Department of Chemistry, University of Alberta                 August 1997
NMR News 97-02
News and tips for users of the Varian NMR systems in the Department.

here are no fixed publishing dates for this newsletter; its appearance solely depends on whether there is a need to present information to the users of the spectrometers or not.

Other content of this NMR News is no longer meaningful and has been removed April 2010.


bullet HMQC coupled or decoupled

sw and sw1: when are they the same and when not

HMQC: coupled or decoupled
The HMQC experiment correlates protons to carbons. It can be run in two fundamentally different ways:


C13 coupled: one-bond carbon proton couplings are active and every CH cross peak appears twice with the distance between the two peaks
1JCH. This method provides the best quality spectra but the analysis is slightly more complicated.


C13 decoupled: during acquisition of the data, the C13 nuclei are decoupled, resulting in only one cross-peak per CH, CH2, CH3 group. The C13 range is very large, even on a 300 MHz spectrometer some 15 kHz. Hence a very strong decoupler field strength is needed to effectively decouple all carbons from the protons over the entire range. This results in some sample heating and lower quality of the spectra. At the same time, hardware limitations to protect the probe require the acquisition time not to be longer than 20% of relaxation plus acquisition time (d1+at). The result of this is lower digital resolution.

The simplified spectra below illustrate the difference between the two methods:


There is no difference in the duration of the experiment in either form and data processing is unchanged as well.

sw and sw1: when are they the same and when not

One of the most common errors when parameter sets are changed by users is the parameter sw1 in two-dimensional experiments. In homonuclear
cases such as GCOSY, GTOCSY, TROESY, sw1 should be the same as sw otherwise you will likely cut off part of the spectrum in F1. When you
change sw  don’t forget to type sw1=sw before you start the acquisition. If you save the parameter set after correctly setting sw and sw1 and use it
in the future again you will not have to worry about this anymore.

In heteronuclear cases (HMQC, HMBC, HSQC) sw1 is not equal to sw. sw1 covers 200 ppm which is sufficient for nearly all cases imaginable so you don’t have to change it.

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