Xenon clathrate hydrate

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. 

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.

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. 

Hydrates of Ar, Kr, and Xe were first synthesized by Villard in 1896 [141]. They were further studied, as well as hydrates of krypton and xenon, by de Forcrand [142]. Several structures for noble gas hydrates are known [143-146]. All the hydrate structures are different from that of ordinary hexagonal ice. In the two fundamental structures, the water molecules form pentagonal dodecahedra which are stacked with different degrees of distortion from their ideally regular forms [146]. The two types of structures are shown in Fig. 26a and 26b [140]. One structure contains 46 water molecules in the unit cell with 2 small and 6 larger cavities. The other structure has 136 water molecules in the unit cell with 16 small and 8 larger cavities. The formation of the two fundamental types of hydrates depends mainly on the size of the guest species. More detailed data for the two principal clathrate hydrate structures are available from the literature [147]. 

Until 1962 only physical inclusion compounds were known. Argon, krypton, and xenon form cage or clathrate compounds with water (clathrate hydrates) and with some organics such as quinol. The host molecules are arranged in such a way that they form cavities that can physically trap the noble gas atoms, referred to as guests. The noble gas will be released upon dissolution or melting of the host lattice. 

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.

Further, following up on Jeffrey s early work on amine hydrates (amine semiclathrates), numerous amines remain to be explored for hydrate phases. In particular, since it has also been observed that some amine clathrates as weU as stoichiometric alcohol hydrates show phase transitions to double clathrate hydrates in the presence of helpgases such as xenon, H2S, and methane, it becomes a worthwhile area for exploration. 

Clathrates were the first systems investigated by Xe NMR of natural xenon gas. The xenon atom is of the same size and shape as methane and it also forms a clathrate hydrate with water. The xenon shielding is much more sensitive (by a factor of about 30) than the shielding of methane to the... 

Figure 17 Calculated vibrational density of states of water molecules in S-I (a) xenon and (b) fluoromethane hydrate. Reprinted from H. Tanaka, The Stability of Xe and CF4 Clathrate Hydrates. Vibrational Frequency Modulation and Cage Distortion, Chem. Phys. Lett., 202, 345. Copyright 1993, with permission from Elsevier.

Another independent study was performed by Handa and Tse (48). The empty lattice of clathrate hydrates, which serves as a hypothetical reference state, is unstable and has never been synthesized in the laboratory. Xenon forms si hydrates and the fugacity of xenon is 1.467 bar for the hydrate-ice-gas equilibrium at 273.15 K. In the case of small and spherical monatomic species like xenon, this experimental condition of xenon hydrates is very close to that of the empty lattice (70=273.15 K, Pa=0 bar). Therefore, Eq. 6 was used to... 

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). 

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. 

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