Quantum Dynamics of Hydrogen Molecules Inside Cages of Clathrate Hydrates

1
Quantum Dynamics of Hydrogen Molecules Inside
Cages of Clathrate Hydrates
Zlatko Bacic, Department of Chemistry, New York
University, New York, NY 10003
Clathrate hydrates with hydrogen molecules as
guests have been synthesized recently. Hydrogen
hydrates have received a great deal of attention
because of their potential as clean, efficient,
and safe materials for hydrogen storage.
Moreover, they offer a unique opportunity to
investigate novel aspects of the highly quantum
dynamics of the coupled translational and
rotational motions of one or several H2 molecules
confined in the clathrate hydrate nanocages. We
have performed diffusion Monte Carlo (DMC)
calculations of the size evolution of the
energetics and the vibrationally averaged spatial
distributions of small (H2)n and (D2)n clusters
inside the large cage of the structure II
clathrate hydrate. For n1-4 the DMC ground-state
energies (top figure) are negative, implying that
the clusters are truly bound and stable. Addition
of the fifth H2 or D2 molecule to the cage causes
the ground-state energy to rise to high positive
values, virtually ruling out their inclusion. The
maximum predicted quadruple H2/D2 occupancy of
the large cage, in agreement with experiments, is
determined largely by the cluster zero-point
energy, a purely quantum effect. The 3D
probability distribution function (PDF) from the
DMC calculations for 4 D2 molecules (bottom
figure) shows their tetrahedral arrangement
inside the large cage. The four distinct lobes of
the 3D PDF are centered at the c.m. positions of
the hydrogen molecules in the n4 global minimum.
Each lobe points at the center of one of the four
hexagonal faces of the large cage, as observed in
the neutron diffraction experiments.