Understanding the HMQC Pulse Sequence

The HMQC experiment gives exactly the same result as the HSQC, and the data is processed in the same way. There are some differences in sensitivity and peak shape that depend on the size and complexity of the molecule, and the pros and cons of the two experiments are the subject of some debate in the literature. Because it relies on double-quantum and zero-quantum coherences (DQC and ZQC) during the evolution (t ) period, the HMQC is more difficult to explain and understand than HSQC, which uses only the familiar singlequantum transitions that can be diagramed and analyzed using vectors. We discuss it here because it forms the basis of the HMBC (multiple-bond) experiment. 

The second 90° 13C pulse converts the ZQC and DQC back into antiphase H SQC, which is refocused by the final 1/(2J) delay just as it is in the HSQC experiment. This is an important theme that occurs in many of the more sophisticated NMR experiments multiple-quantum coherences (DQC and ZQC) cannot be directly observed, but they can be created from SQC, allowed to precess, and converted back to measurable SQC. The multiple-quantum coherences can be detected then, but only indirectly. Multiple quantum coherence is essential to the DQF-COSY and DEPT experiments as well, so even though it is difficult to understand it is very important in modern NMR and cannot be ignored. 

Once we have diagramed the desired coherence order pathway, it is easy to add gradients to select that pathway. One simple solution is to use the 3,4,5 relationship of a right triangle 3x3 + 4x4 = 5x5. Put a gradient in the first half of t of relative amplitude G = 5, another in the second half with amplitude G2 = 3, and a third in the refocusing delay with amplitude G3 = 4. As the coherence order p is 5, —3, and —4, respectively, during these three periods, we have a total twist of  

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