Zhou Bing, Sun Wei, Zhao Biao-Chun, Mi Jin-Xiao, Laskowski Robert, Terskikh Victor, Zhang Xi, Yang Lingyun, Botis Sanda M, Sherriff Barbara L, Pan Yuanming
College of Materials Science and Engineering, Tongji University , Shanghai 200092, China.
Department of Geological Sciences, University of Saskatchewan , Saskatoon, SK S7N 5E2, Canada.
Inorg Chem. 2016 Mar 7;55(5):1970-7. doi: 10.1021/acs.inorgchem.5b02071. Epub 2016 Feb 25.
Borates are built from the [Bϕ3] planar triangles and the [Bϕ4] tetrahedral groups, where ϕ denotes O or OH. However, the [Bϕ4] groups in some borates are highly distorted to include three normal B-O bonds and one anomalously long B-O bond and, therefore, are best described as the [OBO3] pyramids. Four synthetic borates of the boracite-type structures (Mg3B7O13Br, Cu3B7O13Br, Zn3B7O13Cl, and Mg3B7O13Cl) containing a range of [OBO3] pyramids were investigated by multifield (7.05, 14.1, and 21.1 T) (11)B magic-angle spinning nuclear magnetic resonance (MAS NMR), triple quantum (3Q) MAS NMR experiments, as well as density functional theory calculations. The high-resolution (11)B MAS NMR spectra supported by theoretical predictions show that the [OBO3] pyramids are characterized by isotropic chemical shifts δiso((11)B) from 1.4(1) to 4.9(1) ppm and nuclear quadrupole parameters CQ((11)B) up to 1.3(1) MHz, both significantly different from those of the [BO4] and [BO3] groups in borates. These δiso((11)B) and CQ((11)B) values indicate that the [OBO3] pyramids represent an intermediate state between the [BO4] tetrahedra and [BO3] triangles and demonstrate that the (11)B NMR parameters of four-coordinate boron oxyanions are sensitive to local structural environments. The orientation of the calculated unique electronic field gradient tensor element Vzz of the [OBO3] pyramids is aligned approximately along the direction of the anomalously long B-O bond, corresponding to B-2pz with the lowest electron density.
硼酸盐由[Bϕ3]平面三角形和[Bϕ4]四面体基团构成，其中ϕ表示O或OH。然而，一些硼酸盐中的[Bϕ4]基团高度扭曲，包含三个正常的B - O键和一个异常长的B - O键，因此，最好将其描述为[OBO3]金字塔结构。通过多场（7.05、14.1和21.1 T）（11）B魔角旋转核磁共振（MAS NMR）、三量子（3Q）MAS NMR实验以及密度泛函理论计算，研究了四种含有一系列[OBO3]金字塔结构的硼镁铁矿型结构的合成硼酸盐（Mg3B7O13Br、Cu3B7O13Br、Zn3B7O13Cl和Mg3B7O13Cl）。理论预测支持的高分辨率（11）B MAS NMR光谱表明，[OBO3]金字塔结构的特征是各向同性化学位移δiso（（11）B）为1.4(1)至4.9(1) ppm，核四极参数CQ（（11）B）高达1.3(1) MHz，这两者均与硼酸盐中的[BO4]和[BO3]基团有显著差异。这些δiso（（11）B）和CQ（（11）B）值表明，[OBO3]金字塔结构代表了[BO4]四面体和[BO3]三角形之间的中间状态，并证明了四配位硼氧阴离子的（11）B NMR参数对局部结构环境敏感。计算得到的[OBO3]金字塔结构的唯一电子场梯度张量元素Vzz的方向大致沿异常长的B - O键方向排列，对应于电子密度最低的B - 2pz。
