，是基于控制小体积LC介质中的取向和移动有序性。因此，了解限制对液晶相行为的影响至关重要。溶解在LC中的（129）Xe原子的核磁共振屏蔽构成了对LC环境细节非常敏感的探针。将实验结果与微观现象联系起来需要分子模拟。在这项工作中，通过粗粒化（CG）等压蒙特卡罗（MC）模拟，使用量子化学（QC）预参数化的成对加和模型计算Xe核屏蔽张量，计算了溶解在限制于纳米级圆柱腔中的单轴热致液晶中的原子（129）Xe的核磁共振屏蔽。我们报告了（129）Xe核屏蔽的结果及其与适合两种不同腔尺寸的LC的结构和有序性的关系，以及与体相（非限制）模拟结果的比较。我们发现限制显著改变了LC相结构，并导致不同程度的LC有序性共存，这反映在Xe屏蔽中。此外，我们定性地再现了溶解在限制于可控孔径玻璃材料中的LC中的原子Xe的平均（129）Xe化学位移相对于温度的行为。在小半径腔中，向列相 - 顺列相转变仅通过（129）Xe核屏蔽的各向异性分量揭示。在较大的腔中，在Xe屏蔽中可以清楚地看到向列相 - 顺列相 - 各向同性转变。模拟的（129）Xe核磁共振屏蔽对近晶A相 - 向列相转变不敏感，因为在近晶A相中，Xe原子主要占据腔壁附近的不完美层结构。腔壁对（129）Xe核屏蔽的直接贡献取决于腔尺寸，但与温度无关。我们的结果表明，CG模拟和QC预参数化的（129）Xe核磁共振屏蔽的结合允许对包含数千个分子的复杂系统的相行为和结构进行有效研究，并使我们更接近核磁共振实验的模拟。
