Ludwig Martin, Hillebrecht Harald
Institut für Anorganische und Analytische Chemie, Albert-Ludwigs-Universität, Albertstr. 21, 79104 Freiburg, Germany.
Phys Chem Chem Phys. 2021 Feb 19;23(6):3883-3897. doi: 10.1039/d0cp06073a.
Based on the work on referencing 11B nuclear magnetic resonance (NMR) spectra for molecular icosahedral boranes and the subsequent transfer to the rhombohedral boron-rich borides of the α-rB12 type, we show that the magic angle spinning (MAS) NMR spectra of boron-rich borides with four or five symmetry-independent boron atoms can also be calculated. The calculations are performed on the level of density functional theory (DFT) using the gauge-including projector-augmented wave (GIPAW) approach. As model compounds o-MgB12C2 and MgB7 are used, for which the experimental spectra could be calculated in excellent agreement with a deviation of 1 to 2 ppm. Based on the calculations, the different B atoms can be assigned to the respective signals, taking into account the quadrupolar coupling constants Cq from computation of the electric field gradient (EFG) with its main axis Vzz. It is shown that due to the specific geometric conditions of icosahedra, the magnitudes of Vzz for the boron atoms involved in exohedral B-B bonds to neighbouring icosahedra depend only on the valence electron density of the bond critical point and the distance. This also applies to the bonds to the interstitial B2 unit in MgB7, but not to bonds to the heteroatom of the C2 dumbbell in o-MgB12C2. Both results are in line with our previous observations for the rhombohedral species (α-rB12; B12X2 with X = P, As, O). Finally, the spectrum of γ-B28 was calculated, whose structure also contains B12 icosahedra and interstitial B2 dumbbells. Here, a very similar bonding situation is found for the icosahedron, but the calculations show that the situation for the B2 unit is clearly different. In general, the only parameter that needs to be varied to fit calculated and measured spectra is the linewidth, as this cannot be calculated. For the cases of o-MgB12C2 and MgB7 signal areas are related to corresponding site multiplicities. A prerequisite for the successful application of the chosen method seems to be the presence of a semiconductor with a sufficiently large band gap, which is the case for the compounds investigated.
基于对二十面体分子硼烷的11B核磁共振（NMR）光谱的研究工作以及随后向α-rB12型菱面体富硼硼化物的转移，我们表明具有四个或五个对称独立硼原子的富硼硼化物的魔角旋转（MAS）NMR光谱也可以计算。这些计算是在密度泛函理论（DFT）水平上使用包含规范的投影增强波（GIPAW）方法进行的。使用o-MgB12C2和MgB7作为模型化合物，对于它们可以计算出与实验光谱非常吻合的结果，偏差为1至2 ppm。基于这些计算，考虑到通过电场梯度（EFG）计算得到的四极耦合常数Cq及其主轴Vzz，可以将不同的B原子分配到各自的信号上。结果表明，由于二十面体的特定几何条件，参与与相邻二十面体的外表面B-B键的硼原子的Vzz大小仅取决于键临界点的价电子密度和距离。这也适用于与MgB7中的间隙B2单元的键，但不适用于与o-MgB12C2中C2哑铃状杂原子的键。这两个结果与我们之前对菱面体物种（α-rB12；B12X2，其中X = P、As、O）的观察结果一致。最后，计算了γ-B28的光谱，其结构也包含B12二十面体和间隙B2哑铃状结构。在这里，发现二十面体的键合情况非常相似，但计算表明B2单元的情况明显不同。一般来说，为了使计算光谱与测量光谱拟合而需要改变的唯一参数是线宽，因为线宽无法计算。对于o-MgB12C2和MgB7的情况，信号面积与相应的位点多重性相关。成功应用所选方法的一个先决条件似乎是存在具有足够大带隙的半导体，所研究的化合物就是这种情况。
