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

Abstract

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

Global-density fluctuations in methane clathrate hydrates in externally applied electromagnetic fields

“…The dominant fluctuation occurred at ∼720 cm 1 for all simulations and it was attributed to the librational motion of water molecules. [17][18][19] The other two density fluctuations were of much smaller magnitude and of lower frequency. One of these fluctuations occurred at 10-12 cm 1 for all simulations, while the last density fluctuation was only observed for higher-intensity e/m fields and it had a frequency of twice the applied e/m field.…”

“…In comparison, the (partial) rotation induced relatively readily by e/m fields in liquid-like water (or dipolar liquid) environments ensures dissipation of the mechanical work done on the dipoles by the field's torques into re-orientational motion, primarily, albeit with some extent of roto-translational coupling 62 (e.g., enhancement of translational self-diffusivity). 51,63,64 However, in the case of hydrates, whether in bulk-crystal or nanocrystallite form, below the intensity threshold needed for e/m-induced electro-dissociation (which will vary naturally with external-field frequency), 35 the dissipation of torquemediated work is targeted primarily into librational (rotationoscillation 18,19 ) modes (and vibrational modes, if a flexible model is used). 50 In this case, then, there is primarily an "enhanced rattling" effect on the hydrogen-bonding network, rather than outright re-orientational motion leading to rupture of stabilising bonds.…”

“…These were computed for the oxygen and hydrogen atoms in water and for the carbon atoms of the UA methane molecules. Cosine Fourier transformation (power spectra) of the VACFs reveals the densities of states; [16][17][18][19][20]23,54 the oxygen atoms in water dominate the translational motion of the lattice, whilst the hydrogen atoms reflect librational motion. 17 The translational vibrations of the encaged methane molecules may be studied in the power spectrum of the methane carbon atoms (centre-of-mass).…”

“…17 The translational vibrations of the encaged methane molecules may be studied in the power spectrum of the methane carbon atoms (centre-of-mass). [16][17][18][19] In any event, we may also study e/mfield effects on the DOS, as has been done for VACF-spectra in chiral liquids and their racemic mixtures, 55 as well as for liquid water 56 and water adsorbed at metal-oxide surfaces. 57…”

“…[3][4][5] Naturally, this has led to a wide host of molecular-dynamics (MD) simulations studying methane-clathrate extraction methods, typically from marine milieux. 6 More generally, molecular simulation has proved to be a rather invaluable tool in revealing much about equilibrium properties of clathrate hydrates, including structure, [6][7][8][9][10] a "resonant-scattering" picture of phonons and guest motion in cavities, [11][12][13] hydrogen-bonding characteristics, 14,15 and dynamical [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] and energetic properties. 10,16 Such molecularsimulation studies on dynamical properties have often focused on vibrational properties of the host lattice and on the cavities, with "overlapping" between these acoustic and optic a) Present address: Department of Chemical Engineering, University College London, London, United Kingdom.…”