Direct Determination of Motional Correlation Times by 1D MAS and 2D Exchange NMR Techniques

✍ Scribed by D.E. Favre; D.J. Schaefer; B.F. Chmelka

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 342 KB
• Volume: 134
• Category: Article
• ISSN: 1090-7807
• DOI: 10.1006/jmre.1998.1506

✦ Synopsis

One

-and two-dimensional static and magic-angle spinning (MAS) exchange NMR experiments for quantifying slow ( c > 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 2 H and 13 C 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) 13 C 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.