Xenon hydrates

In xenon hydrate, methane hydrate, and other hydrates involving small molecules the eight polyhedron centers per unit cube are occupied by the xenon or methane molecules, their composition accordingly being Xe-5JH 0 and CHi-SjHiO. 

The above molecular dynamics results have been confirmed by incoherent inelastic neutron scattering (IINS) measurements on xenon hydrate (Tse et al., 2001 Gutt et al., 2002). In earlier measurements on methane hydrate, the dominant... 

A number of studies have indicated that a labile cage-like (512) cluster of 20 water molecules around a hydrate guest former may not form preferentially at the initial stages of nucleation. MD simulations of xenon hydrate formation from a xenon-ice system showed that there is no preferential formation of cavities with 20 water molecules, which would be similar to the small hydrate cage. Rather, the statistical mean cage size distribution was found to be between 24 and 27 water molecules. Tse et al. (2002) suggest that this supports the experimental observation that sll SF6 hydrate formation does not require occupation of small cages. 

Coexistence of si and sll carbon dioxide hydrate has been detected from x-ray diffraction measurements during hydrate growth (Staykova and Kuhs, 2003). Similarly, metastable sll hydrate phases were detected using NMR spectroscopy during si xenon hydrate formation (Moudrakovski et al., 2001a) and during si methane/ethane hydrate formation (Bowler et al., 2005 Takeya et al., 2003). 

Nevertheless the analogy with clathrate compounds (p. 179) does not go further since it is just the xenon hydrate (1 at. press, at — 3.40 G) which is very much more stable than the argon hydrate (1 at. at —42.8°) likewise the bromine hydrate is more stable than the chlorine hydrate. 

Fig. 17.1 The structure of the xenon hydrate clathrate. The xenon atoms occupy the centers of regular pentagonal dodecahedra of water molecules (cf. Fig. 8.8).

This framework of water molecules is found in many crystals, such as xenon hydrate, Xe-5fH20 (or 8Xe-46H20, the contents of the unit cube outlined in the drawing), and methane hydrate, CH4 -5f H20. The drawing shows the xenon molecules (xenon atoms) in the chambers. Xenon hydrate can be classified with Prussian blue (plates 27 and 28) as a clathrate crystal. 

C. Gutt, J. Baumert, W. Press, J.S. Tse S. Janssen (2002). J. Chem. Phys., 116, 3795-3799. The vibrational properties of xenon hydrate An inelastic incoherent neutron scattering study. 

Fig. 14. Formation of xenon hydrates as shown in the time development of the xenon spectrum for the reaction of HP xenon with powdered ice. (Courtesy of Igor Moudrakovski. Reprinted from ref. 249 with permission. Copyright 2001, American Chemical Society.)...

Fig. 5 Hyperpolarized Xe-NMR spectroscopy of the formation of xenon hydrate on ice. (From Ref [28].) (A) Single-scan spectra taken as a function of time, showing the development of Xe clathrate hydrate when HP Xe is admitted to powdered ice at 243 K. There is an initial induction period where only the gas line is observed, followed by a rapid rise of the lines due to Xe in the small and large cages of the clathrate hydrate. (B) The Xe-NMR intensity ratio of largeismall cages shows an initial relative excess of Xe in small-cage environments, before eventually approaching the ratio in equilibrium Xe hydrate.

The most surprising anesthetic agents are the noble gases, such as xenon. Xenon is completely unreactive chemically. It has no ability whatever to form ordinary chemical compounds, involving covalent or ionic bonds. The only chemical property that it has is that of taking part in the formation of clathrate crystals. In these crystals the xenon atoms occupy chambers in a framework formed by molecules that interact with one another by the formation of hydrogen bonds. The crystal of this sort of greatest interest to us is xenon hydrate, Xe 5%HiO. The crys-... 

Fig. 3. The hexaka.idecahedron farmed by 28 water molecules in the 17-A hydrate crystals. The unit cube of these hydrate crystals, such as chloroform xenon hydrate, CHCla 29Ce 17HsO, contains 136 water molecules, which define 8 hexa-kaidecabedra and 16 dodecahedra.

Although only the xenon hydrate will be described, all these hydrates have common features. The oxygen atoms of a water molecule are surrounded by four hydrogen atoms. One oxygen forms covalent bonds with two hydrogens of its own molecule, and hydrogen bonds with hydrogens of two adjacent water mole-... 

The structure of xenon hydrate and the hydrates of argon, krypton, methane, chlorine, bromine, hydrogen sulfide, and some other substances is shown in Figure 9-10. The cubic unit of structure has edge about 1200 pm and contains 46 water molecules. Chloroform hydrate, CHCla-I7H2O, has a somewhat more complicated structure, in which the chloroform molecule is surrounded by a 16-sided polyhedron formed by 28 water molecules. 

The structure of a clathrate crystal, xenon hydrate. The xenon atoms occupy cavities (eight per unit cube) in a hydrogen-bonded three-dimensional network formed by the water molecules (46 per unit cube). The O—H- -0 distance is 276 pm, as in ice. Two xenon atoms, at 0, 0, 0 and i, are at the centers of nearly regular pentagonal dodecahedra. The other six, at 0, i, i 0, f, i i, 0, i i, 0, f i, i, 0 and i, 0, are at the centers of tetrakaidecahedra. Each tetrakaidecahedron (one is outlined, right center) has 24 comers (water molecules), two hexagonal faces, and 12 pentagonal faces. 

What type of forces would operate in the xenon hydrates Would these forces include either ionic or covalent bonds involving xenon atoms Briefly explain. 

Table 4. Fractional occupancies and Langmuir constants for methane and xenon hydrate.

For a harmonic crystal the phonon lifetime is infinite and there is no scattering of thermal phonons.To understand the mechanism on how the guest-host interactions lead to the anomalous temperature dependence of the thermal conductivity, the lifetimes were calculated for phonon-phonon scatterings as a result of the anharmonic terms in the xenon-water potential of xenon hydrate in the small and large cage. The inverse relaxation time (lifetime), of a lattice vibration with frequency C0j q) (/ is the branch index and q is the direction of the momentum transfer) is related to the transition rate, W, of the lattice wave scattered from state qj q f by a defect according to, ... 

Figure 11 (a) Theoretical phonon dispersion curves for S-I xenon hydrate, (b) Inelastic incoherent neutron scattering spec-... 

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