User Manual Version 002 BRUKER BIOSPIN 89 (327)
5
Decoupling
Techniques 5
Line shapes in solids are often broadened by dipolar couplings between the spins.
If the coupled spins are of the same kind, it is called homonuclear dipolar cou
-
pling. heteronuclear dipolar couplings exist between nuclei of different kind. While
most dipolar couplings between X-range nuclei can be removed by magic angle
spinning, couplings between
1
H,
19
F and X nuclei cannot easily and efficiently be
removed by spinning.
Decoupling of homonuclear and heteronuclear interactions
can be obtained by different forms of rf-irradiation with or without sample spinning.
It is possible to suppress homonuclear couplings without suppressing heteronu
-
clear couplings. Most frequently, the nucleus
1
H must be decoupled when X-nu-
clei like
13
C or
15
N are observed, since it is abundant and broadens the line
shapes of coupled X-nuclei strongly.
Heteronuclear Decoupling 5.1
CW Decoupling 5.1.1
CW decoupling simply means irradiating the decoupled spins (usually protons)
with RF of constant amplitude and phase. The decoupling program is called cw or
cw13 and it uses pl12 or pl13, respectively. The decoupling programs select the
power level and pl12 does not need to be specified in the pulse program, if it is
not used elsewhere. In the decoupling program there is also a statement setting
the RF carrier frequency, according to the parameter cnst21, which is zero (on re
-
sonance) by default. In order to optimize decoupling, one uses the highest permit-
ted rf-field (e.g. 100 kHz for 4mm probes) and optimizes the carrier frequency o2
or o2p using popt.
The cw –decoupling program is written as follows:
0.5µ pl=pl12 ; reset power level to default decoupling power level
1 100up:0 fq=cnst21; reset decoupling carrier frequency to o2+cnst21
jump to 1 ; repeat until decoupler is switched off by do in the
; main ppg
CW decoupling suffers from the fact that protons have different chemical shifts, so
irradiating at a single frequency does not decouple all protons evenly. At higher
magnetic fields this becomes more evident, since the separation due to the mag
-
netic field increases. CW decoupling requires fairly high decoupling power to be
efficient.
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