Bruker BioSpin Solid State NMR, User Manual

Page: 225 / 328

Basic MQ-MAS
User Manual Version 002 BRUKER BIOSPIN 225 (327)
Obtaining Information from Spectra 17.5
The referencing procedure in
xfshear
defines the axis in the MQ dimension such
that:
(Eq. 17.2)
The value of
δ
qis
is given by:
(Eq. 17.3)
In
(Eq. 17.3)
I is the spin quantum number, Q
cc
the quadrupolar coupling con-
stant,
ω
0
the Larmor frequency, and h the asymmetry parameter. This makes
δ
MQ
µ
ω
0
-2
, which causes the MQ positions to be field dependent. An interesting be-
havior results as one compares spectra at different fields. Plots of the function
δ
MQ
over
ω
0
-2
are shown in
Figure 17.9.
for an arbitrary sample with two sites. If
the isotropic chemical shifts of the two sites are identical, then it is obvious that
the separation of the two lines increases as the field decreases (plot A). In the op-
posite case of identical quadrupole couplings separation increases as the field is
increased (plot B). In cases where a difference in isotropic chemical shift
δ
iso
ex-
ists and the sites have different quadrupole couplings the relative positions de-
pend on which site has the larger quadrupole coupling. The separation of the lines
will always increase as the field decreases (plots C and D), but in some cases a
crossover of the shift positions may be observed as the field
B
0
is altered (plot C).
Figure 17.9. Calculated Shift Positions
δ
MQ
Calculated shift positions
δ
MQ
as function of the static magnetic field B
0
for two dif-
ferent sites with arbitrary
δ
iso
and
δ
qis
. The x axis in each plot is the static magnet-
ic field B
0
increasing from left to right; the y axis
δ
MQ
increases from bottom to top.
Plot A is for identical
δ
iso
, plot B for identical quadrupole coupling and. In plots C
qis iso MQ
δ δ δ
17
10
− =
( ) ( )
( ) ( )
5
2
2
0
2
2
10
3
1
1 2 4
3 1 4 3
∗
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+ ∗
−
− +
− =
η
ω
δ
cc
qis
Q
I I
I I