Dynamical and energetic properties of hydrogen and hydrogen–tetrahydrofuran clathrate hydrates

MacElroy

2013

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Classical equilibrium molecular dynamics simulations have been performed to investigate the diffusive properties of inter-cage hydrogen migration in both pure hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 0.05 kbar from 200 K and up to 250–260 K. For mixed H2-THF systems in which there is single H2 occupation of the small cage (labelled “1S1L”), we found that no H2 migration occurs. However, for more densely filled H2-THF and pure-H2 systems, in which there is more than single H2 occupation in the small cage, there is an onset of inter-cage H2 migration events from the small cages to neighbouring cavities at around 200 K. The mean square displacements of the hydrogen molecules were fitted to a mathematical model consisting of an anomalous term and a Fickian component, and nonlinear regression fitting was conducted to estimate long-time (inter-cage) diffusivities. An approximate Arrhenius temperature relationship for the diffusion coefficient was examined and an estimation of the hydrogen hopping energy barrier was calculated for each system.

Section: Gorman Et Al Have Studied the Energetics And Dynamical Propmentioning

confidence: 99%

Diffusive hydrogen inter-cage migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates

Cao

MacElroy

2013

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“…The radii of the doubly-occupied small cages is around 1.5 -4.5 % larger than their singly-occupied counterparts, with the disparity increasing at higher temperatures; the corresponding result is about 1 -2 % for large cavities. The general lattice expansion at higher temperatures (as characterised by decreasing overall system densities [45] ) leads to a marginally greater local expansion in doubly-occupied small cavities, albeit limited by the lattice and cage structures remaining intact. The cage radii are in good agreement with respective experimental data for small and large sII cage radii of 3.90 and 4.68 Å [46] and 3.91 and 4.73 Å.…”

Section: Resultsmentioning

confidence: 99%

“…1a); the double occupation of the small cage in this case serves to lead to closer hydrogen-lattice contact and reduces the frequency of cage 'breathing'. The shoulder in the 100-115 and 115-150 cm -1 range manifests, to some extent, interaction of the small cages' radial modes with acoustic modes of the lattice translational density of states (i.e., of water oxygen atoms from power spectra derived from their velocity ACFs [45] ). However, these would also be expected to reflect overlap with rattling and O-O modes (which may be gleaned from the power spectra of velocity ACFs [45] ) -it is difficult to isolate conclusively which type of overlap of the small cages' radial modes dominates.…”

Section: Resultsmentioning

confidence: 99%

Dynamical cage behaviour and hydrogen migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates

Gorman

MacElroy

2012

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Classical equilibrium molecular dynamics simulations have been performed to investigate dynamical properties of cage radial breathing modes and intra- and inter-cage hydrogen migration in both pure hydrogen and mixed hydrogen–tetrahydrofuran sII hydrates at 0.05 kbar and up to 250 K. For the mixed H2–THF system in which there is single H2 occupation of the small cage (labelled “1SC 1LC”), we find that no H2 migration occurs, and this is also the case for pure H2 hydrate with single small-cavity occupation and quadruple occupancy for large cages (dubbed “1SC 4LC”). However, for the more densely filled H2–THF and pure-H2 systems, in which there is double H2 occupation in the small cage (dubbed “2SC 1LC” and “2SC 4LC,” respectively), there is an onset of inter-cage H2 migration events from the small cages to neighbouring cavities at around 200 K, with an approximate Arrhenius temperature-dependence for the migration rate from 200 to 250 K. It was found that these “cage hopping” events are facilitated by temporary openings of pentagonal small-cage faces with the relaxation and reformation of key stabilising hydrogen bonds during and following passage. The cages remain essentially intact up to 250 K, save for transient hydrogen bond weakening and reformation during and after inter-cage hydrogen diffusion events in the 200–250 K range. The “breathing modes,” or underlying frequencies governing the variation in the cavities’ radii, exhibit a certain overlap with THF rattling motion in the case of large cavities, while there is some overlap of small cages’ radial breathing modes with lattice acoustic modes.

“…Naturally, guests larger than H 2 lead to acoustic-mode coupling with the lattice, but, however, have less serious effects on host librational dynamics. [20][21][22][23][24][25][26][27] Each water molecule's angular velocity ω was decomposed into rotational-velocity components about local molecule-body-frame axes x, y, z (denoted "b"), wherein the z-axis is perpendicular to the plane, say, the x-axis coincides with the dipole vector, and the y-axis is along the H-H vector,…”

Section: Methodsmentioning

confidence: 99%

“…17 MD, with various potentials, has probed vibrational density of states (DOS) of liquid water, 18 ice, 19 and hydrates. [20][21][22][23][24][25][26][27] More recently, liquid-water MP2-based MD has offered prospects of better agreement with INS-DOS, 28 whilst Density Functional Theory (DFT)-MD has shown qualitative DOS agreement. 25 DFT calculations on ice Ih have also shown reasonably good DOS agreement with INS.…”

Section: Introductionmentioning

confidence: 99%

Communication: Librational dynamics in water, sI and sII clathrate hydrates, and ice Ih: Molecular-dynamics insights

Burnham

2016

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Articles you may be interested inStudy of clathrate hydrates via equilibrium molecular-dynamics simulation employing polarisable and nonpolarisable, rigid and flexible water models The Journal of Chemical Physics 144, 164503 (2016) Equilibrium molecular-dynamics simulations have been performed for liquid water, and on metastable sI and sII polymorphs of empty hydrate lattices, in addition to ice Ih, in order to study the dynamical properties of librational motion (rotation oscillation) depicted by protons in water molecules. In particular, hydrate lattices were found to display prominent "bifurcated" features, or peaks, at circa 70 and 80-95 meV (or ∼560 and 640-760 cm −1 , respectively), also displayed by ice, in essentially quantitative agreement with experimental neutron-scattering data. However, observed differences in dispersion between these librational modes between these two structures (both hydrate polymorphs vis-à-vis ice), owing primarily to density effects, have been decomposed into contributions arising from angular-velocity dynamics about axes in the local molecular frame of water molecules, with in-plane "wagging" and "twisting" rationalising one mode at ∼70 meV, and out-of-plane motion for the higher-frequency band. This was confirmed explicitly by a type of de facto normalmode analysis, in which only immediate layers of water molecules about the one under consideration were allowed to move. In contrast, liquid water displayed no marked preference for such local in-or out-of-plane modes characterising librational motion, owing to the marked absence of rigid, pentamers or hexamers therein. C 2016 AIP Publishing LLC. [http://dx