DIRECT DETERMINATION OF MOTIONAL CORRELATION TIMES BY 1D MAS AND 2D EXCHANGE NMR TECHNIQUES

One- and two-dimensional static and magic-angle spinning (MAS) exchang e NMR experiments for quantifying slow (tau(c) > 1 ms) molecular reori entation dynamics are analyzed, emphasizing the extent to which motion al correlation times can be extracted directly from the experimental d ata. The static two-dimensional (2D) exchange NMR experiment provides geometric information, as well as exchange time scales via straightfor ward and model-free application of Legendre-type orientational autocor relation functions, particularly for axially symmetric interaction ten sors, as often encountered in solid-state H-2 and C-13 NMR. Under cond itions of MAS, increased sensitivity yields higher signal-to-noise spe ctra, with concomitant improvement in the precision and speed of corre lation time measurements, although at the expense of reduced angular ( geometric) resolution. For random jump motions, one-dimensional (1D) e xchange-induced sidebands (EIS) C-13 NMR and the recently developed OD ESSA 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 simpl e exponential fit to the integrated peak intensities measured as a fun ction of mixing time. This is demonstrated on polycrystalline dimethyl sulfone, 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. (C) 1998 Academic P ress.