Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie Pvt., Ottawa, Ontario, Canada.
J Phys Chem A. 2010 Feb 11;114(5):2102-16. doi: 10.1021/jp909106j.
Bromine-79/81 solid-state NMR (SSNMR) spectroscopy is established as a tool to characterize the local structure and symmetry about bromide ions in inorganic systems. Benchmark experimental (79/81)Br SSNMR data are acquired for CaBr(2), SrBr(2), BaBr(2), MgBr(2).6H(2)O, SrBr(2).6H(2)O, BaBr(2).2H(2)O, and CaBr(2).xH(2)O using the Solomon echo and/or QCPMG pulse sequences in magnetic fields of 11.75 and 21.1 T. Analytical line-shape analysis provides (79/81)Br electric field gradient (EFG) tensor parameters (including (79)Br quadrupolar coupling constants, C(Q)((79)Br), of up to 75.1(5) MHz in CaBr(2)), chemical shift tensor parameters (including the largest reported anisotropy), and the relative orientation of the tensor principal axis systems. These data are interpreted in terms of structure and symmetry. Our results indicate that ionic bromide systems should be generally accessible to characterization by (79/81)Br SSNMR despite sizable quadrupolar interactions. The resolving capabilities of (79/81)Br SSNMR spectroscopy are illustrated, using samples which possess up to four magnetically inequivalent sites, and through a rare example of (79)Br magic-angle spinning NMR for a Br in a noncubic lattice. Bromine-79/81 SSNMR spectroscopy is demonstrated to be sensitive to the presence of hydrates (i.e., pseudopolymorphism), via drastic changes in C(Q) and delta(iso). The changes are diagnostic to an extent that the composition of the mixture CaBr(2).xH(2)O is determined for the first time. This technique should therefore be applicable to characterize other unknown mixtures or polymorphs. Important instances where (79)Br nuclear quadrupole resonance data were found to be deficient are noted and corrected. GIPAW DFT computations are shown to be generally in very good agreement with the experimental (79/81)Br SSNMR observations. Finally, it is demonstrated that the origin of the EFG at the Br nuclei cannot be described quantitatively using a point charge model, even after including Sternheimer antishielding effects.
溴-79/81 固态 NMR（SSNMR）光谱已被确立为一种用于表征无机体系中溴化物离子局部结构和对称性的工具。使用 Solomon 回波和/或 QCPMG 脉冲序列，在磁场为 11.75 和 21.1 T 下，获得了 CaBr2、SrBr2、BaBr2、MgBr2·6H2O、SrBr2·6H2O、BaBr2·2H2O 和 CaBr2·xH2O 的基准实验（79/81）Br SSNMR 数据。分析谱线形状分析提供了（79/81）Br 电场梯度（EFG）张量参数（包括高达 75.1（5）MHz 的 CaBr2 中的（79）Br 四极耦合常数 C(Q)（79）Br）、化学位移张量参数（包括报告的最大各向异性）和张量主轴系统的相对取向。这些数据根据结构和对称性进行解释。我们的结果表明，尽管存在相当大的四极相互作用，但离子溴化物体系通常应该可以通过（79/81）Br SSNMR 进行表征。（79/81）Br SSNMR 光谱的分辨能力通过具有多达四个磁不等价位点的样品以及通过非立方晶格中 Br 的罕见（79）Br 魔角旋转 NMR 示例进行了说明。通过 C(Q)和 delta(iso) 的剧烈变化，证明溴-79/81 SSNMR 光谱对水合物（即假多晶型）的存在敏感。这种变化在一定程度上是诊断性的，以至于首次确定了混合物 CaBr2·xH2O 的组成。因此，该技术应该适用于表征其他未知混合物或多晶型物。注意并纠正了发现（79）Br 核四极共振数据存在缺陷的重要实例。GIPAW DFT 计算通常与实验（79/81）Br SSNMR 观察结果非常吻合。最后，证明即使包括 Sternheimer 反屏蔽效应，也不能使用点电荷模型定量描述 Br 核处的 EFG 起源。
