th  - 1988 - 51st ENC Conference

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SOLID STATE BACK PROJECTION IMAGING
JOHN LISTERUD AND GARY DROBNY
DEPARTMENTS OF ELECTRICAL ENGINEERING AND CHEMISTRY
UNIVERSITY OF WASHINGTON, SEATTLE, WA 98195
Abstract
The requirements of an NMR imaging system dedicated to materials science will be
quite distinct from those of medical imaging. Not the least of these differences will be the
degree of flexibility demanded of a research laboratory system as compared to the turn-
key philosophy of the clinical imager. In particular, the materials sciences challenge the
spectroscopist to combine the classic NMR spectroscopies with the imaging experiment.
To these ends we describe the construction of a multi-purpose microscopic NMR imaging
probe for use on a standard spectrometer, and the efficient adaptation of standard two
dimensional NMR data processing utility to image processing. The probe is capable of
a variety of experiments, including the Kumar-Welti- Ernst experiment, backprojection
by mechanical rotation of the sample, and backprojection by electronic rotation of gradi-
ents. Because of its simplicity, backprojection promises to be especially straightforward
to combine with spectroscopic techniques such as chemical shift and multiple quantum
spectroscopy. Furthermore, "macro" feature of the standard two dimensional NMR data
processing utility has a natural extension to tailored image processing, as demonstrated
here by Tl and diffusion weighting of image grey scales.
2o3 I
LONG-RANGE SHIELDING AND CHEMICAL SHIFT IN SILICON CARBIDE
POLYTYPES. M. F. Richardson, J. S. Hartman*, and D. Guo, Department of
Chemistry, Brock University, St. Catharines, Ontario L2S 3AI, Canada.
Silicon carbide, which has many polytyplc modifications of a very simple and
symmetric structure, is an excellent model system for exploring relationships
between chemical shift and crystal structure in network solids. A simple
McConnell equation treatment of bond anlsotropy effects (H. M. McConnell, J.
Chem. Phys., 1957, 27, 226) predicts chemical shifts for s ilicon and carbon
sites which agree well with experiment (J. S. Hartman et al ., J. Amer. Chem.
Soc., 1987, 109, 6059), provided that contributions from bonds up to i00 A from
the site are included in the calculation. The calculated shlf ts depend on both
the layer stacking sequence (i.e., the polytype) and on the spacings between
silicon and carbon layers. Unambiguous assignment of peaks to lattice sites
should now be possible for all polytypes, but chemical shifts are so sensitive
to layer spacings that our calculations are limited by the accuracy of layer
spacing values determined by careful X-ray diffraction work. It appears that
chemical shifts in network solids can in principle be more sensitive to atomic
positions than the most carefully obtained X-ray data. While X-ray diffraction
is necessary to determine the polytype, the most accurate values of layer
spacings in polytypes and other highly correlated structures should in future be
derived from nmr chemical shift values rather than from the X-ray data.
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