NMR imaging of components and materials for DOE application Page: 11 of 19
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The indirect dipolar interaction or J-coupling is present in the solid state as it is in the liquid
state. Its magnitude is insignificant compared to the direct interaction. The techniques,
therefore, which average the larger dipolar interactions to zero most certainly cause the
smaller interactions to disappear.
Another complicating factor is a mechanism called the chemical shift anisotropy. This
mechanism, which scales in magnitude directly with field strength, is averaged to its isotropic
value (the chemical shift) in solution. In a solid sample, the chemical shift of a given nucleus
depends upon the orientation that the particular nucleus' host molecule lies with respect to
the external magnetic field. Each orientation gives rise to a unique electronic environment.
In a crystalline solid, where all orientations with respect to the external field are possible, the
resonances for all orientations of a given nuclei then lie within a given chemical shift range
(i.e., the chemical shift anisotropy). The total value of the chemical shift anisotropy may be
several tens of kHz.
The final complicating line-broadening mechanism is the quadrupolar interaction. For half
integral spins whose I ; %, a non-spherical charge distribution arises at the nucleus. The
non-spherical charge density gives rise to an electric quadrupole moment which couples with
electric field gradients. In electronic environments of high symmetry, there are no electric
field gradients and thus no quadrupole interaction. However, such symmetry is rarely the
case, so for most quadrupolar nuclei the quadrupolar interaction exists. We assume a "high
field" case, where the magnitude of the Zeeman interaction predominates and controls the
magnitude of the energy gap between the %/2 - /z states. The quadrupolar interaction slightly
raises or lowers their energy level and thus the resonance frequency. The frequency change
is angularly dependent, and a powdered sample gives rise to all possible angles with respect
to the external field. The range of resulting frequencies gives rise to broad lines for
quadrupolar nuclei; however, imaging of quadrupolar nuclei are not as prevalent as I = %
nuclei. Some examples of imaging were mentioned earlier in this review.(21-23)
Electronic and mechanical means can be used to minimize the line broadening caused by the
above interactions. The 'H-'3C dipolar interaction (and its smaller indirect coupling
counterpart) can be averaged to zero through the use of high-power proton decoupling. High-
power proton decoupling, of course, works well for the observation of 13C nuclei(27), but
could not be used for 'H imaging since one would be equalizing the population between the
'H spin states, thus eliminating the signal.
For eliminating the 'H-'H dipolar interaction, an examination of the Hamiltonian governing the
interaction reveals another term which can be exploited. The Hamiltonian contains both a
geometric part and a spin-space term. Application of "coherent signal averaging techniques,"
another name for a specific multiple-pulse sequence, moves the macroscopic magnetization
through states that when summed over a cycle of pulses renders the dipolar Hamiltonian (to
zeroth order) to zero. The first such multiple-pulse sequence was named WAHUHA for its
creators, Waugh, Huber, and Haeberlin.(4) More efficient pulse sequences have been
developed such as the MREV-8(28), BR-24(29), and CMG-48(30)(vide supra). When used
properly, these sequences average the dipolar interaction to zero and yield linewidths that can
be profitably imaged.-8-
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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/m1/11/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.