Makrinich M, Sambol M, Goldbourt A
School of Chemistry, Tel Aviv University, Tel Aviv, 69978, Israel.
Phys Chem Chem Phys. 2020 Sep 30;22(37):21022-21030. doi: 10.1039/d0cp01162b.
Solid-state nuclear magnetic resonance has long been used to probe atomic distances between nearby nuclear spins by virtue of the dipolar interaction. New technological advances have recently enabled simultaneous tuning of the radio-frequency resonance circuits to nuclei with close Larmor frequencies, bringing great promise, among other experiments, also to distance measurements between such nuclei, in particular for nuclei with a spin larger than one-half. However, this new possibility has also required modifications of those experiments since the two nuclei cannot be irradiated simultaneously. When measuring distances between a spin S = 1/2 and a quadrupolar spin (S > ½), this drawback can be overcome by splitting the continuous-wave recoupling pulse applied to the quadrupolar nucleus. We show here that a similar adjustment to a highly-efficient phase-modulated (PM) recoupling pulse enables distance measurements between nuclei with close Larmor frequencies, where the coupled spin experiences a very large coupling. Such an experiment, split phase-modulated RESPDOR, is demonstrated on a 13C-81Br system, where the difference in Larmor frequencies is only 7%, or 11.2 MHz on a 14.1 T magnet. The inter-nuclear distances are extracted using an unscaled analytical formula. Since bromine usually experiences particularly high quadrupolar couplings, as in the current case, we suggest that the split-PM-RESPDOR experiment can be highly beneficial for research on bromo-compounds, including many pharmaceuticals, where carbon-bromine bonds are prevalent, and organo-catalysts utilizing the high reactivity of bromides. We show that for butyl triphenylphosphonium bromide, the solid-state NMR distances are in agreement with a low-hydration compound rather than a water-caged semi-clathrate form. The split-PM-RESPDOR experiment is suitable for distance measurements between any quadrupolar ↔ spin-1/2 pair, in particular when the quadrupolar spin experiences a significantly large coupling.
长期以来，固态核磁共振一直通过偶极相互作用来探测相邻核自旋之间的原子距离。最近的新技术进展使得能够同时将射频共振电路调谐到具有相近拉莫尔频率的原子核，这带来了巨大的前景，除了其他实验之外，对于此类原子核之间的距离测量也是如此，特别是对于自旋大于二分之一的原子核。然而，这种新的可能性也需要对那些实验进行修改，因为两个原子核不能同时被照射。当测量自旋(S = 1/2)和四极自旋（(S > ½)）之间的距离时，这个缺点可以通过将施加到四极原子核的连续波再耦合脉冲分开来克服。我们在此表明，对高效相位调制（PM）再耦合脉冲进行类似的调整，可以实现具有相近拉莫尔频率的原子核之间的距离测量，其中耦合自旋经历非常大的耦合。这样一个实验，即分裂相位调制RESPDOR，在一个(^{13}C - ^{81}Br)系统上得到了证明，在(14.1 T)磁体上，拉莫尔频率的差异仅为(7%)，即(11.2 MHz)。使用未缩放的解析公式提取核间距离。由于溴通常会经历特别高的四极耦合，就像当前这种情况，我们认为分裂PM - RESPDOR实验对于溴化合物的研究可能非常有益，包括许多药物，其中碳 - 溴键很普遍，以及利用溴化物高反应性的有机催化剂。我们表明，对于丁基三苯基溴化鏻，固态核磁共振距离与低水合化合物一致，而不是水笼半笼形。分裂PM - RESPDOR实验适用于任何四极(\leftrightarrow)自旋 - 1/2对之间的距离测量，特别是当四极自旋经历显著大的耦合时。
