通过固态HMQC NMR实验间接探测~（14）N的激发方案评价
Evaluation of excitation schemes for indirect detection of 14N via solid-state HMQC NMR experiments
摘要：
先前已经表明，${}^{14}$ N NMR光谱可以通过HMQC实验的间接检测可靠地获得。该方法利用了单量子（SQ）或双量子（DQ）${}^{14}$ N相干性和适当自旋1/2“间谍”核的SQ相干性之间的相干性转移，例如，${}^1$ H.必须注意的是，SQ-SQ方法需要仔细优化的设置以最小化与一阶四极相互作用相关的加宽（即，一个非常好的调整魔角和一个高度稳定的旋转速度），而DQ-SQ的没有。在这项工作中，四个${}^{14}$ N激发方案（DANTE，XiX，硬脉冲（HP），和选择性长脉冲（SLP））的效率进行了比较，使用基于J-HMQC的数值模拟和SQ-SQ或DQ-SQ ${}^1$ H-{ ${}^{14}$ N} D-HMQC实验L-组氨酸盐酸盐和N-乙酰基-L-缬氨酸在18.8 T和62.5 kHz MAS。结果表明，DANTE和SLP都提供了比XiX和HP更有效的14 N激发分布。此外，它表明，SLP计划：（i）是有效的，在大范围的四极相互作用，（ii）是高度稳健的偏移和射频脉冲长度和幅度，（iii）是非常简单的设置。这些因素使SLP非常适合于含氮材料的固态NMR分析中的广泛，非专业用途。
英文摘要：
It has previously been shown that ${}^{14}$N NMR spectra can be reliably obtained through indirect detection via HMQC experiments. This method exploits the transfer of coherence between single- (SQ) or double-quantum (DQ) ${}^{14}$N coherences, and SQ coherences of a suitable spin-1/2 'spy' nucleus, e.g., ${}^1$H. It must be noted that SQ-SQ methods require a carefully optimized setup to minimize the broadening related to the first-order quadrupole interaction (i.e., an extremely well-adjusted magic angle and a highly stable spinning speed), whereas DQ-SQ ones do not. In this work, the efficiencies of four ${}^{14}$N excitation schemes (DANTE, XiX, Hard Pulse (HP), and Selective Long Pulse (SLP)) are compared using J-HMQC based numerical simulations and either SQ-SQ or DQ-SQ ${}^1$H-{${}^{14}$N} D-HMQC experiments on L-histidine HCl and N-acetyl-L-valine at 18.8 T and 62.5 kHz MAS. The results demonstrate that both DANTE and SLP provide a more efficient 14N excitation profile than XiX and HP. Furthermore, it is shown that the SLP scheme: (i) is efficient over a large range of quadrupole interaction, (ii) is highly robust to offset and rf-pulse length and amplitude, and (iii) is very simple to set up. These factors make SLP ideally suited to widespread, non-specialist use in solid-state NMR analyses of nitrogen-containing materials.
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