Jealott's Hill Reseach Centre, Syngenta, Berkshire, UK.
Magn Reson Chem. 2020 Jan;58(1):77-83. doi: 10.1002/mrc.4927. Epub 2019 Aug 19.
The Perfect Echo sequence, originally proposed in the late 1980s, has recently been popularised with many applications in the field of small-molecule proton NMR spectroscopy. The Perfect Echo refocuses all homonuclear J-couplings for AX spin systems and refocuses magnetization in-phase for more complex weakly coupled spin systems, albeit with some intensity reduction. In contrast to suggestions in previous publications, spectra acquired in our laboratory showed that the Perfect Echo caused intensity distortions in strongly coupled systems where the chemical shift difference between the coupled spins was not large compared to the J-coupling. This paper reports experimental observations and theoretical analysis of strongly coupled spins to confirm the distortions are real and that they originate principally from transfer of magnetization caused by the final inversion pulse of the Perfect Echo. The intensity changes are not large, but because of them, identifications of coupling partners based on resonance intensities ("roofing") can no longer be relied on when the Perfect Echo is used. However, theory and experiment confirm that adding an orthogonal excitation pulse at the end of the Perfect Echo greatly reduces the distortions.
完美回波（Perfect Echo）序列最初是在 20 世纪 80 年代末提出的，近年来在小分子质子 NMR 波谱学领域得到了广泛应用。完美回波对 AX 自旋系统中的所有同核 J 耦合进行重聚焦，并对更复杂的弱耦合自旋系统中的磁化强度进行同相重聚焦，尽管强度有所降低。与之前出版物中的建议相反，我们实验室采集的光谱表明，在化学位移差值与 J 耦合相比不太大的强耦合系统中，完美回波会导致强度失真。本文报告了对强耦合自旋的实验观察和理论分析，以确认这些失真确实存在，并且它们主要源于完美回波的最后反转脉冲引起的磁化转移。强度变化不大，但由于这些变化，当使用完美回波时，基于共振强度的耦合伙伴识别（“屋顶”）不再可靠。然而，理论和实验都证实，在完美回波的末尾添加正交激发脉冲可以大大减少这些失真。
