Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovsky Square 2, 16206 Prague, Czech Republic.
Molecules. 2019 May 3;24(9):1731. doi: 10.3390/molecules24091731.
The H chemical shielding anisotropy (CSA) is an NMR parameter that is exquisitely sensitive to the local environment of protons in crystalline systems, but it is difficult to obtain it experimentally due to the need to concomitantly suppress other anisotropic interactions in the solid-state NMR (SSNMR) pulse sequences. The SSNMR measurements of the H CSA are particularly challenging if the fast magic-angle-spinning (MAS) is applied. It is thus important to confront the results of both the single-crystal (SC) and fast-MAS experiments with their theoretical counterparts. Here the plane-waves (PW) DFT calculations have been carried out using two functionals in order to precisely characterize the structures and the H NMR chemical shielding tensors (CSTs) of the solid forms of maleic, malonic, and citric acids, and of L-histidine hydrochloride monohydrate. The level of agreement between the PW DFT and either SC or fast-MAS SSNMR H CSA data has been critically compared. It has been found that for the eigenvalues of the H CSTs provided by the fast-MAS measurements, an accuracy limit of current PW DFT predictions is about two ppm in terms of the standard deviation of the linear regression model, and sources of this error have been thoroughly discussed.
H 化学位移各向异性（CSA）是 NMR 参数，对晶体系统中质子的局部环境非常敏感，但由于需要在固态 NMR（SSNMR）脉冲序列中同时抑制其他各向异性相互作用，因此很难在实验中获得。如果应用快速魔角旋转（MAS），则对 H CSA 的 SSNMR 测量尤其具有挑战性。因此，用理论对照来对比单晶（SC）和快速 MAS 实验的结果非常重要。在这里，使用两种泛函进行了平面波（PW）DFT 计算，以便精确地描述马来酸、丙二酸和柠檬酸的固体形式以及 L-组氨酸盐酸盐一水合物的结构和 H NMR 化学屏蔽张量（CST）。仔细比较了 PW DFT 与 SC 或快速 MAS SSNMR H CSA 数据之间的一致性。结果发现，对于快速 MAS 测量提供的 H CST 的本征值，当前 PW DFT 预测的精度极限约为 2 ppm，这是线性回归模型的标准偏差，并且彻底讨论了该误差的来源。
