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This afternoon, I was having some trouble with an HSQC (Heteronuclear Single Quantum Coherence Spectroscopy) spectrum I was trying to acquire, and the spectroscopist recommended I try HMQC (Heteronuclear Multiple Quantum Coherence) spectra, as they are more robust. I did so and got a very nice spectrum in about half the time an equivalent HSQC would have taken, and didn't have to phase it, to boot.

So, what is the downside? Why doesn't everyone use HMQC spectra instead of HSQC? I know they are rather different on a quantum-mechanical level, but as an end-user what, if any, is the difference in the data the two techniques give me, what are the advantages and disadvantages of each, and in what situations might I prefer one over the other?

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  • $\begingroup$ Most likely, for you the main difference is that HSQC gives better $\ce{^13C}$ or $\ce{^15N}$ resolution than HMQC (as long as the experiment is well set up). $\endgroup$ – Chris May 25 '12 at 23:33
  • $\begingroup$ Very interesting; I didn't notice that. I was at 500 MHz, so I'm hoping that won't matter this time, but it is good to keep in mind. Too bad you didn't write this as an answer or I'd upvote it. $\endgroup$ – Canageek May 25 '12 at 23:37
  • $\begingroup$ The quantum mechanical details matter a lot; usually long-range HSQC is preferable to HMQC or HMBC. See: Mandal, P. K.; Majumdar A. A comprehensive discussion of HSQC and HMQC pulse sequences. Concepts Magn. Reson., Part A 2004, 20A (1), 1–23. DOI: 10.1002/cmr.a.10095. $\endgroup$ – scott Aug 21 '13 at 20:36
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You get a better resolution in the indirect dimension with an HSQC compared to an HMQC. The HSQC signal is a singlet while the HMQC is usually a multiplet as you see certain couplings on it. So generally you should prefer the HSQC over the HMQC.

One advantage of the HMQC is that it uses significantly fewer pulses than the HSQC. This means that errors in the pulses don't accumulate as much as in the HSQC, you'll lose less signal if your pulses are wrong than in the HSQC.

If you had trouble setting up an HSQC, that could mean that the probe wasn't tuned and matched properly for your sample, or that your pulse lengths were wrong. The proton pulse should be determined on each sample individually; the $\ce{^13C}$ and $\ce{^15N}$ pulses should be set up correctly already. If you're not using a probe with automatic tuning and matching you should also do that manually before the measurement.

There are a lot more HSQC and HMQC variants that are used, e.g. Sofast-HMQC or TROSY sequences that are used for larger molecules like proteins. Another interesting variant you should consider is the multiplicity-edited HSQC, where $\ce{CH2}$ signals will have the opposite sign from $\ce{CH}$ or $\ce{CH3}$ signals.

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