Bruker BioSpin Solid State NMR, User Manual

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Double-CP
User Manual Version 002 BRUKER BIOSPIN 261 (327)
4. Now we select the shape to use for the C-N contact. To find the HH contact
more rapidly (it is a very narrow condition) it is recommended to start with a
ramp shape. In order to find a HH condition independent of the type of shape,
it is recommended to select shapes which are all centred around 50% ampli-
tude, which allows arbitrary amplitude modulation without changing the HH
condition. For a start, generate a ramp shape from 45 to 55% with 100 slices,
using shape tool (
stdisp
). Store the ramp as ramp4555.100. Select this ramp
as
spnam1
.
Figure 20.4. Shape Tool display with ramp shape from 45 to 55%.
The amplitude factor is 50%, corresponding to 50% RF field or a power level
change of 6 dB, since the amplitude corresponds directly to the pulse voltage.
5. Since the shape is centred around 50%, the RF voltage here is down by a fac-
tor of 2 (= 6 dB in voltage), the power must be increased by 6 dB to get the
same RF field as with a 100% square pulse.
6. Usually, the ramp shape is set on
13
C (but it can also be used on
15
N). Set
pl5
to 46 or 24 kHz RF field on
15
N. Set
spnam1
to
ramp4555.100,sp1
to 35 kHz
RF field on
13
C - 6 dB. Set
pl13
=
pl12
for a start, set
p16
to 5 msec. Optimise
the power level
pl5
. A variation over -1 to +1 dB in steps of 0.2 dB should be
ample. In order to be sure, one can optimise
sp1
and
pl5
as an array,
sp1
in
steps of 0.5 dB,
pl5
in steps of 0.2 dB. Use a full phase cycle to avoid signal
from a direct proton to carbon transfer (which is cancelled by the phase cycle).
Optimise
p16
between 5 and 15 msec. See fig. 6 for an optimization of
pl5
(
15
N square pulse power).
7. With a ramp shape for the N-C transfer, one should get 40-50% DCP efficiency
(
Figure 20.7.
), compared to the reference direct
13
C CP spectrum. If this can-
not be achieved, even with careful HH matching, the following parameters
should be checked:
8. Re-optimize
p16,
the optimum should be > 10 msec. If the signal gets worse
with longer contact time, there is a loss due to direct
13
C-
1
H contact. Minimise
this loss in the following way: