th  - 1987 - 51st ENC Conference

Wednesday - AM
THE DETERmiNATION OF THE STRUCTURE OF PARTIALLY-ORIENTED SOLIDS
USING 2D-MAGIC-ANGLE-SPINNING N}~
Gerard S. Harbison
Department of Chemistry, SUNY Stony Brook, Stony Brook, NY 11794.
V.-D. Vogt, C. Boeffel, B. Bluemich and H.W. Spiess
Max-Planck-Institut fuer Polymerforschung, Postfach 3148, D-6500 Mainz, FRG.
High-field magic-angle-spinning (MAS) spectra of oriented samples display
variations in the phases and intensities of the MAS centerbands and sidebands,
which depend on the position of the sample order axis about the rotor axis at
the time the precession of the nuclear magnetization begins. This phenomenon
may be observed by synchronizing the spectral excitation with the sample
rotation (I), and we have shown (2) that it may be made the basis of a 2D-NMR
experiment, in which the first time dimension is the (time-dependent) rotor
position, and the second is routine unrestricted acquisition. The 2D spectra
thus obtained consist of MAS centerbands and sidebands in two dimensions,
whose intensities depend on the size of the chemical shielding tensor, its
orientation relative to the sample order axis, and the degree of disorder of
the sample about that axis. Using a spherical harmonic expansion of the
shielding tensor orientation about two Euler angles in the frame of reference
of the order axis (3), we can, without recourse to model-building, determine
the complete orientational distribution function of the residue in question
relative to the sample axis. Thus, for each distinct, resolvable resonance in
a complex sample, we can determine the preferred orientation of that residue,
and its statistical distribution about that preferred orientation. Obviously,
determining these quantities for a real sample, coupled with the geometrical
restrictions imposed by the laws of chemistry, is tantamount to determining
its structure on the atomic scale.
We have applied our experimental and theoretical protocols to a number of
oriented polymers, among them polyethylene terephthalate (PET) and several
liquid-crystalline polymers. We shall illustrate their use with a discussion
of the structure of the crystalline and amorphous phases of PET, in which the
results obtained by our method are in close agreement with existing x-ray
studies. We shall also consider their potential application to other systems.
(1) M. blaricq & J.S. Waugh (1979) J. Chem. Phys. 70:3300
(2) G.S. Harblson & H.W. Spiess (1986) Chem. Phys. Lett. 124:128
(3) G.S. Harbison, V-D. Vogt & H.W. Spiess (1987) J. Chem. Phys., in press.