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 Oct 14;114(40):10810-23. doi: 10.1021/jp108237x.
Central-transition (127)I solid-state nuclear magnetic resonance (SSNMR) spectra are presented for several anhydrous group 2 metal iodides (MgI(2), CaI(2), SrI(2), and BaI(2)), hydrates (BaI(2)·2H(2)O and SrI(2)·6H(2)O), and CdI(2) (4H polytype). Variable offset cumulative spectrum data acquisition coupled with echo pulse sequences and an 'ultrahigh' applied field of 21.1 T were usually suitable to acquire high-quality spectra. Spectral analysis revealed iodine-127 nuclear quadrupole coupling constants (C(Q)((127)I)) ranging in magnitude from 43.5 (CaI(2)) to 214 MHz (one site in SrI(2)). For very large C(Q), analytical second-order perturbation theory could not be used to reliably extract chemical shifts and a treatment which includes quadrupolar effects exactly was required (Bain, A. D. Mol. Phys. 2003, 101, 3163). Differences between second-order and exact modeling allowed us to observe 'higher-order' quadrupole-induced effects for the first time. This finding will have implications for the interpretation of SSNMR spectra of quadrupolar nuclei with large quadrupole moments. In favorable situations (i.e., C(Q)((127)I) < 120 MHz), measurements were also performed at 11.75 T which when combined with the 21.1 T data allowed us to measure iodine chemical shift (CS) tensor spans in the range from 60 (BaI(2)·2H(2)O) to 300 ppm (one site in BaI(2)). These measurements represent the first complete characterizations (i.e., electric field gradient and CS tensors as well as their relative orientation) of noncubic iodide sites using (127)I SSNMR. In select cases, the SSNMR data are supported with (127)I NQR measurements. We also summarize a variety of trends in the halogen SSNMR parameters for group 2 metal halides. Gauge-including projector-augmented wave DFT computations are employed to complement the experimental observations, to predict potential structures for the two hydrates, and to highlight the sensitivity of C(Q)((127)I) to minute structural changes, which has potential applications in NMR crystallography.
呈现了几种无水 2 族金属碘化物（MgI₂、CaI₂、SrI₂ 和 BaI₂）、水合物（BaI₂·2H₂O 和 SrI₂·6H₂O）和 CdI₂（4H 多型）的中心跃迁（127）I 固态核磁共振（SSNMR）谱。通常，采用可变偏移累积谱数据采集与回波脉冲序列以及 21.1 T 的“超高”外加磁场，可获得高质量的光谱。光谱分析揭示了碘-127 核四极耦合常数（C(Q)((127)I)）的幅度范围从 43.5（CaI₂）到 214 MHz（SrI₂中的一个位点）。对于非常大的 C(Q)，分析二阶微扰理论不能可靠地提取化学位移，需要进行包括四极效应的确切处理（Bain，A. D. Mol. Phys. 2003, 101, 3163）。二阶和精确建模之间的差异使我们能够首次观察到“更高阶”四极诱导效应。这一发现将对具有大四极矩的四核核磁共振谱的解释产生影响。在有利的情况下（即，C(Q)((127)I) < 120 MHz），还在 11.75 T 下进行了测量，将其与 21.1 T 数据结合使用，使我们能够测量碘化学位移（CS）张量跨度范围从 60（BaI₂·2H₂O）到 300 ppm（BaI₂中的一个位点）。这些测量代表了首次使用（127）I SSNMR 对非立方碘化物位点进行完整的特征描述（即电场梯度和 CS 张量及其相对取向）。在某些情况下，SSNMR 数据得到了（127）I NQR 测量的支持。我们还总结了 2 族金属卤化物卤化物 SSNMR 参数的各种趋势。使用包含规范的投影增强波 DFT 计算来补充实验观察结果，预测两种水合物的潜在结构，并强调 C(Q)((127)I)对微小结构变化的敏感性，这在 NMR 晶体学中有潜在的应用。
