SOLID-STATE DYNAMIC PROCESSES IN COMPLEX-SYSTEMS ANALYZED BY 2-DIMENSIONAL ISOTROPIC-ANISOTROPIC CORRELATION NUCLEAR-MAGNETIC-RESONANCE

We describe the application of a recently developed two-dimensional nu clear magnetic resonance (2D NMR) technique, variable-angle correlatio n spectroscopy, to the analysis of molecular motions in complex unlabe led solids. This technique separates the broad anisotropic chemical sh ift line shapes of nuclei in a sample according to the isotropic shift of each site. It can therefore be used to characterize molecular reor ientations by monitoring the changes that these processes introduce in the resolved powder patterns as a function of temperature. Using the C-13 NMR anisotropies of dimethylsulfone as a test case, we explored t he potential applications of following such an approach. It was found that in contrast to what happens in nonexchanging systems, the anisotr opic line shapes resolved by the variable-angle technique on an exchan ging solid are different from line shapes that at similar temperatures can be recorded from a nonrotating sample. An explanation for these d ifferences is presented, and the complete theory required to extract k inetic and geometric information from the experimental 2D line shapes is introduced and illustrated with computer simulations. The capabilit y of this approach to analyze motions in complex systems is further de monstrated with a natural-abundance C-13 variable-temperature NMR anal ysis of L-tyrosine ethyl ester; a reorienting compound possessing up t o 11 inequivalent carbon sites in the solid phase.