Ludwig Martin, Hillebrecht Harald
Institut für Anorganische und Analytische Chemie, Albert-Ludwigs-Universität, Albertstr. 21, 79104 Freiburg, Germany.
Phys Chem Chem Phys. 2021 Jan 6;23(1):470-486. doi: 10.1039/d0cp04061d.
In the present study, solid-state nuclear magnetic resonance (NMR) spectra under magic angle spinning conditions of the rhombohedral structures α-B and B12P2 are reported together with the corresponding parameter sets from first principles calculations on α-B B12X2 (X = P, As, O). With the combination of density functional theory (DFT) and the gauge-including projector-augmented wave (GIPAW) approach as the theoretical tools at hand the computed 11B parameters lead to unambiguous explanation of the measurements. Thereby, we overcome common obstacles of processing recorded NMR spectra of solid-state compounds with several crystallographic positions, in particular non-trivial signal assignments and parameter determination due to peak overlap or even unexpected intensity/area ratios. In fact, we find very good agreement between the theoretical results and measured spectra without applying fitting procedures. Using the Perdew-Burke-Ernzerhof (PBE) functional, the results of the common construction types for pseudopotentials and referencing methods for the chemical shift determination are compared. Suggestions and conclusions from experimental 11B NMR studies on parameters according to the icosahedral positions are critically discussed, for instance the early suspected correlation to chemical shifts is not confirmed. Regarding the electric field gradient (EFG) a detailed explanation for obtaining small deviations amongst all investigated structures of the icosahedral polar sites compared to the equatorial sites is given. Our results show an important link between the exohedral bonding situation of compounds with icosahedral structure elements and the main axis of the EFG and therefore, also measurable quadrupole coupling constants if certain geometrical conditions are fulfilled. Finally, this work also contributes to establishing the number of unique sites measured by solid-state NMR methods within the modification of β-B.
在本研究中，报道了菱面体结构α - B和B12P2在魔角旋转条件下的固态核磁共振（NMR）光谱，以及对α - B B12X2（X = P、As、O）进行第一性原理计算得到的相应参数集。以密度泛函理论（DFT）和含规范投影增强波（GIPAW）方法相结合作为手头的理论工具，计算得到的11B参数对测量结果给出了明确的解释。由此，我们克服了处理具有多个晶体学位置的固态化合物记录的NMR光谱时的常见障碍，特别是由于峰重叠甚至意外的强度/面积比导致的非平凡信号归属和参数确定问题。事实上，我们发现理论结果与测量光谱之间非常吻合，无需应用拟合程序。使用Perdew - Burke - Ernzerhof（PBE）泛函，比较了赝势的常见构造类型和化学位移确定的参考方法的结果。批判性地讨论了根据二十面体位置对11B NMR参数进行实验研究得出的建议和结论，例如早期怀疑的与化学位移的相关性未得到证实。关于电场梯度（EFG），给出了在二十面体极位与赤道位的所有研究结构之间获得小偏差的详细解释。我们的结果表明，具有二十面体结构元素的化合物的外表面键合情况与EFG的主轴之间存在重要联系，因此，如果满足某些几何条件，也与可测量的四极耦合常数有关。最后，这项工作也有助于确定在β - B变体中通过固态NMR方法测量的独特位点的数量。
