Department of Physics, University of Warwick, Gibbet Hill Rd., Coventry CV4 7AL, UK.
Chemistry. 2010 Mar 8;16(10):3222-39. doi: 10.1002/chem.200901581.
A variable B(0) field static (broadline) NMR study of a large suite of niobate materials has enabled the elucidation of high-precision measurement of (93)Nb NMR interaction parameters such as the isotropic chemical shift (delta(iso)), quadrupole coupling constant and asymmetry parameter (C(Q) and eta(Q)), chemical shift span/anisotropy and skew/asymmetry (Omega/Deltadelta and kappa/eta(delta)) and Euler angles (alpha, beta, gamma) describing the relative orientation of the quadrupolar and chemical shift tensorial frames. These measurements have been augmented with ab initio DFT calculations by using WIEN2k and NMR-CASTEP codes, which corroborate these reported values. Unlike previous assertions made about the inability to detect CSA (chemical shift anisotropy) contributions from Nb(V) in most oxo environments, this study emphasises that a thorough variable B(0) approach coupled with the VOCS (variable offset cumulative spectroscopy) technique for the acquisition of undistorted broad (-1/2<-->+1/2) central transition resonances facilitates the unambiguous observation of both quadrupolar and CSA contributions within these (93)Nb broadline data. These measurements reveal that the (93)Nb electric field gradient tensor is a particularly sensitive measure of the immediate and extended environments of the Nb(V) positions, with C(Q) values in the 0 to >80 MHz range being measured; similarly, the delta(iso) (covering an approximately 250 ppm range) and Omega values (covering a 0 to approximately 800 ppm range) characteristic of these niobate systems are also sensitive to structural disposition. However, their systematic rationalisation in terms of the Nb-O bond angles and distances defining the immediate Nb(V) oxo environment is complicated by longer-range influences that usually involve other heavy elements comprising the structure. It has also been established in this study that the best computational method(s) of analysis for the (93)Nb NMR interaction parameters generated here are the all-electron WIEN2k and the gauge included projector augmented wave (GIPAW) NMR-CASTEP DFT approaches, which account for the short- and long-range symmetries, periodicities and interaction-potential characteristics for all elements (and particularly the heavy elements) in comparison with Gaussian 03 methods, which focus on terminated portions of the total structure.
对一系列铌酸盐材料进行了变量 B(0) 场静（宽带）NMR 研究，从而能够高精度测量（93）Nb NMR 相互作用参数，如各向同性化学位移（δiso）、四极耦合常数和不对称参数（C(Q)和η(Q)）、化学位移跨度/各向异性和倾斜/不对称（Ω/Δδ和 κ/η(Δ)）和描述四极和化学位移张量框架相对取向的欧拉角（α、β、γ）。这些测量结果通过使用 WIEN2k 和 NMR-CASTEP 代码的从头算 DFT 计算得到了补充，这些计算结果与报告的值相符。与之前关于在大多数氧环境中无法检测 Nb(V) 的 CSA（化学位移各向异性）贡献的断言不同，这项研究强调，彻底的变量 B(0) 方法与 VOCS（可变偏移累积光谱学）技术相结合，用于获取未失真的宽带（-1/2<-->+1/2）中心转换共振，有助于在这些（93）Nb 宽带数据中明确观察到四极和 CSA 贡献。这些测量结果表明，（93）Nb 电场梯度张量是 Nb(V) 位置的直接和扩展环境的一个特别敏感的测量指标，测量到的 C(Q) 值在 0 到>80 MHz 范围内；类似地，δiso（覆盖约 250 ppm 范围）和Ω值（覆盖 0 到约 800 ppm 范围）特征也与这些铌酸盐系统的结构有关。然而，由于涉及构成结构的其他重元素的远程影响，它们在 Nb-O 键角和距离定义直接 Nb(V) 氧环境方面的系统合理化变得复杂。在这项研究中还确定，这里生成的（93）Nb NMR 相互作用参数的最佳分析计算方法是全电子 WIEN2k 和包含规范的投影增强波（GIPAW）NMR-CASTEP DFT 方法，这些方法考虑了所有元素（特别是重元素）的短程和远程对称性、周期性和相互作用势特征，与仅关注总结构的终止部分的 Gaussian 03 方法相比。
