14N overtone NMR spectra under magic angle spinning: Experiments and numerically exact simulations
14N overtone NMR spectra under magic angle spinning: Experiments and numerically exact simulations
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DOI：
10.1063/1.4775592
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发表时间：
2013-02-14
影响因子：
4.4
通讯作者：
Brinkmann, Andreas
中科院分区：
文献类型：
--
作者：
O'Dell, Luke A.;Brinkmann, Andreas
最近的研究表明，在魔角旋转（MAS）条件下，可以直接获得高分辨率的N-14泛音核磁共振光谱[L]。A. O'Dell和C. I. Ratcliffe，化学。理论物理。科学通报，2014 (5)[j]。初步实验结果显示，粉末图案宽度变窄，频率移位取决于MAS速率，并且明显没有旋转侧带，这些观察结果似乎与先前的理论处理不一致。在这里，我们使用数值精确的模拟再现了这些效应，并考虑了完整的核自旋哈密顿量。在样品纺丝下，N-14泛音信号被纺丝频率分隔成5个（0、+/- 1、+/- 2）泛音边带。对于以魔角旋转的粉末样品，+2 ω (r)边带占主导地位，而其他边带显示出明显较低的信号强度。所得的MAS粉末图案显示出特征的四极性线形，从中可以确定N-14的四极性参数和各向同性化学位移。以其他角度旋转样品可以改变五个泛音边带的形状和相对强度，MAS提供了平均偶极耦合和屏蔽各向异性的好处。为了证明这种实验方法的优势，我们展示了从l -组氨酸获得的N-14泛音MAS光谱，其中所有三个氮位点的粉末模式都被清楚地分解了。[http://dx.doi.org/10.1063/1.4775592]
It was recently shown that high resolution N-14 overtone NMR spectra can be obtained directly under magic angle spinning (MAS) conditions [L. A. O'Dell and C. I. Ratcliffe, Chem. Phys. Lett. 514, 168 (2011)]. Preliminary experimental results showed narrowed powder pattern widths, a frequency shift that is dependent on the MAS rate, and an apparent absence of spinning sidebands, observations which appeared to be inconsistent with previous theoretical treatments. Herein, we reproduce these effects using numerically exact simulations that take into account the full nuclear spin Hamiltonian. Under sample spinning, the N-14 overtone signal is split into five (0, +/- 1, +/- 2) overtone sidebands separated by the spinning frequency. For a powder sample spinning at the magic angle, the +2 omega(r) sideband is dominant while the others show significantly lower signal intensities. The resultant MAS powder patterns show characteristic quadrupolar lineshapes from which the N-14 quadrupolar parameters and isotropic chemical shift can be determined. Spinning the sample at other angles is shown to alter both the shapes and relative intensities of the five overtone sidebands, with MAS providing the benefit of averaging dipolar couplings and shielding anisotropy. To demonstrate the advantages of this experimental approach, we present the N-14 overtone MAS spectrum obtained from L-histidine, in which powder patterns from all three nitrogen sites are clearly resolved. [http://dx.doi.org/10.1063/1.4775592]