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

B. L. Trout, B. H. Suits, R. J. Gorte, and D. White
Density functional theory calculations with periodic boundary conditions, Car-Parrinello simulations and multi-nuclear 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 Bronsted-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 2-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.

Journal of Physical Chemistry B104, 11734-11747 (2000).

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