Molecular motion of hydrogen-bonded CH3CN in H-MFI: A H-1, H-2, and C-13 multinuclear nuclear magnetic resonance study

The dynamics associated with the adsorption complex formed by CH3CN at Bron sted sites in the high-silica zeolite H-MFI have been explored using H-1, H -2, and C-13 nuclear magnetic resonance (NMR) spectra measured as a functio n of temperature between 78 and 400 K. A simple NMR line-shape theory, base d on rapid, small-angle reorientations of thr CH3CN molecular axis with a t emperature-dependent amplitude, has been used to account for the data. An a nisotropic, angular motion is observed with a small amplitude at low temper atures increasing to approximately +/-35 degrees from its average position at room temperature. This motion is primarily constrained to a plane in the zeolite, but a distribution in amplitudes for different types of sites is required to fully account for the data. At higher temperatures, the powder line shapes are completely motionally narrowed, presumably due to exchange between physisorbed molecules and those bound to the sites.