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 n=1-4 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 n=4 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.