CPD (Proton) Decoupling
Historically, a Broad-Band (BB-) modulation (or Noise modulation) scheme has been
used for proton decoupling. Most samples were run using 10 mm probes and tubes. When
using BB in a normal 1-pulse experiment or in an .AU program, a hardwired modulator is
used. The efficiency is not great; as a result about 5 to 10 Watts of decoupler power is needed
for carbon spectra in a 10 mm probe. For most phosphorous spectra, less power ( 1 to 2
Watts) is sufficient.
The invention of the Composite Pulse Decoupling (CPD) sequence(s) allows more
efficient decoupling with lower power levels.
CPD consists of a sequence of pulses of a specific flip angle and re-phase. The sequences
require that the rf-power and the pulse width are reasonably well adjusted, otherwise decoupling
will not work very well.
The CPD sequence normally used for 13C spectra with Proton decoupling is named
Waltz-16. It consists of a sequence of 16 90 degree pulses. For reasonable decoupling,
the 90 degree pulses need to be withing about 10 %. This corresponds to a 1 db change in the
decoupler power level. If necessary, adjust the power level.
On the AM spectrometers the typing 'CPD' activates a CPD sequence. This command is used before
starting an acquisition with the 'ZG' command.
In 'AU'-type sequences, eg. 'POWGATE.AU', the "GO=n CPD"
pulse program statement activates a WALTZ-16 type sequence (as on BVX-300 and UCB-200).
In DISNMR, P9 is always used for the CPD 90o pulse.
(except if the "CPDL" feature is used, then the pulse will be assigned by the
CPD sequence specifically written. To date, we have not made any use of this feature.)
With 5 mm probes the decoupler level switching is not really needed
anymore. On the AM-400, you can use 'DP=17H' (QNP probe) and 'CPD'. An organic
sample in C6D6 warms only 2.5o with the V.T. gas flow at 4 (upper edge of float). With
POWGATE, the temperature rises about 1.5o , not a significant difference.
The Inverse Broad Band probe is even more efficient. DP of 25H is used with P9 = 68 us!
On the AM-500, CPD provides good decoupling at 2 Watt or less. With the Inverse probe, S2
(DP) of 17H and P9 of 97 msec are used. For the 5 mm Broad Band probe, P9 needs to be at
about 105 msec with S2 (DP) of 13H. The 10 mm BB probes use about 110 msec at 12H.
Determining Decoupler Settings for CPD AM-4/500:
Note that CPD critically depends on the pulse width used within the CPD sequence as
well as the decoupler power. These values depend on the probe in use. If either setting is
off, or the probe de-tuned, decoupling will be incomplete. This manifests itself as residual
proton coupling; signals coupled to protons can be broadened or even visibly split.
If the decoupling seems in need of improvement, the 90o decoupler pulse should be
measured with e.g. S3=13H (5 mm BB on AM-500) or whatever power will be used. Use
the DEPT90.AU sequence to measure 90o decoupler pulse.
pulse program uses p3 for the 90 deg decoupler
The DEPT90.AU pulse program uses P1 for the decoupler pulse. Be sure that the O2 offset is on
resonance, meaning within a few Hz of the proton signal of e.g. dioxane (ASTM sample)!
(On the AM-400, O2 = 5770; D2 =3.52 ms). After the 90 degree pulse has been determined, use
that value for P9 used by CDP decoupling.
Determining Decoupler Settings for CPD AMX-4/500:
On AMX spectrometers, the same approach is used, excpet that the parameters are different. The
powagte pulse program uses the power levels S2 and S4.
The dept90 pulse program uses p3 for the 90 deg decoupler
pulse at power level S1, S2 is the ecoupler power level for CPD decoupling.
The CPD decoupling sequence may use P31, not P9 (some pulse program comments are
incorrect, BRuker changed the convention after some of the the pulseprograms were written...)
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Last Update: 2/9/95.