Dynamical properties of hydrogen sulphide motion in its clathrate hydrate from ab initio and classical isobaric-isothermal molecular dynamics.

Equilibrium ab initio (AI) and classical constant pressure-constant temperature molecular dynamics (MD) simulations have been performed to investigate the dynamical properties in (type I) hydrogen sulphide hydrate at 150 and 300 K and 1 bar, and also lower temperatures, with particular scrutiny of guest motion. The rattling motions of the guests in the large and small cavities were around 45 and 75-80 cm(-1), respectively, from AIMD, with the corresponding classical MD modes being 10-12 cm(-1) less at 150 K and around 5 cm(-1) lower at 300 K. The rattling motion in the small cavity overlapped somewhat with the translational motion of the host lattice (with modes at circa 85 and 110 cm(-1)), due in part to a smaller cage radius and more frequent occurrences of guest-host hydrogen bonding leading to greater coupling in the motion. The experimentally determined H-S stretch and H-S-H bending frequencies, in the vicinity of 2550-2620 and 1175 cm(-1) [H.R. Richardson et al., J. Chem. Phys.1985, 83, 4387] were reproduced successfully in the AIMD simulations. Consideration of Kubic harmonics for the guest molecules from AIMD revealed that a preferred orientation of the dipole-vector (or C(2)-axis) exists at 150 K vis--vis the [100] cube axis in both the small and large cavities, but is markedly more significant for the small cavity, while there is no preferred orientation at 300 K. In comparison, classical MD did not reveal any preferred orientation at either temperature, or at 75 K (closer to the AIMD simulation at 150 K vis-à-vis that approach's estimated melting point). Probing rotational dynamics of the guests reinforced this temperature effect, revealing more rapid rotational time scales at 300 K with faster decay times of dipole-vector (C(2)) and H-H-vector (C(2)(y)) being similar for each cage, at around 0.25 and 0.2 ps, respectively, versus approximately 0.45 and 0.5 ps (large) and 0.8 ps (small) at 150 K. It was found that the origin of the observed preferred orientations, especially in the small cages, at 150 K via AIMD was attributable to optimization of the dipolar interaction between the guest and outward-pointing water dipoles in the cavity, with guests "flitting" rotationally between various such configurations, forming occasionally hydrogen bonds with the host molecules.