Department of Chemistry, University of Ottawa, Ontario, Canada.
Chemistry. 2012 Apr 27;18(18):5748-58. doi: 10.1002/chem.201103478. Epub 2012 Mar 20.
Multinuclear ((31)P and (79/81)Br), multifield (9.4, 11.75, and 21.1 T) solid-state nuclear magnetic resonance experiments are performed for seven phosphonium bromides bearing the triphenylphosphonium cation, a molecular scaffold found in many applications in chemistry. This is undertaken to fully characterise their bromine electric field gradient (EFG) tensors, as well as the chemical shift (CS) tensors of both the halogen and the phosphorus nuclei, providing a rare and novel insight into the local electronic environments surrounding them. New crystal structures, obtained from single-crystal X-ray diffraction, are reported for six compounds to aid in the interpretation of the NMR data. Among them is a new structure of BrPPh(4), because the previously reported one was inconsistent with our magnetic resonance data, thereby demonstrating how NMR data of non-standard nuclei can correct or improve X-ray diffraction data. Our results indicate that, despite sizable quadrupolar interactions, (79/81)Br magnetic resonance spectroscopy is a powerful characterisation tool that allows for the differentiation between chemically similar bromine sites, as shown through the range in the characteristic NMR parameters. (35/37)Cl solid-state NMR data, obtained for an analogous phosphonium chloride sample, provide insight into the relationship between unit cell volume, nuclear quadrupolar coupling constants, and Sternheimer antishielding factors. The experimental findings are complemented by gauge-including projector-augmented wave (GIPAW) DFT calculations, which substantiate our experimentally determined strong dependence of the largest component of the bromine CS tensor, δ(11), on the shortest Br-P distance in the crystal structure, a finding that has possible application in the field of NMR crystallography. This trend is explained in terms of Ramsey's theory on paramagnetic shielding. Overall, this work demonstrates how careful NMR studies of underexploited exotic nuclides, such as (79/81)Br, can afford insights into structure and bonding environments in the solid state.
多核（31）P 和（79/81）Br）、多场（9.4、11.75 和 21.1 T）固态核磁共振实验在七种含有三苯基膦阳离子的季鏻溴化物上进行，三苯基膦阳离子是许多化学应用中发现的分子支架。这是为了充分表征它们的溴原子电场梯度（EFG）张量，以及卤素和磷核的化学位移（CS）张量，为它们周围的局部电子环境提供了罕见而新颖的见解。从单晶 X 射线衍射获得了六种化合物的新晶体结构，以帮助解释 NMR 数据。其中包括 BrPPh(4)的新结构，因为之前报道的结构与我们的磁共振数据不一致，从而证明了非标准核的 NMR 数据如何纠正或改进 X 射线衍射数据。我们的结果表明，尽管存在相当大的四极相互作用，（79/81）Br 磁共振波谱是一种强大的表征工具，可以区分化学相似的溴原子位点，这可以通过特征 NMR 参数的范围来证明。获得类似的季鏻氯化物样品的（35/37）Cl 固态 NMR 数据为晶胞体积、核四极耦合常数和 Sternheimer 反屏蔽因子之间的关系提供了深入了解。实验结果得到了包含量子化学计算（GIPAW DFT）的补充，证实了我们实验确定的溴 CS 张量最大分量δ(11)与晶体结构中最短的 Br-P 距离之间的强烈依赖关系，这一发现可能在 NMR 晶体学领域有应用。这种趋势可以用 Ramsey 关于顺磁屏蔽的理论来解释。总的来说，这项工作表明，对（79/81）Br 等未充分利用的外来核进行仔细的 NMR 研究，可以深入了解固态中的结构和键合环境。
