NMR imaging of components and materials for DOE application Page: 10 of 19
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crystalline structure across the crystal face. Enough data were obtained to get an image of
some detail and ammonia diffusion information.
NMRI OF SOLID SAMPLES
So far, this review has addressed liquid samples, liquid-like samples, or samples that could be
made to act more liquid-like by increasing the sample temperature or adsorbing the material
of interest into a porous solid matrix. Indeed, such an approach tries to study every possible
application of conventional NMRI and pushes conventional NMRI to its limit. But where does
the realm of conventional NMRI end and the more sophisticated instrumentation and pulse
programming of solid-state NMRI/NMR begin? The answer to this question, again, is entirely
sample-related. There must be sufficient motion of the molecules in question to create
nuclear relaxation times (T, or T2) long enough to give natural linewidths of tens of Hertz (Hz)
(with conventionally chosen gradient strengths). If this is not the case, then the NMR image
will be extensively blurred unless some type of solid-state line narrowing technique is used
to initially mimic the isotropic motion of molecules in the liquid state. Line narrowing can be
combined with higher gradient field strengths to theoretically produce images of solid samples.
These experiments can be performed on conventional solid-state NMR spectrometers,
equipped with accessories to provide field gradients, imaging, and pulse programming. But
with all these complications and increasing complexity of the experiment, what continues to
motivate the NMRI of solids? As pointed out by Jezzard(2), solid-state imaging can a) provide
a means of morphological study of polymer blends and composites, b) theoretically allow any
microscopic/macroscopic NMR-detectable property to be imaged, and c) reveal information
on aggregation and blending of highly complex solids in a non-invasive way. The high
explosive blends and other composite materials used in the DOE weapons complex need such
an examination so that the properties of the material in its native, solid state are revealed
instead of performing solvent digestion/dissolution prior to characterization. Such sample
treatment may destroy the very information we require. To understand the line narrowing
techniques that are available, a brief introduction to solid-state line broadening mechanisms
follows.
Line broadening is caused from one or more mechanisms. The most important, which is
averaged to zero by rapid isotropic molecular motion, is the dipole-dipole interaction. For most
organic systems, the most important dipole-dipole interactions are the 'H-'H and the 'H-13C
dipolar interactions. Their magnitudes are approximately 50 and 20 kilohertz (kHz),
respectively. This compares to the more widely known indirect or J-coupling, whose
magnitude is from approximately 1 to 270 Hz. In solution, molecules and sections of
molecules move much faster than this; by doing so, the direct dipole-dipole interactions are
averaged to zero. This is not so with solid samples, and a single peak may be split into a large
number of multiplets, the size and magnitude of which is dependent on the total number of
dipolar coupled nuclei, the angle between them and the external field, and the inverse of the
distance between them cubed.-7-
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Richardson, B. R. NMR imaging of components and materials for DOE application, report, December 1, 1993; United States. (https://digital.library.unt.edu/ark:/67531/metadc1318990/m1/10/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.