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2.1
INTERACTIONS INFLUENCING SOLID-STATE NMR
SPECTRUM
Usually, the interactions in the NMR are expressed as the sum of the seven interactions
shown below.
H = H
H
:
Z
H
:
σ
H
:
J
H
:
CSA
H
:
JA
H
:
D
H
:
Q
H
is the main interaction that determines the fundamental resonance frequency of the
Z
nucleus, and other interactions are expressed as perturbations of H
can observe the perturbation terms. In the six perturbation terms, H
tropic terms, and H
tween anisotropic and isotropic terms is whether the interaction is dependent on the direc-
tion of the molecule or not.
Although anisotropic terms change greatly depending on the orientation of the molecule,
they become 0 (zero) if the orientation of the molecule is changed uniformly and aver-
aged. If the molecule is carrying out rotational Brownian motion, the anisotropic terms
disappear and only the isotropic terms remain.
Spectroscopy of solution NMR is constructed under the assumption that the averaging is
carried out by such molecular motion. However, in the solution NMR method, since ani-
sotropic terms may affect a relaxation phenomenon, and anisotropic terms may appear in
a spectrum by partial orientation of molecules in high-magnetic-field NMR, we cannot
ignore it completely.
On the other hand, in solid NMR that treats a solid sample that has no molecular motion
or is constructed from small molecules, anisotropic terms become rather dominant terms
in the interaction.
How to deal with these anisotropic terms is the main subject of solid-state NMR.
Usually, in solid-state NMR, interactions are eliminated by the following two methods.
a.
Magic angle spinning(MAS)
b.
High-power decoupling
By using these techniques, H
be eliminated by magic angle spinning and high-power decoupling. Moreover, H
only in a nucleus whose nuclear spin is larger than one half, and can be eliminated by the
MQMAS (Multiple Quantum-Magic Angle Spinning) method (
Since H
JA
NMECAXS_V50-SLD-2
2 FUNDAMENTALS OF SOLID-STATE NMR MEASUREMENT
+H
+H
+H
+H
+H
σ
Z
J
CSA
JA
Zeeman interaction
Chemical shift isotropy
Spin-spin interaction (J coupling) isotropy
Chemical shift anisotropy
Spin-spin interaction anisotropy
Magnetic dipole-dipole interaction
Quadrupole interaction
, H
, H
, and H
CSA
JA
D
can be eliminated by magic angle spinning, and H
CSA
can hardly be observed, we usually ignore it.
+H
D
Q
are called anisotropic terms. The difference be-
Q
. On a spectrum, you
Z
and H
are called iso-
σ
J
can
D
exists
Q
Chap. 7).
2-1
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