Bruker BioSpin Solid State NMR - Homonuclear Decoupling; Multiple Pulse NMR: Observing Chemical Shifts of Homonuclear Coupled Nuclei; Multiple Pulse Decoupling; Br-24, Mrev-8, Blew-12

92 (327) BRUKER BIOSPIN User Manual Version 002
Decoupling Techniques
Homonuclear Decoupling 5.2
Homonuclear decoupling refers to methods which decouple dipolar interactions
between like spins. Those are only prominent between abundant spins like
1
H,
19
F and
31
P (and potentially some others). This interaction cannot easily be spun
out in most cases and renders NMR-parameters like chemical shifts of the homo
-
nuclear coupled spins or heteronuclear couplings and J-couplings to other (X-)nu-
clei unobservable.
Multiple Pulse NMR: Observing Chemical Shifts of Homonuclear Coupled Nuclei 5.2.1
Multiple pulse NMR methods are covered in the chapters about CRAMPS of this
manual collection. The principle of those methods (CRAMPS, if MAS is used to
average CSA interactions simultaneously), is to set the magnetization of the spins
into the magic angle, using a suitable pulse sequence. In this case, the dipolar
couplings between those spins are suppressed. Short observation windows be
-
tween pulses allow observation of the signal from the decoupled nuclei.
Reference:
1. S. Hafner and H.W. Spiess, Multiple-Pulse Line Narrowing under Fast Magic-Angle Spinning, J.
Magn. Reson. A 121, 160-166 (1996) and references therein.
Multiple Pulse Decoupling 5.2.2
Multiple Pulse Decoupling: Observing dipolar couplings and j-couplings to homo-
nuclear coupled nuclei.
Homonuclear couplings between abundant spins (usually protons) superimpose
their heteronuclear dipolar couplings to X-spins and J-couplings to X-spins so
these (distinct) couplings are not observable. homonuclear decoupling protons
while observing X-spins makes these couplings observable. Any method used in
multiple pulse NMR (section
5.2.1) may be used to achieve this.
BR-24, MREV-8, BLEW-12
Used as heteronuclear decoupling methods, the window between pulses may be
shortened or omitted (semi-windowless or windowless sequences). These se
-
quences work well, but have rather long cycle times and are therefore not suitable
for fast spinning samples. Else they work in a similar fashion as the sequences
covered in the following. BLEW-12 decoupling is supplied as a standard cpd-pro
-
gram. It consists of a windowless sequence of 90° pulses with suitable phases.
High RF levels for decoupling provide better resolution.
FSLG Decoupling
The Frequency Switched Lee Goldburg (FSLG) sequence may be used at spin ra-
tes up to 15 kHz. It is a homonuclear decoupling sequence which rotates the inter-
action Hamiltonian around an effective field, aligned at the magic angle (arctan
) with respect to the Zeeman field in the rotating frame. The tilt is achieved by
off resonance irradiation at the Lee Goldburg frequency f
LG
according to the Lee
Goldburg condition.
2