Computer simulations of the dynamics of doubly occupied N2 clathrate hydrates

We have studied the dynamical properties of doubly and singly occupied structure II N2 clathrate hydrates by performing a series of classical molecular dynamics calculations. The intermolecular vibrational density of states of the guest molecules show the most explicit changes when going from single to double occupancies. Instead of the narrow peak for single occupancy (at 20 cm−1), a broad range of frequencies occurs between 0 and 200 cm−1 with four modes, which have been assigned to the possible vibrations in the cage. This spectrum provides a unique fingerprint of double occupancy. For the host lattice, the libron band and the optic phonon branch show lower frequencies as compared to single fillings. In contrast, the acoustic phonon branch shifts to lower frequencies for a single filling of the large cases. We find that there is a large vibrational guest–host coupling for both single and double occupancies, although the frequency range is broader in the latter case. There is a significant translational diffusion of the N2 molecules in the doubly occupied cages. Occasionally both molecules simultaneously migrate from one local energy minimum to another. An orientational order of the N2 molecules occurs only within the small cages. However, the relative orientation of the two molecules in the doubly occupied cages is preserved in time.