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Is perfection ever attainable? Strong coupling effects in the Perfect Echo
Author(s) -
Howe Peter W.A.
Publication year - 2020
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.4927
Subject(s) - homonuclear molecule , spins , chemistry , pulse sequence , spin echo , echo (communications protocol) , magnetization , magnetization transfer , coupling (piping) , nuclear magnetic resonance , computational physics , physics , magnetic field , molecule , quantum mechanics , condensed matter physics , magnetic resonance imaging , computer science , medicine , mechanical engineering , computer network , organic chemistry , engineering , radiology
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.