Clathrate cages

As mentioned before, clathrates are highly dynamic structures. Not only are hydrogen bonds forming the clathrate cages constantly disrupted and reformed, but also guest molecules can exhibit considerable mobility. The X-ray structure of bromine hydrate reported by the Ripmeester group illustrates this point [36]. 

The zeolites are aluminosilicate framework minerals of general formula M", [AI4Sil0lr+>JJ -zH20.y They are characterized by open structures that permit exchange of catioas and water molecules (Fig. 16.2). In the synthetic zeolites the aperture and channel sizes may sometimes be controlled by a sort of template synthesis—the zeolite is synthesized around a particular organoammonium canon. This yields channels of the desired size. The zeolite framework thus behaves in some ways like a clathrate cage about a guest molecule (Chapter 8). The synthesis of zeolites also involves several other factors such as the Al/Si ratio, the pH. the temperature and pressure, and the presence or absence of seed crystals - ... 

Clathrate-Cage Model. The final water model which is of major interest is based on clathrate hydrate cage structures. It was originally proposed by Pauling (116), who noted the existence of clathrate hydrates of many inert gases and suggested, by analogy to the chlorine hydrate,... 

Figure 11 Diagrammatic representation of the ellipsoidal clathrate compound (10)4 (chloroform) projected in the ab plane. The black and white coding indicates the host enantiomers. Diol molecules are represented as two oxygen atoms (solid spheres) joined by their carbocyclic framework (solid rod). Dashed lines represent the hydroxy hydrogen bonds. The three 4 screw axes involve only one enantiomer of 10 (surrounded by white rods), and the three 43 screw axes the other (black rods). Each ellipsoidal clathrate cage site is, however, centrosymmetric.

While the clathrate model is attractive, it is not correct to assume that the water is organized in some long-lived structure the observation that the self-diffusion coefficient for co-sphere water is larger than that for the solute rules this out. However, the rotational correlation time is shorter for ethanol and t-butyl alcohol in water (in the clathrate cage ) than in the pure liquid (Goldammer and Hertz, 1970 Goldammer and Zeidler, 1969). Nmr experiments show that in water the solvent dipole moments point away from the apolar groups (Hertz and Radle, 1973). 

Figure 8.10 The distribution of the number of oxygen atoms within 5.1 A of the Kr atom in aqueous solution at an elevated temperature in the region of the entropy convergence temperature (LaViolette et al, 2003). These results were obtained to investigate the possibilities of clathrate nucleation upon quenching see Filipponi etal. (1997) and Bowron etal. (1998). Note that the coordination numbers n = 20 or n = 24, which are associated with clathrate cages, are unexceptional in this distribution for the liquid solution. The subtle structure in this distribution for n below the mode may be reflective of possibilities for alternative thermodynamic phases, e.g. the coexisting gas phase, or structures with commodious cages.

Hydrophobic molecules do not dissolve readily, however, in water, and form a clathrate, cage-like structure (Figure 2.13) when mixed with water. 

The crystal structures of most of the ices have now been determined with reasonable certainty and will be discussed in turn with some detail. For convenience this information is assembled in table 3.4, all the data being reduced to atmospheric pressure and a temperature of —175 °C. The structures of some of the lower ices bear a close resemblance to the allotropic forms of quartz while the highest-pressure forms have some analogy with the clathrate cages formed by water molecules about inert gas atoms. 

In water there are many possible hydrogen-bonded structures other than those like Ice I and indeed truly ice-like clusters must be comparatively rare to account for the observed metastability of supercooled water. Some clusters in water may correspond to bulk structures having a higher free energy than liquid water and hence are described by a curve like (b) in fig. 4.5. Others, like the clathrate cages of Pauling, have very favourable bonding for particular numbers of molecules but the structure cannot be extended continuously to a bulk phase. Such clusters have a behaviour described by a curve like (c). 

Fig. 4 Stereoviews illustrating (a) the construction of a clathration cage in unsolvated 3-hydroquinone, and (b) more extended portions of two identical, but displaced, three-dimensional networks that constitute the (3-hydroquinone host structure.

Only at first glance, the two approaches, the clathrate cage model and the cavity-based model, looked very different, the former based on the hydrogen bonding of water, and the later on the hard core of water. But taken all results together it would appear that both are just different perspectives on the same physics with different diagnostics reporting consequences of the same shifted balance between H bonds and vdW interactions. Actually, in a... 

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