Xenon clathrates

Figure 7.43 (top) 129Xe CP-MAS NMR solid-state spectra for a the xenon clathrate of Dianin s compound obtained at 1 atmosphere pressure (bottom) the DQ filtered spectrum showing only the resonance due to the doubly occupied cavity. (Reproduced by permission of The Royal Society of Chemistry). 

Here, we examine the origin of unusually large thermal expansivity of xenon clathrate hydrate (structure I). Xenon interaction is described by an LJ potential whose parameters are given in Table 3[25]. The method is similar to the calculation for ice. The densities of state of water molecules for occupied and empty hydrates are shown in Figure 21. Clearly, frequencies of some modes shift to higher side upon encaging guest molecules. 

Special interest attaches to the chemical state of the Xe daughter atom of the jff-decay. The single-line sources, K 104 and NasHa IOg, show small but significant chemical isomer shifts when compared with a xenon clathrate absorber [118, 119]. Numerical values are collected in Table... 

Finally, the excited-state magnetic moment of Xe has been measured by applying a 78-kG external magnetic field to a xenon clathrate absorber [123]. Computer analysis of the broadened spectrum gave = -f0-68(30) n.m. 

Sv Fig. 18.1 Part of the solid state structure of tris(P-hydroquinone) xenon clathrate showing the arrangement of hydrogen-bonded organic molecules around a xenon atom [T. Birchall et al. (1989) Acta Crystallogr., Seel. C, vol. 45, p. 944]. Colour code Xe, yellow C, grey O, red H, white. 

Figure 12 The sxe NMR spectra of the formation of a xenon clathrate hydrate at 233 K and time f after admission of the xenon to the powdered ice sample. The signal at 160 ppm is attributed to xenon in the large tetrakaidecahedral cages and the one at 240 ppm to xenon in the smaller dodecahedral cages. Adapted with permission of the American Chemical Society from Pietrass T, Gaede HC, Bifone A, Pines A and Ripmeester JA (1995) Journal of the American Chemical Society, 117 7520-7525.

Figure 18 Dissociation pressure over a temperature range from 123.15 to 273.15 K. Solid and dashed lines show the calculated dissociation pressure for S-I argon and xenon clathrates, respectively. Dash-dot line shows the dissociation pressure for the argon hydrate. Open and black circles show the experimental results for argon and xenon clathrate hydrates, respectively. Reprinted by permission of Taylor Francis Ltd, http //www.tandf.co.uk/joumals from H. Tanaka and K. Nakanishi, The Stability of Clathrate Hydrates Temperature Dependence of Dissociation Pressure in Xe and Ar Hydrate, Molecular Simulation, 1994.

Radon forms a series of clathrate compounds (inclusion compounds) similar to those of argon, krypton, and xenon. These can be prepared by mixing trace amounts of radon with macro amounts of host substances and allowing the mixtures to crystallize. No chemical bonds are formed the radon is merely trapped in the lattice of surrounding atoms it therefore escapes when the host crystal melts or dissolves. Compounds prepared in this manner include radon hydrate, Rn 6H20 (Nikitin, 1936) radon-phenol clathrate, Rn 3C H 0H (Nikitin and Kovalskaya, 1952) radon-p-chlorophenol clathrate, Rn 3p-ClC H 0H (Nikitin and Ioffe, 1952) and radon-p-cresol clathrate, Rn bp-CH C H OH (Trofimov and Kazankin, 1966). Radon has also been reported to co-crystallize with sulfur dioxide, carbon dioxide, hydrogen chloride, and hydrogen sulfide (Nikitin, 1939). 

Handa, Y.P. (1986b). Calorimetric determinations of the compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of xenon and krypton. J. Chem. Thermodynamics, 18 (9), 891-902. 

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. 

Radon forms a series of clathrate compounds similar to those of krypton, xenon, and argon (e.g. Rn-6H20, RnGCeHsOH, Rn-3/ -ClC6H40H). 

Although inert-gas clathrates have been described, this compound is believed to be the first xenon charge-transfer compound which is stable at room temperatures. Lattice-energy calculations for the xenon compound, by means of Kapustinskii s equation, give a value 110 kcal. mole", which is only 10 kcal. mole" smaller than that calculated for the dioxygenyl compound. These values indicate that if the compounds are ionic the electron affinity of the platinum hexafluoride must have a minimum value of 170 kcal. mole". ... 

Another example of crystals containing more than one component is provided by the clathrates. In clathrates of, 3-quinol, three quinol molecules are hydrogen bonded together to form an approximately spherical cavity of radius 4 A (Figure 15.13). Any molecule of appropriate size such as oxygen, nitrogen, krypton, xenon, methane, sulfur dioxide, or methyl alcohol can be trapped, and if it is not disordered within the clathrate, its location and orientation can be determined in the crystalline state by X-ray diffraction methods. In most cases, when a clathrate is... 

When quinol is crystallised from aqueous solution in the presence of argon at 40 atmospheres, the solid has the properties of quinol but contains argon which is set free when the quinol is melted or dissolved. The gas molecule is trapped inside a cage of hydrogen-bonded quinol molecules (Powell, 1948). Clathrates are formed by krypton, xenon and such gases as... 

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