Bruker BioSpin Solid State NMR, User Manual

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User Manual Version 002 BRUKER BIOSPIN 127 (327)
9
Modifications of
FSLG HETCOR
9
The basic HETCOR sequence can be improved in several respects. The protons
which are observed are all coupled to
13
C carbons (since we observe these). So
the proton shifts evolve also under the residual dipolar coupling and the J-cou-
pling to
13
C. This can be refocused by a
13
C π -pulse in the middle of the proton
evolution. The pulse program
lghetfqpi
will serve this purpose.
Furthermore, it may be desirable to compare the proton shift spectrum obtained
with X-observation (HETCOR) with the proton spectrum obtained by CRAMPS
techniques (see chapters 22, 23, and 24), observing the protons directly. Usually,
these experiments use phase modulated shapes (PMLG or DUMBO). In order to
make both experiments comparable, it is favored to use the same type of proton
shift evolution in both sequences. The pulse programs which use phase gradient
shapes to achieve homonuclear proton dipolar decoupling are
pmlghet
and
wp-
mlghet
. If DUMBO decoupling is desired, the pulse programs are dumbohet or
edumbohet. These pulse programs use either windowless pulse trains, or win-
dowed pulse trains which can be timed in exact analogy to the CRAMPS-type se-
quences wpmlg*2 and dumbo*2. These sequences also suppress the center ridge
efficiently so that the carrier frequency need not be shifted out of the proton range
during evolution (cnst24=0). In contrary, it is possible to shift the carrier to the pro-
ton shift range center.
A third modification addresses the problem of poor discrimination between sites
which are strongly and weakly coupled to protons. In the standard sequence this
is solely achieved setting contact times short. Of course, this reduces cross peaks
from remote couplings more than it reduces cross peaks from directly bonded pro-
tons. However, the remote couplings are always present through the homonuclear
coupling between all protons. These couplings can however be suppressed by ex-
ecuting the contact with a Lee-Goldburg proton offset. Then the protons are
homonuclear decoupled, and the transfer from protons to X only follows the heter-
onuclear dipolar coupling between those. The pulse program
lghetfqlgcp
works
completely in analogy to
lghetfq
, but executes the contact at a proton offset cal-
culated from the proton RF field during the spin lock contact pulse. This modifies
the HH condition, which must be reestablished using the pulse program lgcp.
In the following sections, the specifics of these modified sequences are dis-
cussed.