
no signal can be observed (Wind et al. 1988). In these
cases, obtaining a spectrum with sample spinning
sidebands is usually preferred. Alternatively, Dixon
(1981) has devised a CPMAS NMR experiment to
separate spinning sidebands from isotropic reso-
nances, although this experiment may not always
produce the desired results, sometimes distorting
intensities due to relaxation effects.
Finally, in some cases, CP spin dynamics may not
be suitable for a successful experiment. As shown
in Fig. 2, if the T
1r
/T
IS
ratio is too small, CP may
become difficult or impossible to detect (Alemany
et al. 1983b).
5. Summary
While
13
C and
29
Si CPMAS NMR was the focus of
this discussion, other nuclei can be observed, includ-
ing
15
N,
119
Sn, and
31
P. It is also possible to generate
CP intensity from nuclei other than
1
H. For example,
13
C—
19
F CPMAS spectra can be obtained for fluor-
inated polymers. In a number of respects CPMAS
NMR is experimentally more demanding than con-
ventional liquid NMR. However, CPMAS NMR is
invaluable in the study of solids, providing both static
(structural) and dynamic (motional) information.
While there are some samples which cannot be stud-
ied by CPMAS NMR, this technique can be applied
to a wide variety of materials, regardless of purity,
crystallinity, or other physical attributes.
Acknowledgments
E. A. Williams provided helpful discussions and
P. Donahue helped proof-read the manuscript.
Ye-Feng Wang of GE Silicones, Waterford, NY,
provided the heat-cured elastomer.
See also: High-temperature Superconductors, Cup-
rate: Magnetic Properties by NMR/NQR; Nuclear
Magnetic Resonance Spectrometry; Nuclear Reso-
nance Scattering
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General Electric Company, Niskayuna
New York, USA
Spin Fluctuations
1. Introduction
In general, the term spin fluctuations refers to fluc-
tuations of the magnetic (electron spin) moment in
magnetic systems, both ordered and paramagnetic
ones. In the ground state (zero temperature, zero
1094
Spin Fluctuations