Through-space contributions to two-dimensional double-quantum J correlation NMR spectra of magic-angle-spinning solids.
Through-space contributions to two-dimensional double-quantum J correlation NMR spectra of magic-angle-spinning solids.
复制标题
魔角旋转固体二维双量子 J 相关 NMR 谱的空间贡献。
作者：
F. Fayon;D. Massiot;M. Levitt;J. Titman;D. Gregory;L. Duma;L. Emsley;S. Brown
当解释用通常使用的双量子（DQ）魔角自旋（MAS）脉冲序列获得的二维NMR光谱时，常规使用的假设被称为重新聚焦的令人难以置信的天然丰度双量子转移实验（INADEQUATE）[A. Lesage，M. Bardet和L. Emsley，J. Am. Chem.Soc.121，10987（1999）]的一个重要的发现是，相关峰仅在具有通过键连接性的成对核中观察到。这个假设的有效性在这里解决的理论，实验和计算机模拟。如果两个核的各向同性化学位移不同，MAS频率远离转动共振，理论描述表明DQ相关峰确实指示J耦合。然而，如果各向同性的化学位移是相同的，它表明，DQ峰可以出现对的核，即使在没有通过键J耦合。这些峰值出现在一对具有非零的空间偶极-偶极耦合和化学位移各向异性张量的原子核的特定情况下，所述化学位移各向异性张量具有不同的主幅度或取向，前提是MAS频率与化学位移各向异性相当或小于化学位移各向异性。实验31 P光谱记录的样品上的TiP 2 O 7和计算机模拟表明，这些异常峰的幅度增加与增加B 0磁场和它们减少与MAS频率增加。从理论上解释了这种行为。
A routinely used assumption when interpreting two-dimensional NMR spectra obtained with a commonly used double-quantum (DQ) magic-angle-spining (MAS) pulse sequence referred to as the refocused incredible natural abundance double-quantum transfer experiment (INADEQUATE) [A. Lesage, M. Bardet, and L. Emsley, J. Am. Chem. Soc. 121, 10987 (1999)] has been that correlation peaks are only observed for pairs of nuclei with a through-bond connectivity. The validity of this assumption is addressed here by theory, experiment, and computer simulations. If the isotropic chemical shifts of the two nuclei are different and the MAS frequency is far from rotational resonance, the theoretical description demonstrates that DQ correlation peaks are indeed indicative of a J coupling. However, if the isotropic chemical shifts are the same, it is shown that DQ peaks can appear for pairs of nuclei even in the absence of a through-bond J coupling. These peaks appear in the specific case of a pair of nuclei with a nonzero through-space dipole-dipole coupling and chemical shift anisotropy tensors having different principal magnitudes or orientations, provided that the MAS frequency is comparable to or smaller than the chemical shift anisotropies. Experimental 31P spectra recorded on a sample of TiP2O7 and computer simulations show that the magnitude of these anomalous peaks increases with increasing B0 magnetic field and that they decrease with increasing MAS frequency. This behavior is explained theoretically.