11B MAS NMR and First-Principles Study of the [OBO3] Pyramids in Borates

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• 作者：Bing Zhou ; Wei Sun ; Biao-Chun Zhao ; Jin-Xiao Mi ; Robert Laskowski ; Victor Terskikh ; Xi Zhang ; Lingyun Yang ; Sanda M. Botis ; Barbara L. Sherriff ; Yuanming Pan
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：March 7, 2016
• 年：2016
• 卷：55
• 期：5
• 页码：1970-1977
• 全文大小：421K
• ISSN：1520-510X

文摘

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) ¹¹B magic-angle spinning nuclear magnetic resonance (MAS NMR), triple quantum (3Q) MAS NMR experiments, as well as density functional theory calculations. The high-resolution ¹¹B MAS NMR spectra supported by theoretical predictions show that the [OBO₃] pyramids are characterized by isotropic chemical shifts δ_iso(¹¹B) from 1.4(1) to 4.9(1) ppm and nuclear quadrupole parameters C_Q(¹¹B) up to 1.3(1) MHz, both significantly different from those of the [BO₄] and [BO₃] groups in borates. These δ_iso(¹¹B) and C_Q(¹¹B) values indicate that the [OBO₃] pyramids represent an intermediate state between the [BO₄] tetrahedra and [BO₃] triangles and demonstrate that the ¹¹B NMR parameters of four-coordinate boron oxyanions are sensitive to local structural environments. The orientation of the calculated unique electronic field gradient tensor element V_zz of the [OBO₃] pyramids is aligned approximately along the direction of the anomalously long B–O bond, corresponding to B-2p_z with the lowest electron density.