Department of Chemistry, University of Alberta           September 1998
NMR News 98-07
News and tips for users of the Varian NMR systems in the Department.
Editor:
Albin.Otter@ualberta.ca          http://nmr.chem.ualberta.ca

There 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.

Contents

bullet FAQ 98-07.1 why is temperature control important in NMR?

 

FAQ 98-07.1 Why is temperature control important in NMR?

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Chemical shifts are temperature-dependent. Consequently, if a sample is measured first on i300 and then for some more experiments on i600 or u500, the temperature should ideally be the same, otherwise the shifts will be different. Although the deviations might be small, it can cause unnecessary and very unpleasant confusion. By using the EZ NMR buttons, 27.0C is automatically achieved on all spectrometers. There might be a significant deviation between the reading on the remote status display and the actual temperature in the probe.

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Shims are temperature-dependent. All shims available in the general shim library are for 27C. The more the temperature is different form 27C, the less the shims will work.

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Quite a number of 1D and 2D experiments available use solvent suppression by presaturation of the solvent peak. The decoupler (or transmitter) can only be effective if the solvent peak is really where the irradiation frequency is set to. Example: the shift of the HDO resonance (4.75 ppm at 27.0C) changes by about 0.01 ppm per C. Because the decoupler is set to irradiate at exactly 4.75 ppm, the effectiveness of the presaturation decreases substantially with every degree the temperature is off, i.e. without manually readjusting the irradiation point the presaturation will only be optimum at 27.0C. This is less of a concern for solvents such as CDCl3, CD2Cl2 and C6D6. In those cases the solvent does not typically require any special treatment.

If a different temperature and presaturation are needed, a spectrum should first be run with dm='nnn', then after Fourier transformation the cursor should be set on the top of the solvent peak, followed by typing sd (set decoupler) and dm='ynn'. The last statement turns the decoupler on again and is very often forgotten. To start the experiment, a go command is needed.


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