David M. Grant
- Research Interest
Professor Grant's research interests are
in the area of nuclear magnetic resonance with special emphasis on
the carbon-13 magnetic isotope. For many years, his group has been
interested in the correlation of both chemical shifts and spin-spin
coupling with both the electronic and geometrical structure of
molecules. Quantum mechanical calculations of these parameters using
both semi-empirical and more sophisticated wave functions provide a
theoretical basis for the correlations.
Nuclear spin relaxation depends intimately
on internuclear distances and the molecular orientational dynamics.
Spin relaxation data obtained in Professor GrantŐs laboratory have
been used to measure distances and to characterize anisotropic
molecular diffusion. The temperature dependence of such data
provides information on liquid state dynamics and the associated
thermodynamics of intermolecular interactions. The research is
focusing on coupled relaxation of multiple spins as revealed in spin
multiple relaxation. Careful preparation of the nonequilibrium spin
states allows one to reap a wealth of information not available from
a single resonance line. Such results can be used to characterize
not only anisotropic molecular reorientation but also segmental
motion in macromolecules of both synthetic and biological origin.
Recent activity in the laboratory has been
focused on the NMR of organic solids. Using cross polarization
techniques, solid carbon-13 data can be obtained under magic-angle
spinning conditions and on stationary samples. In the first instance
relatively high resolution solid spectra are obtained which contain
information not only on intramolecular but also intermolecular
features. In many systems the break in molecular symmetry, due to
crystal environmental effects, is accompanied by an increase in the
number of resonance lines. Solid NMR work on single crystals
provides a useful relationship with X-ray crystallography results
and parallel studies have been initiated.
For many solids, single crystal samples
are either impossible or unduly hard to obtain and new 2D slow
turning NMR methods have been found to provide high grade tensor
values in the acquisition dimension, with isotropic shifts appearing
in the evolution dimension. A 2D spatial correlation method for the
study of polymers provides information on the ordering found in
polymers and other incommensurate solids. As the chemical shift
tensor contains important three-dimensional information on molecular
solids, shift tensor data has high promise in future NMR studies in
Chemistry.
Without rapid spinning techniques, powder
patterns are obtained in solid samples. Work using cryogenic
equipment provides a way to obtain NMR shielding tensors for small
molecules embedded in argon matrices at cryogenic temperatures (4-20
K). Under this degree of isolation, doubly enriched molecules in two
carbon-13 isotopes yield dipolar coupled spectra from which the
carbon-carbon distances can be obtained, as well as the chemical
shielding tensor components. Even for a powder or matrix-isolated
molecule the dipole-dipole axis fixes the orientation of these
shielding tensors within an arbitrary rotation about the
carbon-carbon vector. Such information has previously required the
acquisition of data on a single crystal. A variety of solid
applications in both the fossil fuel area and the biomedical area
have proven the value of this new technique.
Work on instrumental development of the
NMR technique in both solids and liquids is also under way.
Especially important is the improvement of probes and their
performance in multiple dimension NMR experiments. Recent work has
led to the construction of a high pressure, variable temperature
cell that can be used at 500 MHz for the study of molecules that may
be dissolved in super critical fluids such as CO2 and related
materials. Molecules dissolved in super critical fluids have
motional regimes that lie between gases and liquids and such data
allow for continuity between these states of matter.
©ACW
Revised: August 09, 2001.