Max-Planck-Institute of Solid-State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany.
J Magn Reson. 2010 Apr;203(2):226-35. doi: 10.1016/j.jmr.2009.12.021. Epub 2010 Jan 4.
To efficiently obtain multiple-quantum magic-angle spinning (MQMAS) spectra of the nuclide 45Sc (I=7/2), we have combined several previously suggested techniques to enhance the signal-to-noise ratio and to improve spectral resolution for the test sample, scandium sulphate pentahydrate (ScSPH). Whereas the 45Sc-3QMAS spectrum of ScSPH does not offer sufficient resolution to clearly distinguish between the 3 scandium sites present in the crystal structure, these sites are well-resolved in the 5QMAS spectrum. The loss of sensitivity incurred by using MQMAS with 5Q coherence order is partly compensated for by using fast-amplitude modulated (FAM) sequences to improve the efficiency of both 5Q coherence excitation and conversion. Also, heteronuclear decoupling is employed to minimise dephasing of the 45Sc signal during the 5Q evolution period due to dipolar couplings with the water protons in the ScSPH sample. Application of multi-pulse decoupling schemes such as TPPM and SPINAL results in improved sensitivity and resolution in the F(1) (isotropic) dimension of the 5QMAS spectrum, the best results being achieved with the recently suggested SW(f)-TPPM sequence. By numerical fitting of the 45Sc-NMR spectra of ScSPH from 3QMAS, 5QMAS and single-quantum MAS at magnetic fields B(0)=9.4 T and 17.6 T, the isotropic chemical shift delta(iso), the quadrupolar coupling constant chi, and the asymmetry parameter eta were obtained. Averaging over all experiments, the NMR parameters determined for the 3 scandium sites, designated (a), (b) and (c) are: delta(iso)(a)=-15.5+/-0.5 ppm, chi(a)=5.60+/-0.10 MHz, eta(a)=0.06+/-0.05; delta(iso)(b)=-12.9+/-0.5 ppm, chi(b)=4.50+/-0.10 MHz, eta(b)=1.00+/-0.00; and delta(iso)(c)=-4.7+/-0.2 ppm, chi(c)=4.55+/-0.05 MHz, eta(c)=0.50+/-0.02. The NMR scandium species were assigned to the independent crystallographic sites by evaluating their experimental response to proton decoupling, and by density functional theory (DFT) calculations using the PAW and GIPAW approaches, in the following way: Sc(1) to (c), Sc(2) to (a), and Sc(3) to (b). The need to compute NMR parameters using an energy-optimised crystal structure is once again demonstrated.
为了高效地获取核素 45Sc（I=7/2）的多量子魔角旋转（MQMAS）谱，我们结合了几种先前提出的技术，以提高测试样品硫酸氧钪五水合物（ScSPH）的信噪比和光谱分辨率。虽然 ScSPH 的 45Sc-3QMAS 谱没有提供足够的分辨率来清楚地区分晶体结构中存在的 3 个钪位点，但在 5QMAS 谱中这些位点得到了很好的分辨。使用具有 5Q 相干顺序的 MQMAS 会导致灵敏度降低，但使用快速幅度调制（FAM）序列来提高 5Q 相干激发和转换的效率，部分弥补了这一损失。此外，采用异核去耦技术可以最小化由于 ScSPH 样品中与水质子的偶极相互作用而导致的 45Sc 信号在 5Q 演化期间的退相。应用多脉冲去耦方案，如 TPPM 和 SPINAL，可提高 5QMAS 谱 F（1）（各向同性）维度的灵敏度和分辨率，其中最近提出的 SW（f）-TPPM 序列效果最佳。通过对磁场 B（0）=9.4 T 和 17.6 T 下 ScSPH 的 3QMAS、5QMAS 和单量子 MAS 的 45Sc-NMR 谱进行数值拟合，得到了各向同性化学位移 delta（iso）、四极耦合常数 chi 和不对称参数 eta。对所有实验进行平均后，确定了标记为（a）、（b）和（c）的 3 个钪位点的 NMR 参数为：delta（iso）（a）=-15.5+/-0.5 ppm，chi（a）=5.60+/-0.10 MHz，eta（a）=0.06+/-0.05；delta（iso）（b）=-12.9+/-0.5 ppm，chi（b）=4.50+/-0.10 MHz，eta（b）=1.00+/-0.00；和 delta（iso）（c）=-4.7+/-0.2 ppm，chi（c）=4.55+/-0.05 MHz，eta（c）=0.50+/-0.02。通过评估它们对质子去耦的实验响应以及使用 PAW 和 GIPAW 方法的密度泛函理论（DFT）计算，将 NMR 钪物种分配到独立的结晶学位置，方式如下：Sc（1）至（c），Sc（2）至（a），和 Sc（3）至（b）。再次证明了使用能量优化的晶体结构计算 NMR 参数的必要性。
