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Free-Energy Calculations of the Intercage Hopping Barriers of Hydrogen Molecules in Clathrate Hydrates

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journal contribution
posted on 18.07.2016 by Christian J. Burnham, Niall J. English
Clathrate hydrates are nonstoichiometric crystalline inclusion compounds in which a water host lattice encages small guest atoms or molecules in cavities, and they have potential utility as a hydrogen-storage vector. In spite of the anomalous mechanistic nature of guest-diffusivity in clathrate hydrates, characterizing the precise mechanisms of intercage diffusive migration therein remains an elusive challenge. Also, nuclear quantum effects are particularly important for small guests such as H2, and cannot realistically be neglected in the host lattice in any rigorous dynamical treatment of H2 intercage diffusivity. Here we compute free-energy profiles and barriers, showing that quantal delocalization increases these barriers dramatically vis-à-vis classical dynamics for intercage H2 diffusion, by combining umbrella sampling with path-integral molecular dynamics in the extended solid. Results are compared to earlier DFT ab initio molecular dynamics calculations of Trinh et al, who found that the free-energy barriers decrease with increasing temperature.