3-DIMENSIONAL VARIABLE-ANGLE NUCLEAR-MAGNETIC-RESONANCE EXCHANGE SPECTROSCOPY WITHOUT ROTOR AXIS HOPPING

Slow, large-amplitude chain motions play an important role in determin ing the macroscopic mechanical properties of polymers. Although such m otions have been studied quantitatively by two-dimensional (2D) nuclea r: magnetic resonance (NMR) exchange experiments, overlapping anisotro pic patterns hamper spectral analysis, and limit applications. Variabl e angle correlation spectroscopy (VACSY) has proven useful in resolvin g such problems for rapidly spinning samples by separating anisotropic spectral patterns according to isotropic chemical shifts. In a previo us study [J. Am. Chem. Sec. 115, 4825 (1993)], we described a three-di mensional (3D) NMR experiment that incorporates; the VACSY method and a hop of the rotor axis to correlate the isotropic chemical shifts to 2D anisotropic exchange patterns. The hop of the rotor axis, however, presents experimental difficulties and limits the range of motional ra tes that may be studied. We present in this paper a new 3D VACSY excha nge experiment that obtains the same correlations without the need for the rotor axis hop. A series of 2D exchange spectra are recorded with the sample spinning at different rotation axis angles.' Then using th e scaling of the anisotropic frequency at the different angles, we con struct the data onto a 3D matrix so that a Fourier transformation dire ctly yields the desired correlations. The technique is applied to C-13 exchange NMR to study the slow molecular motion of ordered isotactic polypropylene.