Computer simulations of the dynamics of doubly occupied N2 clathrate hydrates

Abstract

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.

Linking microscopic guest properties to macroscopic observables in clathrate hydrates: Guest-host hydrogen bonding

“…17 This choice of force field for the clathrate and guests gives good estimates for the lattice vectors and other experimental data available in the equilibrated clathrates. 22,23 Isotropic constant pressure/temperature NpT molecular dynamics simulations on periodic simulation cells were performed using the DL_POLY software program version 2. 16.…”

Cage Occupancies in Nitrogen Clathrate Hydrates from Monte Carlo Simulations

“…[14][15][16] Only few simulations results for a N 2 clathrate can be found in the literature. [17][18][19][20][21][22] Horikawa et al, and later van Klaveren et al, have studied the dynamical behavior of encaged N 2 molecules using molecular dynamics (MD). 17,21 van Klaveren et al have shown also that a N 2 hydrate with sII at high pressure remains mechanically stable even for the case of a full double occupancy of the large cages.…”

Microscopic observation and in situ Raman scattering studies on high-pressure phase transformations of a synthetic nitrogen hydrate

“…5 These results support the double occupation determined by the above neutron diffrac-tion measurements. Klaveren et al also calculated the vibrational frequencies for sII nitrogen hydrate, 6,7 in which they showed three N-N stretching peak frequencies for the single occupied S cages, the double occupied L cages, and the single occupied L cages. 6 Although a number of Raman experiments for the N-N stretching vibrations in natural or synthetic air hydrates have been performed, [8][9][10][11][12][13][14][15] there is no evidence for the double occupancy.…”