Isotropic NMR spectra of half-integer quadrupolar nuclei using satellite transitions and magic-angle spinning
Isotropic NMR spectra of half-integer quadrupolar nuclei using satellite transitions and magic-angle spinning
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DOI：
10.1021/ja9939791
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发表时间：
2000-04-05
影响因子：
15
通讯作者：
Gan, ZH
中科院分区：
文献类型：
--
作者：
Gan, ZH
核磁共振 (NMR) 波谱已成为许多重要材料结构表征的重要工具，例如含有 11B、17O、23Na、27Al、71Ga 和 87Rb 等元素的矿物、陶瓷、玻璃、半导体和催化剂。自旋 S> 1/2 的核的 1 NMR 谱主要由核四极自旋相互作用主导。对于半整数四极核，通过仅观察中心 1/2 T-1/2 NMR 跃迁可以获得相对较窄的谱线，此时一阶四极效应消失。二阶四极效应包含高阶各向异性项，这些项未通过魔角样本旋转 (MAS) 完全平均。 2 虽然高磁场可以减少二阶效应，但要完全消除谱线展宽，需要双旋转 (DOR) 3 或动态角旋转 (DAS) 4 才能实现各向同性光谱。最近，弗里德曼和哈伍德推出了一种利用多量子跃迁的优雅方法。 5 多量子魔角旋转（MQMAS）方法仅需要传统的MAS即可获得半整数四极核的各向同性光谱。 MQMAS 的灵敏度和定量特性在很大程度上取决于多量子激发的效率，对高效率的追求推动了脉冲序列优化的持续努力。 6 在此贡献中，提出了一种基于卫星过渡的不同方法。四极核自旋流形中的卫星跃迁可以使用短射频脉冲有效激发，并在 MAS 下直接观察。 7 因此，正如此处所示，卫星跃迁 MAS 方法可高效生成半整数四极核的各向同性谱。在高磁场下，自旋态的一阶和二阶四极效应 - Se mS e S 可以用球谐函数展开，其中 ωQ) e2qQ/2S (2S-1) p 和 ωL 是拉莫尔频率，a2, m (1) 和 al, m (2) 仅取决于电场梯度 (EFG) 张量的不对称因子，(θ, φ) 是磁场的极角EFG 张量的主轴框架。一阶项 ES, mS Q (1) 对 1/2 没有影响
Nuclear magnetic resonance (NMR) spectroscopy has become an important tool for structural characterization of many important materials such as minerals, ceramics, glasses, semiconductors, and catalysts which contain elements such as 11B, 17O, 23Na, 27Al, 71Ga, and 87Rb. 1 NMR spectra of nuclei with spin S> 1/2 are dominated by the nuclear quadrupolar spin interaction. For half-integer quadrupolar nuclei, relatively narrow spectral lines can be obtained by limiting observation only to the central 1/2 T- 1/2 NMR transition for which the first-order quadrupolar effect vanishes. The second-order quadrupolar effect contains high-rank anisotropic terms which are not completely averaged by magic-angle sample spinning (MAS). 2 Although high magnetic fields can reduce the second-order effect, a complete removal of line broadening requires double rotation (DOR) 3 or dynamic angle spinning (DAS) 4 to achieve isotropic spectra. Recently, Frydman and Harwood have introduced an elegant method which utilizes multiple-quantum transitions. 5 The multiple-quantum magic-angle spinning (MQMAS) method requires only conventional MAS to obtain isotropic spectra of half-integer quadrupolar nuclei. The sensitivity and the quantitative features of MQMAS largely depend on the efficiency of multiple-quantum excitation and the quest for high efficiency drives an ongoing effort for pulse sequence optimization. 6 In this contribution, a different approach based on satellite transitions is proposed. The satellite transitions in the spin manifold of quadrupolar nuclei can be efficiently excited using short radio frequency pulses and directly observed under MAS. 7 Therefore, as will be shown here the satellite transition MAS method generates isotropic spectra of half-integer quadrupolar nuclei with high efficiencies. At high magnetic fields, the first-and second-order quadrupolar effects to spin states-S e mS e S can be expanded in terms of spherical harmonics where ωQ) e2qQ/2S (2S-1) p and ωL is the Lamor frequency, a2, m (1) and al, m (2) depend only on the asymmetry factor of the electrical field gradient (EFG) tensor, and (θ, φ) are the polar angles of the magnetic field in the principal axis frame of the EFG tensor. The first-order terms ES, mS Q (1) have no effect on the 1/2