Basic MQ-MAS
User Manual Version 002 BRUKER BIOSPIN 227 (327)
We see that a 5Q experiment has a 5 times smaller range than the 3Q experiment
and therefore, folding of peaks will always occur even at fast spinning. For even
higher quantum orders the shift ranges are 7 and 30 times smaller for 7Q and 9Q
than for the 3Q experiment, respectively.
Table 17.6. summarizes ppm ranges for
the maximum spinning frequencies of 2.5, 3.2, and 4 mm probes, respectively. A
Larmor frequency of 100 MHz is assumed. One can see that the ranges become
less than the typical chemical shift range for many nuclei. The expression
⎜R-p ⎜ acts like a scaling factor that scales the frequency scale directly. Mathemat-
ically this is solved in the AU program xfshear in such a way that the observe Lar-
mor frequency is multiplied by the factor ÁR-p Á to redefine an apparent Larmor
frequency in the MQ dimension.
Figures are calculated for a Larmor frequency of 100 MHz.
From the isotropic shift and the shift position in the MQ dimension the so-called
SOQE parameter can be calculated, d
qis
being given by equation 2:
(Eq. 17.4)
with
(Eq. 17.5)
f(I) equals 4, 16.67, 39.2, and 72 for I=3/2, 5/2, 7/2, and 9/2, respectively. So one
can see that for a given value of Q
cc
the second order quadrupole induced upfield
shift d
qis
decreases as the spin I increases. With d
qis
always being negative this
has a direct influence on the appearance of the sheared 2D spectra.
Figure 17.10. shows 2D
17
O 3QMAS spectra at 11.7 T and 18.8 T where the Lar-
mor frequency of this nucleus is 67.8 and 108.4 MHz, respectively. The sample is
Table 17.6. Chemical Shift Ranges for all MQ Experiments for All Spins I
Spin I and MQ
Experiment
15 kHz
[4 mm probe]
25 kHz
[3.2 mm probe]
35 kHz
[2.5 mm probe]
3/2 39.6 ppm 66.0 ppm 92.4 ppm
5/2 3Q
5/2 5Q
105.6 ppm
26.1 ppm
176.0 ppm
35.2 ppm
246.4 ppm
49.3 ppm
7/2 3Q
7/2 5Q
7/2 7Q
197.4 ppm
39.5 ppm
14.1 ppm
329.0 ppm
65.8 ppm
23.5 ppm
460.6 ppm
92.1 ppm
32.9 ppm
9/2 3Q
9/2 5Q
9/2 7Q
9/2 9Q
319.2 ppm
63.2 ppm
45.6 ppm
10.6 ppm
532.0 ppm
160.4 ppm
76.0 ppm
17.7 ppm
744.8 ppm
144.9 ppm
106.4 ppm
24.8 ppm
()
5
2
0
2
2
103
1
ω
δ
η
IfQSOQE
qiscc
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+=
()
()()
()()
3143
124
2
−+
−
−=
II
II
If
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