Clathrate hydrates of triisoamylbutylammonium iodide

The phase diagram of the (i-C₅H₁₁)₃C₄H₉NI?H₂O system is reported. Triisoamylbutylammonium iodide forms polyhydrates with hydration numbers of 36 (mp 12.5°C) and 32 (mp 13.2°C) and a dihydrate. Crystals of both polyhdrates have been isolated, and their compositions have been determined.     

Clathrate hydrates of tetrabutylammonium carboxylates and dicarboxylates

Phase diagrams of the binary aqueous systems with tetra-n-butylammonium (TBA) carboxylates ((C₄H₉)₄NC_nH_{2 n+1}CO₂, where n=0÷5) and dicarboxylates ([(C₄H₉)₄N]₂(CH₂)_nC₂O₄, where n=1÷3) including some branched carboxylate anions, have been studied in the field of crystallization of clathrate hydrates. Monocrystals of many hydrates have been prepared and their composition, densities, melting points and X-ray data have been determined. In the set of TBA carboxylate hydrates the stability increases towards TBA propionate or butyrate hydrates (for different structures) and then it decreases as sizes of anions grow, This is explained by additional stabilization of the framework caused by small cavities being occupied until the hydrophobic part of the anion is able to go in the cavity. In the set of hydrates of TBA dicarboxylates the change of the stability is easily accounted for by the modelling of the inclusion of dicarboxylate ions in the cavities, within known structures, different ways of hydrophilic inclusion being taken into account.     

Clathrate hydrates of hexagonal structure III at high pressures: Structures and phase diagrams

It was shown in this work that the clathrate hydrates of Hexagonal Structure III, formed in the ternary systems 1-methylpiperazine-help gas-water and iso-amyl alcohol-help gas-water are stable in a wide range of pressures. The decomposition curves of these hydrates were studied for the first time up to the pressures 1 GPa. Ar, Kr, Xe and CH₄ were utilized as the help gases. In a number of the systems studied, high pressure phases were revealed that presumably form due to the distortion of the corresponding low pressure hydrate structures.     

Atomic hydrogen production from semi-clathrate hydrates

Atomic hydrogen has received recent attention because of its potential role in energy devices, silicon devices, artificial photosynthesis, hydrogen storage, and so forth. Here, we propose a highly efficient route for producing atomic hydrogen using semi-clathrate hydrates. Two major hydrogen radical sources, derived from guest/host materials, are closely examined.     

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

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 H(2)-THF system in which there is single H(2) occupation of the small cage (labelled "1SC 1LC"), we find that no H(2) migration occurs, and this is also the case for pure H(2) hydrate with single small-cavity occupation and quadruple occupancy for large cages (dubbed "1SC 4LC"). However, for the more densely filled H(2)-THF and pure-H(2) systems, in which there is double H(2) occupation in the small cage (dubbed "2SC 1LC" and "2SC 4LC," respectively), there is an onset of inter-cage H(2) 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.     

Thermodynamic predictions of various tetrahydrofuran and hydrogen clathrate hydrates

Tetrahydrofuran (THF) is one of the most widely used analogues for gas hydrates as well as a commonly used additive for reducing the formation pressure of a given hydrate process. Hydrates are also currently being investigated as storage materials for hydrogen as well as materials for hydrogen separations. Here we present a thermodynamic model, based on the CSMGem framework, that accurately captures the phase behavior of various hydrates containing THF and hydrogen. The model uses previously regressed parameters for components other than THF and H₂, and can reproduce hydrate formation conditions for a number of hydrates containing THF and/or hydrogen (simple THF, THF + CH₄, THF + N₂, THF + CO₂, THF + H₂, CH₄ + H₂, C₂H₆ + H₂ and C₃H₈ + H₂). The incorporation of THF and H₂ within this model framework will serve as a valuable tool for hydrate scenarios involving either of these components.     

On the thermodynamic stability of hydrogen clathrate hydrates

The cage occupancy of hydrogen clathrate hydrate has been examined by grand canonical Monte Carlo (GCMC) simulations for wide ranges of temperature and pressure. The simulations are carried out with a fixed number of water molecules and a fixed chemical potential of the guest species so that hydrogen molecules can be created or annihilated in the clathrate. Two types of the GCMC simulations are performed; in one the volume of the clathrate is fixed and in the other it is allowed to adjust itself under a preset pressure so as to take account of compression by a hydrostatic pressure and expansion due to multiple cage occupancy. It is found that the smaller cage in structure II is practically incapable of accommodating more than a single guest molecule even at pressures as high as 500 MPa, which agrees with the recent experimental investigations. The larger cage is found to encapsulate at most 4 hydrogen molecules, but its occupancy is dependent significantly on the pressure of hydrogen.     

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

Classical equilibrium molecular dynamics (MD) simulations have been performed to investigate the dynamical and energetic properties in hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 30 and 200 K and 0.05 kbar, and also at intermediate temperatures, using SPC/E and TIP4P-2005 water models. The potential model is found to have a large impact on overall density, with the TIP4P-2005 systems being on average 1% more dense than their SPC/E counterparts, due to the greater guest-host interaction energy. For the lightly-filled mixed H(2)-THF system, in which there is single H(2) occupation of the small cage (1s1l), we find that the largest contribution to the interaction energy of both types of guest is the van der Waals component with the surrounding water molecules in the constituent cavities. For the more densely-filled mixed H(2)-THF system, in which there is double H(2) occupation in the small cage (2s1l), we find that there is no dominant component (i.e., van der Waals or Coulombic) in the H(2) interaction energy with the rest of the system, but for the THF molecules, the dominant contribution is again the van der Waals interaction with the surrounding cage-water molecules; again, the Coulombic component increases in importance with increasing temperature. The lightly-filled pure H(2) hydrate (1s4l) system exhibits a similar pattern vis-à-vis the H(2) interaction energy as for the lightly-filled mixed H(2)-THF system, and for the more densely-filled pure H(2) system (2s4l), there is no dominant component of interaction energy, due to the multiple occupancy of the cavities. By consideration of Kubic harmonics, there is some evidence of preferential alignment of the THF molecules, particularly at 200 K; this was found to arise at higher temperatures due to transient hydrogen bonding of the oxygen atom in THF molecules with the surrounding cage-water molecules.     

Competitive cage occupancy of hydrogen and argon in structure-II hydrates

The cage occupancy of hydrogen in the single-crystals of simple hydrogen hydrates and hydrogen + argon mixed-gas hydrates was investigated by means of in situ Raman spectroscopy under the three-phase (hydrate + water + fluid) equilibrium condition. In the equilibrium pressure region higher than approximately 25 MPa, four hydrogen cluster and argon competitively occupied the large cages of structure-II hydrogen + argon mixed-gas hydrates. In addition, Raman spectroscopic analysis at liquid nitrogen temperature (77 K) supports that the clusters of two, three, or four hydrogen molecules occupy large cages.     

Molecular hydrogen storage in binary THF-H2 clathrate hydrates

The hydrogen storage capacity of binary THF-H(2) clathrate hydrate has been determined as a function of formation pressure, THF composition, and time. The amount of hydrogen stored in the stoichiometric hydrate increases with pressure and exhibits asymptotic (Langmuir) behavior to approximately 1.0 wt % H(2). This hydrogen concentration corresponds to one hydrogen molecule occupying each of the small 5(12) cavities and one THF molecule in each large 5(12)6(4) cavity in the hydrate framework. Contrary to previous reports, hydrogen storage was not increased upon decreasing the THF concentration below the stoichiometric 5.6 mol % solution to 0.5 mol %, at constant pressure, even after one week. This provides strong evidence that THF preferentially occupies the large 5(12)6(4) cavity over hydrogen, for the range of experimental conditions tested. The maximum amount of hydrogen stored in this binary hydrate was about 1.0 wt % at moderate pressure (<60 MPa) and is independent of the initial THF concentration over the range of conditions tested.     

Relativistic Gaussian basis sets for the atoms hydrogen to neon

Gaussian basis sets for use in relativistic molecular calculations are developed for atoms and ions with one to ten electrons. A relativistic radial wavefunction coupled to an angular function of l-symmetry is expanded into a linear combination of spherical Gaussians of the form r ^l exp (–r ²). One set of basis functions is used for all large and small components of the same angular symmetry. The expansion coefficients and the orbital exponents have been determined by minimizing the integral over the weighted square of the deviation between the Dirac or Dirac-Fock radial wavefunctions and their analytical approximations. The basis sets calculated with a weighting function inversely proportional to the radial distance are found to have numerical constants very similar to those of their energy-optimized non-relativistic counterparts. Atomic sets are formed by combining l-subsets. The results of relativistic and non-relativistic calculations based on these sets are analyzed with respect to different criteria, e.g. their ability to reproduce the relativistic total energy contribution and the spin-orbit splitting. Contraction schemes are proposed.Dedicated to Prof. Dr. A. Neckel on occasion of his 60th birthday     

Molecular dynamics simulations of binary structure H hydrogen and methyl-tert-butylether clathrate hydrates

Binary structure H (sH) hydrogen and methyl-tert-butylether (MTBE) clathrate hydrates are studied with molecular dynamics simulations. Simulations on a 3 x 3 x 3 sH unit cell with up to 4.7 mass % hydrogen gas are run at pressures of 100 bars and 2 kbars at 100 and 273 K. For the small and medium cages of the sH unit cell, H2 guest molecule occupancies of 0, 1 (single occupancy), and 2 (double occupancy) are considered with the MTBE molecule occupying all of the large cages. An increase of the small and medium cage occupancies from 1 to 2 leads to a jump in the unit cell volume and configurational energy. Calculations are also set up with 13, 23, and 89 of the MTBE molecules in the large cages replaced by sets of three to six H2 molecules, and the effects on the configurational energy and volume of the simulation cell are determined. As MTBE molecules are replaced with sets of H2 guests in the large cages, the configurational energy of the unit cell increases. At the lower temperature, the energy and volume of the clathrate are not sensitive to the number of hydrogen guests in the large cages; however, at higher temperatures the repulsions among the H2 guest molecules in the large cages cause an increase in the system energy and volume.     

Dicyclohexylamine-Thiourea Clathrate

Abstract

The reaction of dicyclohexylamine (DCHA) with thiourea leads to the formation of the inclusion compound DCHA(6 Thiourea). Room temperature, single crystal X-ray diffraction analysis shows the product has a trigonal structure, α=β=90°, γ=120°, a=b=15.801(2)A, c=12.451(3)A, which may be described as a thiourea matrix defining hexagonal cavities where the di-cyclohexylamine molecules are accommodated. ¹³C-cross polarization magic angle spinning (CP-MAS) NMR study indicates the guest inside the cavities has a relatively free rotation and that the channels are, concerning this amine, perfect van der Waals cavities. Thermal studies indicates that the structural identity of the thiourea matrix endures after a partial loss of amine.     

Clathrate formation mechanism of supercritical hydrogen adsorption on copper(II) benzoate pyrazine

The adsorption isotherms of supercritical hydrogen on [Cu2(bz)4(pyz)]n were measured at 77 K up to 10 MPa. The amount of supercritical hydrogen adsorbed on [Cu2(bz)4(pyz)]n at 77 K was 1.4 wt % at 10 MPa. The adsorption isotherms of supercritical hydrogen on [Cu2(bz)4(pyz)]n showed a stepwise adsorption that suggests clathrate formation between [Cu2(bz)4(pyz)]n and hydrogen molecules.     

Concentration studies of collision-induced fundamental absorption of hydrogen dissolved in liquid neon

We report further and more detailed results of our recent investigation [W. A. Herrebout, B. J. van der Veken, and A. P. Kouzov, Phys. Rev. Lett. 101, 093001 (2008)] on the collision-induced fundamental absorption by hydrogen dissolved in liquid neon (T ≈ 25 K). The band shapes were studied in a wide range of concentrations (0.003-0.05 mole fractions) as well as for different ortho/para ratios and at much higher level of accuracy and resolution than before. Due to almost unhindered rotation of the hydrogen molecule and low temperature, an unprecedently rich frequency-domain picture produced by different terms of the interaction-induced polarization was observed. While some of them are conspicuous via fast intracell motion of a light guest (H(2)), others-induced by the electrostatic field of the guest-give rise to lines whose shapes are imprinted by fluctuations of the nearest surrounding. Strong motional narrowing observed on the guest-guest induced lines shows up in their Lorentzian shapes which are signatures of microscopic-scale diffusion. Near-Lorentzian peaks were also detected at the tops of the diffuse lines induced by isolated guests. Their formation may be associated with a long-living defect (vacancy) emerging in the vicinity of the polarization inductor. Altogether, our results give the first unambiguous spectroscopic evidence on the diffusional evolution of isolated binary interactions that emerge in dense chaotic media.     

The influence of pressure on the stability of clathrate hydrates of hydrogen and tetrahydrofuran

Changes in the Gibbs energy of hydration of molecular hydrogen and tetrahydrofuran (THF) at pressures of 0.1, 6.0, and 12.0 MPa over the temperature range 230–300 K were studied by the molecular dynamics method. The Gibbs energy of hydrogen in water-tetrahydrofuran-hydrogen solutions passed minima over the temperature range 235–265 K, which were indicative of a comparatively stable clathrate hydrate state. The Gibbs energy of the hydrogen molecule at the local minimum at 262 K was ∼4.5 kJ/mol; at atmospheric pressure and room temperature, it was ∼2 kJ/mol. An analysis of the radial distribution function and the coordination number of the THF molecule showed that, at 240–257 K, a clathrate hydrate of THF with the structure close to clathrate sII was predominantly formed.