B-11 MAS NMR and First-Principles Study of the [OBO3] Pyramids in Borates
B-11 MAS NMR and First-Principles Study of the [OBO3] Pyramids in Borates
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DOI：
10.1021/acs.inorgchem.5b02071
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发表时间：
2016
影响因子：
4.6
通讯作者：
Pan Yuanming
中科院分区：
文献类型：
--
作者：
Zhou Bing;Sun Wei;Zhao Biao-Chun;Mi Jin-Xiao;Laskowski Robert;Terskikh Victor;Zhang Xi;Yang Lingyun;Botis S;a M.;Sherriff Barbara L.;Pan Yuanming
硼酸盐由[B 3]平面三角形和[B 4]四面体基团构成，其中B表示O或OH。然而，某些硼酸盐中的[B 4]基团高度扭曲，包括三个正常的B-O键和一个异常长的B-O键，因此最好描述为[OBO 3]金字塔。采用多场（7.05，14.1，21.1 T）11B魔角自旋核磁共振（MAS NMR）、三重量子（3Q）MAS NMR实验以及密度泛函理论计算研究了四种含[OBO 3]金字塔结构的硼酸盐（Mg3 B7 O 13 Br，Cu 3B 7 O 13 Br，Zn 3B 7 O 13 Cl和Mg3 B7 O 13 Cl）.高分辨~（11）B MAS NMR谱表明，[OBO_3]金字塔的各向同性化学位移δiso（11 B）为1.4（1）~ 4.9（1）ppm，核四极矩CQ（11 B）为1.3（1）MHz，与硼酸盐中的[BO_4]和[BO_3]基团有显著差异。这些δiso（11 B）和CQ（11 B）值表明，[OBO 3]金字塔代表[BO 4]四面体和[BO 3]三角形之间的中间态，并表明四配位硼氧阴离子的11 B NMR参数对局部结构环境敏感。计算出的[OBO 3]金字塔的独特电子场梯度张量元素Vz的取向大致沿着异常长的B-O键的方向排列，对应于电子密度最低的B-2 pz。
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-resolution11B MAS NMR spectra supported by theoretical predictions show that the [OBO3] pyramids are characterized by isotropic chemical shifts δiso(11B) from 1.4(1) to 4.9(1) ppm and nuclear quadrupole parametersCQ(11B) up to 1.3(1) MHz, both significantly different from those of the [BO4] and [BO3] groups in borates. These δiso(11B) andCQ(11B) values indicate that the [OBO3] pyramids represent an intermediate state between the [BO4] tetrahedra and [BO3] triangles and demonstrate that the11B NMR parameters of four-coordinate boron oxyanions are sensitive to local structural environments. The orientation of the calculated unique electronic field gradient tensor elementVzzof the [OBO3] pyramids is aligned approximately along the direction of the anomalously long B–O bond, corresponding to B-2pzwith the lowest electron density.