156 (327) BRUKER BIOSPIN User Manual Version 002
REDOR
measured, and the reference experiment does not show a decay independent of
the heteronuclear dipolar coupling. Faster spinning may solve the problem. Alter-
natively, the constant time (CT-)REDOR sequence may be used, which can help
to enhance the performance in these cases and to reduce the experimental time
needed dramatically.
Here the refocusing pulse is stepped in small time increments from the beginning
of the rotor period, where it has a small effect on the signal intensity, to the middle
of the rotor period, where it has the maximum effect.
Finally if one or both of the involved nuclei do have a quadrupolar moment, the
here used REDOR sequence should not be used, but there are several different
REDOR like experiments in literature which have an enhanced performance on
observing quadrupolar nuclei. These are for example the Rotational Echo Adia-
batic Passage DOuble Resonance (REAPDOR) or TRAnsfer of Population in
DOuble Resonance (TRAPDOR) sequences.
Usually, the setup is chosen such that the more sensitive nucleus is observed.
The measured coupling is of course independent from the choice of the nucleus,
but there may be reasons to consider carefully, which nucleus is observed. Of
course it is tempting to observe the nucleus with higher isotopic concentration, but
this is usually not recommended, since it is more difficult to observe a small inten-
sity change on a strong signal (which might be caused by fluctuations) than a big-
ger difference on the low abundance nucleus caused by the high abundance of
the coupling partner. An example: The measurement will be more precise observ-
ing
13
C and defocus with
31
P pulses than vice versa, because the effect on
31
P,
caused by 1.1% of
13
C would be max. 1.1%, a very small change which would re-
quire extremely high S/N and extremely high stability in signal generation and spin
rate.
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