Structure and order in partially oriented solids: Characterization by 2D-magic-angle-spinning NMR

Theory and applications of a new 2D-NMR technique for characterizing the molecular structure and order of partially oriented solids are described. After deriving a general expression for the signal intensities in two dimensions in cases of perfect order, the influence of disorder on the spectra is discussed. It is demonstrated that an expansion of the orientational distribution function in terms of Wigner rotation matrices allows spectra from residues with an arbitrary degree of order and arbitrary chemical shielding tensor orientation to be fully characterized. The technique is illustrated by obtaining the orientational distribution function for the crystalline and amorphous regions in semicrystalline polyethylene terephthalate, showing that the molecular orientation is quite different in the two phases.