Clathrates and van der Waals Complexes

Clathrates are host-guest complexes in which a crystalline cage of the host compound holds the guest molecule by weak intermolecular forces. Often, the cavities of the guest molecule are formed by a network of hydrogen bonds between covalently bound compounds. Powell [126] names them clathrates from the Latin word clathratus , which means enclosed. It is interesting to note that the lattice structure of the host in the clathrate is not its normal crystalline form the former becomes thermodynamically more stable than the latter only by the formation of the host-guest complex. 

The first known clathrate was a gas hydrate of chlorine, discovered by Davy in 1811 [127]. By 1900, many clathrate compounds had been prepared [128], 

The best-known noble gas clathrates are hydrates, hydroquinone and phenol clathrates, which have found an increasing number of uses [131]. Clathrates may serve as convenient storage for noble gases. Because of the different affinity hydroquinone clathrate prepared from an equal mixture of krypton and xenon liberates 3 times the amount of Xe than Kr [132]. Clathrates are also of interest for nuclear technology. Radioactive isotopes of argon, xenon and krypton can more easily be handled in the compact form of a solid rather than in gas form [133-136]. 

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]. 

The phenol clathrates of argon, krypton, and xenon have been prepared by Lahr and Williams [148] by direct combination of the gas with crystalline 

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