Molecular Motions of Hydrogen Bonded CH3CN in the Zeolite Chabazite: Comparison of First-Principles Molecular Dynamics Simulations with Results from 1H, 2H, and 13C NMR

Density functional theory calculations with periodic boundary conditions, Car-Parrinello simulations, and multinuclear solid-state NMR experiments at temperatures in the range 77 to 450 K, have been performed to probe the structure and motion of acetonitrile adsorbed at an isolated Brønsted-acid site in chabazite. The 1:1 stoichiometric acetonitrile adsorption complex is hydrogen-bonded to the acid site, and two minima have been found for the position of adsorbed acetonitrile on a proton associated with a single oxygen atom in the zeolite lattice. In agreement with experiment, the results of the Car-Parrinello simulations indicate a free rotation of the methyl group protons about the acetonitrile molecular axis, as well as a motion of this axis that can be described as a two-dimensional libration about the hydrogen bond. The details and temperature dependence of the distributions describing the librational amplitudes as a function of temperature derived from the simulations are, however, not in agreement with experiment. Whereas the experimental distributions reach a limiting value at 300 K, the amplitudes continuously increase in the simulations. The reasons for this are briefly discussed.