Bruker BioSpin Solid State NMR, User Manual

Page: 226 / 328

226 (327) BRUKER BIOSPIN User Manual Version 002
Basic MQ-MAS
and D shift positions for two sites with large and small d
iso
and large and small
δ
qis
and with large and small
δ
iso
and small and large
δ
qis
are plotted, respectively.
This behavior is independent of the spin quantum number and of the order p of
the experiment. Higher quantum order experiments are possible for half integer
spin quantum numbers >3/2, however, corresponding pulse programs are not pro-
vided in the pulse program library. They can easily be derived from the 3Q pulse
program by changing the phase cycle. In the 3-pulse sequence (
mp3qzqf ) e.g.
ph2 should be changed for the 5Q experiment to:
ph2 = 0 0 36 36 72 72 108 108 144 144 180 180 216 216 252 252 288 288
324 324
An 18° phase increment of the phase ph1 of the first pulse is required for States or
States-TPPI phase sensitive acquisition. For a full phase cycle a multiple of 20
scans must be used.
For the 4-pulse sequence ( mp3qzfil ) the phases should be changed to:
ph1 = 0 36 72 108 144 180 216 252 288 324
ph2 = 0*40 90*40 180*40 270*40
ph3 = 0
ph4 = 0*10 90*10 180*10 270*10
receiver = {0 180}*5 {90 270}*5 {180 0}*5 {270 90}*5 {180 0}*5 {270 90}*5{0
180}*5 {90 270}*5.
Again an 18° phase increment of the first pulse for States or States-TPPI phase
sensitive detection in F1 is needed. Thus a full phase cycle can be performed with
a multiple of 160 scans.
The usefulness of such a 5Q experiment is limited, and there are several draw-
backs: Firstly, the sensitivity is much inferior to the 3Q experiment because of the
lower transition probability and a less efficient excitation. Secondly, the shift range
(in ppm) in the indirect dimension is much smaller when a rotor synchronized ex-
periment is performed. The factors
⎜
R-p
⎜
are listed in
Table 17.5.
. The shift posi-
tions in the MQ dimension in a sheared spectrum are the same for all orders p
and therefore, no additional information can be expected. However, the observed
line widths are slightly reduced in the higher order experiments so in special cas-
es some enhancement of resolution can provide additional information.
The spectral width in the MQ dimension of the sheared spectrum is given by spin-
ning speed /
⎜
R-p
⎜
in a rotor synchronized experiment. A 5Q experiment e.g.
gives a 7.08/1.42 = 5 times smaller spectral range in the indirect dimension than a
3Q experiment.
Table 17.5. Values of |R-p| for Various Spins I and Orders p
Spin I R(p=3) |R-p| (p=
±
3) |R-p| (p=
±
5) |R-p| (p=
±
7) |R-p| (p=
±
9)
3/2 -7/9 3.78 - - -
5/2 19/12 1.42 7.08 - -
7/2 101/45 0.76 3.78 10.58 -
9/2 91/36 0.47 2.36 6.61 14.17