User Manual Version 002 BRUKER BIOSPIN 155 (327)
12
REDOR 12
Rotational Echo DOuble Resonance is an experiment based on the heteronuclear
dipolar coupling between the observed nuclei. The REDOR sequence investi-
gates this coupling under high resolution MAS conditions.
Dipolar couplings between I (decoupled spin) and S (observed) spins are spun
out under MAS if there are no strong homonuclear interactions and if the hetero-
nuclear coupling is not too big. In the case of couplings between most hetero-nu-
clei (like
13
C,
15
N and
29
Si) this is usually the case (small couplings of a few kHz
and dilute spins), if the coupled nucleus is
31
P,
19
F or even
1
H, the coupling may
not easily be spun out and the standard REDOR sequence may not be applicable
in these cases.
The REDOR sequence reintroduces the heteronuclear dipolar coupling between
the spin S and I by applying p pulses every half of a rotor period on the second
channel (I), while the S channel is observed. A p pulse at half a rotor period will re-
focus the dipolar interaction averaged by spinning and dephase the magnetiza-
tion, leading to an attenuation of the observed signal. Evaluation requires the
acquisition of 2 data sets, one with refocusing pulse, the other one without, so that
the natural dephasing can be subtracted out from the dipolar dephasing due to
the refocusing pulse. Reference experiment (without I refocusing pulse) and de-
phased experiment (with I refocusing pulse) are subtracted and evaluated. Refer-
ence experiment and dephased experiment can be acquired consecutively or in
an interleaved mode so that experimental drifts will not cause large errors. Usual-
ly, the experiment is set up as a pseudo-2D experiment where the number of rotor
periods with p pulses is increased before detection.
The experiment can either be used to investigate isolated spin systems or multi
speed systems. In both cases the time dependent difference of the echo S
0
(with-
out the reintroduction of the heteronuclear dipolar coupling) and the second echo
experiment S’ (with the reintroducing p pulses applied on the I channel) can be
used for calculating the distance information for the two involved spins or the sec-
ond moment of the spin system respectively.
In isolated two spin systems the measured REDOR (dephasing)-curve can be
used to determine distance information between the two involved spins. In case of
investigating multispin systems the experimental REDOR dephasing curves can
only be used to determine the second moment (M
2
).
(Eq. 12.1)
By the relation of the second moment to the distance by, this information in combi-
nation with theoretical simulations can be used to determine a mean distance be-
tween the involved spins as well.
In the case of very strong dipolar couplings the normal REDOR approach for mul-
tispin systems can not be used without introducing severe errors into the calculat-
ed second moment. With very strong dipolar couplings, the signal intensity may
be lost after very few or only one refocusing pulse, so the decay curve cannot be
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