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.
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.
content of this NMR News is no longer meaningful and has been removed April 2010.
Why is temperature control important in NMR?
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, 27.0°C 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
are temperature-dependent. All shims available in the general shim
library are for 27°C. The more the temperature is different form 27°C,
the less the shims will work.
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 27.0°C) 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 27.0°C. 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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