Molecular Motion of Hydrogen-Bonded CH \u3c inf\u3e 3 CN in H-MFI: A \u3c sup\u3e 1 \u3c/sup\u3e H, \u3c sup\u3e 2 \u3c/sup\u3e H, and \u3c sup\u3e 13 \u3c/sup\u3e C Multinuclear Nuclear Magnetic Resonance Study

The dynamics associated with the adsorption complex formed by CH3CN at Brønsted sites in the high-silica zeolite H-MFI have been explored using 1H, 2H, and 13C nuclear magnetic resonance (NMR) spectra measured as a function of temperature between 78 and 400 K. A simple NMR line-shape theory, based on rapid, small-angle reorientations of the CH3CN molecular axis with a temperature-dependent amplitude, has been used to account for the data. An anisotropic, angular motion is observed with a small amplitude at low temperatures increasing to approximately ±35 ° 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

\u3c sup\u3e 13 \u3c/sup\u3e C chemical shielding anisotropy in the solid phases of CH \u3c inf\u3e 3 \u3c sup\u3e 13 \u3c/sup\u3e CN

The principal elements of the axially symmetric nitrile 13C chemical shift tensor, and the C≡N bond distance of solid CH3CN have been determined from both static and sample spinning NMR experiments at 78 and 140 K. The spectra show no significant differences between the α and β forms of the solid though a large temperature coefficient is observed for both the isotropic chemical shift, σ̄, and the 14N nuclear electric quadrupole interactions as seen through the carbon-nitrogen dipole interaction

Molecular motion of methyl rotor in a zeolite: Proton NMR lineshapes of CH \u3c inf\u3e 3 CN- \u3c sup\u3e 2 \u3c/sup\u3e HZSM-5 adsorption complex

Proton lineshapes of methyl group in spatially isolated stoichiometric 1:1 CH3CN-2HZSM-5 adsorption complexes are shown as a function of temperature. At 78 K, the methyl group dipolar lineshape is characteristic of a fixed-axis tunneling planar rotator. With increasing temperature, one observes motional averaging associated with the reorientation of the symmetry axis of the hydrogen-bonded acetonitrile at the acid site. Since changes in dipolar anisotropy with temperature are determined by confinement interactions, lineshape measurements can be used to explore dispersion interactions in zeolitic frameworks. These interactions are highly dependent on differences in size, geometry and rigidity of zeolite cavities