L. Frydman et al., SOLID-STATE DYNAMIC PROCESSES IN COMPLEX-SYSTEMS ANALYZED BY 2-DIMENSIONAL ISOTROPIC-ANISOTROPIC CORRELATION NUCLEAR-MAGNETIC-RESONANCE, The Journal of chemical physics, 101(1), 1994, pp. 111-117
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.