Clathrate hydrates structures

Less common clathrate hydrates formed by compounds other than natural gas guests (such as Jeffrey s structures III-VII, structure T, complex layer structures) and high pressure hydrate phases are also briefly described to provide a comprehensive account of clathrate hydrate structural properties. 

Figure 7.2 Hydrogen bonded rings of water molecules found in clathrate hydrate structures. Four membered rings are only found in structure H.

Udachin, K. A., Ripmeester, J. A., A complex clathrate hydrate structure showing bimodal guest hydration. Nature 1999, 397, 420-423. 

J. Lipkowski et al., A novel clathrate hydrate structure of tetra-iso-amyl ammonium fluoride. J. Incl. Phenom. Mol. Recogn. Chem. 9, 275-276 (1990)... 

Where explanations of the properties of the liquid mixtures have been couched in terms of either clathrate-hydrate structures or... 

An interesting example where infrared O-H frequencies were used to correlate structures is for choline chloride dihydrate, which is postulated to have a semi-clathrate hydrate structure by analogy with the known crystal structure of tetraethyl ammonium fluoride pentahydrate [162]. 

Water molecules are four-coordinated in the ices but may be three-coordinated in small molecule hydrates. In the ices and in the ice-like clathrate hydrate structures, the water molecules are always four-coordinated. In small molecule hydrates, they display four- and threefold coordination. They always donate two hydrogen bonds, but may accept one or two bonds, or be connected to one or two cations. 

Fig. 21.6 a -c. Other polyhedral voids observed in the clathrate hydrate structures, a tetrakaideca-hedron, 51262 b pentakaidecahedron, 51263 c hexakaidecahedron, 5t264. Unlike the dodecahedron (Fig. 21.3) which is an regular solid, these are semi-regular solids. If the edges and angles in a face are equal, the faces cannot be exactly planar... 

A help gas stabilizes double and mixed hydrates. There is some uncertainty whether these inclusion hydrates are in fact stable without a guest species in the small 512 cages. All the clathrate hydrate structures are known to be stabilized and their melting points raised by including H2S with the guest compounds, which for this reason is called the helpgas [782-786]. Since we know of no experiments where the gas hydrates were prepared in the absence of air or some other gas, this help-gas phenomenon may occur to a more or less degree in all laboratory... 

The original X-ray studies of the gas clathrate hydrate structures were based on powder diffraction data, since the gas hydrates are notoriously difficult to obtain as single crystals. Clathrate hydrates formed by compounds which are liquids above 0°C, such as those of ethylene oxide and tetrahydrofuran, can be readily grown as large single crystals, and X-ray single crystal studies of both these hydrates have been carried out [787, 788]. 

Ripmeester JA, Tse JS, Ratcliffe Cl, Powell BM (1987) A new clathrate hydrate structure. Nature (Lond) 325 135-136... 

Udachin, K.A., Ratcliffe, C.I., and Ripmeester, J.A. (2001) A dense and efficient clathrate hydrate structure with unusual cages. Angew. Chem. Int. Ed., 40, 1303. 

Clathrate hydrates are crystalline but nonstoichiometric compounds and all the cages are not always occupied. Clathrate hydrates are stable only when the interaction between guest and water molecules dominates over sum of the unfavorable two terms (1) entropy decrease arising from confinement of guest molecules in small void cages, and (2) free energy for formation of empty clathrate hydrate structure from ice or liquid water. 

Table 2. Diameters of guest molecules (A) that form clathrate hydrate structure 1 and 11, and the ratios relative to the effective cage sizes for smaller and larger cages. Effective cage size is defined as the cage size -2.9A . A cage occupied by a guest molecule is marked with an asterisk.

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. 

Answers to such difficult questions can be found in applied thermodynamics - in terms of measured, macroscopic values of pressures, temperatures, compositions, volumes, enthalpies, etc. This chapter provides an overview of natural gas clathrate hydrates - structures, phase diagrams, and thermodynamic predictions/measurements that guide our understanding in dealing with such questions. The hydrate historical perspective provides an example of how knowledge advances in a technical field. At the conclusion of the chapter, future thermodynamic challenges are presented. 

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]. 

Table 1. Clathrate Hydrate Structures with Large Cationic Guests (From Refs. 5,6)... 

A clathrate hydrate is a crystalline inclusion compound in which small guest molecules, usually hydrophobic, are trapped in polyhedral cages formed by hydrogen-bonded water molecules. True clathrates are formed by guests that interact with the hydrate lattice only by weak, nondirectional forces. In such cases, the water molecules form a completely hydrogen-bonded network, and the inaterials effectively are ices. A number of structures are known for true clathrate hydrates, including the three major families of clathrate hydrate structures that will be discussed later. 

Fig. 4 Clathrate hydrate structures. Hydrates of cubic Structures I, II. and hexagonal Structure H are illustrated to indicate the stacking of the polyhedra.

Clathrate hydrates are crystalline inclusion compounds composed of host water cages that trap guest molecule-. [-.The three most common types of clathrate hydrates are known as structure I, II, and H, which differ in the type of water cage they contain. The type of clathrate hydrate structure formed depends mainly on the size of the guest molecules present (for further details, see the article Clathrate Hydrates and Refs. [8,9]). The structures of these compounds were first determined from x-ray diffraction studieshowever, as mentioned previously, vibrational spectroscopy can provide important complementary information on the structures and dynamics of these compounds and can also detect the presence of any guest molecule-host lattice interactions. 

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