Department of Chemistry, University of Ottawa, Ottawa K1N 6N5, Canada.
J Chem Phys. 2013 May 7;138(17):174202. doi: 10.1063/1.4802192.
Among the interactions which govern NMR spectra, spin-spin coupling interactions provide the most direct form of structural information which is of interest to chemists. Dipolar coupling may be used to measure internuclear distances directly and J coupling may be used to identify bonding interactions and provide insights into the nature of the bonds. It is well known that the presence of a quadrupolar interaction reintroduces the dipolar interaction in spinning samples; however, similarly to the J coupling, this information is often lost if the observed nucleus is quadrupolar due to quadrupolar spectral broadening. Here we show for multiple spin pairs that double-rotation (DOR) NMR fully removes the effects of the quadrupolar interaction on the NMR spectrum leaving only the effects of dipolar and J couplings. We also demonstrate that the J coupling multiplets do not disappear for quadrupolar A2 spin pairs as they do for spin-1/2 nuclei. With DOR NMR, it is then straightforward to measure homonuclear J coupling constants between magnetically equivalent quadrupolar nuclei. A deeper understanding of the origins of the magnitudes and dominant mechanisms of J coupling for quadrupolar spin pairs in a series of related compounds is obtained by decomposing computed J coupling constants into their major molecular orbital contributions.
在控制 NMR 谱的相互作用中，自旋-自旋耦合相互作用提供了化学家最感兴趣的最直接的结构信息形式。偶极耦合可用于直接测量核间距离，而 J 耦合可用于识别键合相互作用并深入了解键的性质。众所周知，四极相互作用的存在会在旋转样品中重新引入偶极相互作用；然而，如果观察到的核由于四极谱展宽而具有四极矩，则此信息通常会丢失，类似于 J 耦合。在这里，我们展示了多个自旋对的情况，双重旋转（DOR）NMR 完全消除了四极相互作用对 NMR 谱的影响，仅留下偶极和 J 耦合的影响。我们还证明了对于四极矩 A2 自旋对，J 耦合多重峰不会像自旋-1/2 核那样消失。使用 DOR NMR，可以很方便地测量磁等价的四极核之间的同核 J 耦合常数。通过将计算出的 J 耦合常数分解为其主要分子轨道贡献，可以更深入地了解一系列相关化合物中四极矩自旋对的 J 耦合幅度和主要机制的起源。
