Abstract
Theory and application are described of a new rotor-synchronized 3D-NMR (three-dimensional nuclear magnetic resonance) experiment, that correlates molecular structure, order, and dynamics in partially oriented solids. General expressions for the sideband intensities in three frequency dimensions are developed for uniaxially ordered samples undergoing ultraslow molecular motions. For an isotropic powder the exchange pattern is confined to a single plane perpendicular to one of the frequency axes. For a partially ordered solid without exchange the sideband pattern is confined to a cross-diagonal plane. Sidebands in the full 3D cube result only if the residue giving rise to the signal is involved in both, molecular order and dynamics. The experiment, therefore, correlates these two properties and relates them to the chemical structure. Moreover, it offers a means to exploit the superior information about the geometry of the molecular motions provided by partially ordered samples under high resolution magic angle spinning (MAS) conditions. The technique is illustrated by 3D MAS spectra of highly oriented poly(oxymethylene), where strong positive and negative sidebands in the full 3D cube result from the helical jump motion.
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