th  - 1988 - 51st ENC Conference

31 DELAYED REFOCUSSING TWO-DIMENSIONAL NMR IN ROTATING SOLIDS
A.C. Kolbert *'1'2, D.P. Raleigh 1'2, H.B. Levitt 2, R.G. Grlffin 2
1
Department of Chemistry
and
2
Francis Bitter National Magnet Laboratory
Massachusetts Institute of Technology
Cambridge, HA 02139
We describe a new class of two-dimenslonal MASS NHR experiments designed to
measure small coupling tensors. The experiment, in its simplest form, involves
the placement of a K-pulse at tl/2 after cross-polarizatlon, that is, in the
middle of the evolution period, followed by unrestricted sampling during t~.
The effect of the n-pulse is to delay rotational echo formation in t I resulting
in the FID in t. having rotor echoes spaced at 2T . The 2-D spectrum resulting
i
from this experiment will have rotational sideban~s spaced at ~ /2 in the ~.
dimension, while maintaining the effective spinning speed in ~2 ~ A further i
example of experiments in this class is provided by an experiment which yields
rotational sldebands at ~r/3 in ~I' and involves the placement of E-pulses at t 1
and 2ti/3.
32
MEASUREMENTS OF TWO-DIMENSIONAL NMR POWDER PATTERNS IN ROTATING
ISOLIDS.T. Nakal, J. Ashida and T • Terao* • Department ~ of Chemistr- y•
Faculty of Science• Kyoto University, Kyoto 606• Japan.
Switching-angle sample-spinnlng techniques for measuring the heteronuclear
dipolar/chemical shift 2D powder patterns are reported. The techniques have the
advantages of the high signal-to-noise ratio and the low distortion of the spectrum
compared with those in stationary powder samples• Furthermore, for compounds with
more than one chemically distinct nucleus• the individual 2D powder patterns can be
separately obtained by 3D NMR. Practical applications of these techniques are
demonstrated with the 13C 2D powder patterns of calcium formate, polyethylene, and
polyacetylene. The chemical shift tensors and proton positions in calcium formate
were obtained for the two crystallographically inequivalent formate ions, which
agree with the results already reported by single crystal studies of 13C NMR and
neutron diffraction. The chemical shift principal axes in polyethylene were found
to be only approximately along the symmetry directions of the CH 2 group• indicating
a strong perturbation of the electric environment by the crystal field•
Current address: Department of Chemistry• University of California, Berkeley•
CA 94720.
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