Direct determination of motional correlation times by 1D MAS and 2D exchange NMR techniques
One- and two-dimensional static and magic-angle spinning (MAS) exchange NMR experiments for quantifying slow (tauc > 1 ms) molecular reorientation dynamics are analyzed, emphasizing the extent to which motional correlation times can be extracted directly from the experimental data. The static two-dimensional (2D) exchange NMR experiment provides geometric information, as well as exchange time scales via straightforward and model-free application of Legendre-type orientational autocorrelation functions, particularly for axially symmetric interaction tensors, as often encountered in solid-state 2H and 13C NMR. Under conditions of MAS, increased sensitivity yields higher signal-to-noise spectra, with concomitant improvement in the precision and speed of correlation time measurements, although at the expense of reduced angular (geometric) resolution. For random jump motions, one-dimensional (1D) exchange-induced sidebands (EIS) 13C NMR and the recently developed ODESSA and time-reverse ODESSA experiments complement the static and MAS two-dimensional exchange NMR experiments by providing faster means of obtaining motional correlation times. For each of these experiments, the correlation time of a dynamic process may be obtained from a simple exponential fit to the integrated peak intensities measured as a function of mixing time. This is demonstrated on polycrystalline dimethylsulfone, where the reorientation rates from EIS, ODESSA, time-reverse ODESSA, and 2D exchange are shown to be equivalent and consistent with literature values. In the analysis, the advantages and limitations of the different methods are compared and discussed. Copyright 1998 Academic Press.
Favre
DE
Department of Chemical Engineering, University of California, Santa Barbara, California, 93106, USA.
Schaefer
DJ
Chmelka
BF