th  - 1987 - 51st ENC Conference

Sunday - PM
MF33 - POSTERS
INCREASED RESOLUTION FOR PROTON NMR SPECTRA OF SOLID MATERIALS
Thomas G. Neiss and James E. Roberts
Department of Chemistry #6
Lehigh University
Bethlehem, PA 18015
Sophisticated multiple-pulse techniques must be used to
obtain "high resolution" proton spectra of most solid
materials, or a single broad peak is observed as a consequence
of the large homonuclear couplings of the proton reservoir.
When Magic Angle Sample Spinning (MASS) is included with the
multiple-pulse experiment, the typical proton peak width is
still between 1 and 2 ppm. When several isotropic chemical
shifts are present, the resulting spectrum is often not
interpretable due to significant spectral overlap. Two more
complicated experiments are available for increasing the
resolution obtained in proton NMR spectra of solid materials.
Two-dimensional heteronuclear chemical shift correlation
NMR has been applied to liquids to connect the proton and
carbon chemical shifts through J couplings. The J couplings
in solids are usually not resolved. However, with appropriate
implementation during MASS, the 2-D experiment correlates the
proton and carbon chemical shifts, yielding better overall
resolution in the proton dimension, even though the proton
linewidths are still 1 to 2 ppm.
An alternative to the full two dimensional technique
utilizes selective coherence transfer to observe only specific
spin systems in a one-dimensional experiment. The selective
coherence transfer occurs through the strong heteronuclear
dipolar interaction between bonded spins, so some protons are
not readily observed with this technique. The gain in proton
spectrum resolution is comparable to that obtained with
two-dimensional chemical shift correlation, but data
accumulation and processing takes less time. Although several
I-D selective experiments might be needed to fully
characterize a molecule, it is still a viable approach in many
situations.
Acknowledgement is made to the donors of the Petroleum
Research Fund for partial support of this work. Supported by
NSF-Solid State Chemistry program grant # DMR-8553275.
Additional support through the Presidential Young Investigator
program was obtained from Cambridge Isotope Laboratories; Doty
Scientific; General Electric Corporate Research and
Development; General Electric NMR Instruments; IBM
Instruments; Merck, Sharp and Dohme; and Monsanto Company.