A series of well-characterized magnesium borate minerals and synthetic analogues have been studied via ultrahigh-field ²⁵Mg solid-state NMR spectroscopy. Correlations between ²⁵Mg NMR parameters and the local structure at the magnesium site(s) are highlighted and discussed. First-principles density functional theory calculations of ²⁵Mg NMR parameters carried out with the WIEN2k software package support our experimental ²⁵Mg NMR data. Experimental ²⁵Mg C_Q values range from 0.7 ± 0.1 MHz in hungchaoite to 18.0 ± 0.5 MHz in boracite-type Mg₃B₇O₁₃Br. To our knowledge, the latter C_Q value is the largest reported ²⁵Mg nuclear quadrupole coupling constant. In general, C_Q values correlate positively with the degree of geometrical distortion at the Mg site, despite the diversity in nearest-neighbor ligands (O^2–, OH^–, H₂O, F^–, Cl^–, and Br^–) across the series of magnesium borates. Experimental δ_iso values range from 0.2 ± 0.5 ppm in hungchaoite to 23 ± 3 ppm in grandidierite, which are within the expected chemical shift range for diamagnetic magnesium borates.

中文翻译：

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硼酸镁矿物的超高场²⁵ Mg NMR和DFT研究

通过超高场²⁵ Mg固态NMR光谱研究了一系列特性良好的硼酸镁矿物和合成类似物。²⁵ Mg NMR参数与镁位点的局部结构之间的相关性得到强调和讨论。使用WIEN2k软件包对²⁵ Mg NMR参数进行第一性原理密度泛函理论计算，可支持我们的实验性²⁵ Mg NMR数据。实验性的²⁵ Mg C _Q值范围从红潮石中的0.7±0.1 MHz到硼铁矿型Mg ₃ B ₇ O ₁₃ Br中的18.0±0.5 MHz 。就我们所知，后者C_Q值是报道的最大²⁵ Mg核四极耦合常数。通常，尽管最近邻配体（O ^2–，OH ^–，H ₂ O，F ^–，Cl ^–和Br ^–）的多样性，C _Q值与Mg处的几何变形程度呈正相关。硼酸镁系列。实验δ_异值的范围从在hungchaoite 0.2±0.5ppm至在grandidierite 23±3ppm的，这是预期的化学位移范围为反磁性镁的硼酸盐内。