Intra-Cage Structure, Vibrations and Tetrahedral-Site Hopping of H2 and D2 in Doubly-Occupied 51264 Cages in sII Clathrate Hydrates from Path-Integral and Classical Molecular Dynamics

Abstract

The intra-cage behaviour of guest H2 and D2 molecules in doubly occupied 51264 cages in structure-II (sII) clathrate hydrates were investigated using classical and path-integral molecular dynamics at 100 K. We probed the structure of tetrahedral sites, proton vibrations, localised molecular rattling timescales at sites, and the jump-diffusion travel of H2 and D2 molecules between sites. The site-diffusion model was correlated with experimental neutron scattering data, and the cage occupancies were then discussed in light of recent state-of-the-art experimental and theoretical findings in the literature.

Hydrogen and Deuterium Molecular Escape from Clathrate Hydrates: “Leaky” Microsecond-Molecular-Dynamics Predictions

“…Migration occurs in the main through hexagonal faces, as refs ( 45 − 48 ). make clear from free-energy-barrier considerations.…”

“…Certainly, in the case of quadruple large-cage occupation, the level of molecular “crowding” is quite elevated, given that there is a growing experimental and simulation body of consensus that large cages are typically no more than doubly occupied. 45 Therefore, more elevated concentrations need to overcome larger free-energy barriers to jump from one cage to another, with all large cages having high occupation. 9 , 41 , 42 , 46 , 47 …”

“…On comparing Figure 3 (H 2 ) versus Figure 4 (D 2 ), although the rate of D 2 leakage is generally somewhat slower than that of H 2 (although understood in a first-order isotopic sense by double the mass), there is a greater ultimate reduction in D 2 content vis-à-vis H 2 , ceteris paribus; this disparity becomes more marked at higher temperatures. As will be discussed below, the guest–cage interactions become more marked at higher temperatures, with a larger scope for greater-amplitude intracage rattling, 48 together with faster intracage “tetrahedral-site-swapping” motions, 45 and so heavier D 2 molecules exhibit a greater degree of thermal activation, due to greater propensity for temperature-induced intracage vibrational and rattling activation. Although double in mass, the greater amplitude of D 2 collisions with surrounding cages, and magnitude of momentum-transfer “leakage,” leads to cages’ greater “flexing” and time-dependent distortions, facilitating escape.…”

“… 48 The highest E a arises for double occupancy vis-à-vis other occupancies in both deuterium and hydrogen cases, and this is also supported by the occupancy percentage of the gas molecules. For nominal double occupation, refs ( 45 ). and ( 48 ) highlight the more highly stable intramolecular configurations in terms of the general tetrahedral structure vis-à-vis other occupancies (both thermodynamically and structurally and with respect to intra- and inter-cage-hopping propensity).…”

“…In terms of specific subpicosecond cage-centric dynamical phenomena, e.g., cage-rattling and distortion, these do play an important role, of course in cage-to-cage hopping within the hydrate phase. 41 , 42 , 45 , 48 For instance, ref ( 41 ). considers cage-radii, distortion, flexing, and vibrational frequency for these processes before and during cage-hop events, while ref ( 48 ).…”