th  - 1988 - 51st ENC Conference

MON i0:25
NMR STRATEGI'ES AND HIGH-SPEED MAS, G.E. Maciel * C.E. Bronnimann
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S.F.Dec, B.L. Hawkins and R.A. Wind, Department of Chemistry,
Colorado State Univesity, Ft. Collins, CO 80523
With MAS speeds inching toward 30 KHz, a variety of important possibilities
and issues arise in solid-state NMR. One of the most direct benefits of high-speed
MAS is the ability to reduce spinning sidebands and r~e~move them from spectral
regions of interest. This is especially beneficial in ~'Al NMR, allowing the use
of high fields with the corresponding reduction in the second-order quadrupole
effect. Examples will be shown.
The temptation to employ high-speed MAS as a high-resolution ~oli~-state IH
NMR technique must be considered in relation to the nature of the~H-~H dipolar
interactions in each sla~_P1~e ~ Direct comparisons of CRAMPS and MAS only r,~ults
show that when strong ipolar interactions are present, as expected, the
CRAMPS approach provides far superior results.
The anticipated interference of high-speed MAS with CP efficiency i ~H r ~ dily
demonstrated, even in systems with strong dipolar interactions. In - C CP
experiments carried out as a function of MAS speed, the Hartmann-Hahn match curves
differ dramatically for carbons with directly attached hydrogens relative to carbons
without. Hence, the use of high-speed MAS to overcome spinning sideband
problems in high-field CP-MAS experiments seems problematical. One potential
approach to this kind Of problem may be stop-and-go (STAG) spinning, in which CP
occurs during a static-sample period in the STAG-sequence. Some STAG results will
be shown.
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