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
Abstract
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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