Bruker BioSpin Solid State NMR, User Manual

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User Manual Version 002 BRUKER BIOSPIN 179 (327)
14
Symmetry Based
Recoupling
14
Sample rotation averages most anisotropic interactions, and therefore removes
the information available from them. Therefore, selective recoupling of anisotropic
interactions is desired for structural analysis (re-coupling,
re introduction of aniso-
tropic interactions, like e.g. dipolar coupling ), in order to regain specific informa-
tion. The topic has been thoroughly reviewed, by E.A. Bennett et al, and by S.
Dusold et al. One strategy is the use of symmetry based recoupling sequences;
see M. Hohwy et al (1998) et al. and A. Brinkmann et al. (2000). In these se-
quences, double quantum coherence are excited via the dipolar homonuclear di-
polar coupling. Single quantum coherence are suppressed by phase cycling. The
size of the dipolar coupling can be determined from the build-up rate of DQ signal
intensity, measured after reconversion into SQ coherence. It should also be men-
tioned that there are recoupling sequences that do not generate double quantum
coherences (DRAWS, DRAMA, and MELODRAMA).
Symmetry-based recoupling sequences recouple specific spin interactions, using
cyclic sequences composed of N phase-shifted repetitions of either 2
π
(C se-
quences) or
π
(R sequences) rotation elements. Which interaction(s) are recou-
pled by a given sequence is determined by the relationship between the sample
rotation rate, the spin rotation rate, and the rate of phase shift between the ele-
ments. The sequences are denoted as e.g. CN , where N is the number of ele-
ments in the cycle, n is the number of rotor periods spanned by the N elements,
and the total phase rotation between the elements is 2
π
/
ν
. In the simplest imple-
mentation of a C sequence, the 2
π
rotation element is simply a 2
π
pulse, but other
elements are possible. Thus the sequence C7
1
2
consists of 7 consecutive 2
π
pulses, with the phase of each pulse shifted by 2
π
/7 from the previous one. The
whole sequence takes two rotor periods, each 2
π
pulse thus takes 2/7
th
rotor peri-
od. The spin nutation frequency and sample rotation frequency are thus related by
ν
RF
= (7/2)*
ν
rotor
. In practice, the original C7 sequence uses an additional
π
-phase
alternation for every second pulse, so that 14 pulses are executed during 2 rotor
periods, requiring
ν
RF
= 7*
ν
rotor
.
For all C and R sequences, the spin nutation frequency must be accurately
matched to the sample rotation rate. Since X-X dipolar couplings are usually
small, long mixing times are required to reintroduce the dipolar coupling. When
1
H
decoupling is required, it is important to avoid any transfer of magnetization to or
from the proton spin system (HH condition), which would destroy the desired in-
formation. This means that the effective fields on X and H must be very different.
However, proton decoupling must still be efficient as well. It has been shown that
the two RF fields should differ by a factor of 3, which in practice is extremely diffi-
cult to meet. It has also been shown that at very high spin rates (>16 kHz) decou-
pling is not necessary at all. A possible trick is also to use off-resonant LG
decoupling during the recoupling sequence. This enhances the effective proton
field (vector sum of RF field and offset), and sharpens the HH condition since the
homonuclear couplings are suppressed.
ν
η
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