Molecular motion of methyl rotor in a zeolite: proton NMR lineshapes of CH3CN-2HZSM-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.