Clathrate-like cages

Mixture Models Broken-Down Ice Structures. Historically, the mixture models have received considerably more attention than the uniformist, average models. Somewhat arbitrarily, we divide these as follows (1) broken-down ice lattice models (i.e., ice-like structural units in equilibrium with monomers) (2) cluster models (clusters in equilibrium with monomers) (3) models based on clathrate-like cages (again in equilibrium with monomers). In each case, it is understood that at least two species of water exist—namely, a bulky species representing some... 

There are important differences between the literature models and our results. In our case, (i) the number of monomers is smaller than that in the Pauling model (where they are present in clathrate-like cages), and (ii) they coexist with a disturbed but still infinite, not disintegrated network of water molecules. In contrast, the models in refs 11 and 32 do not involve a network but only a distribution of clusters. 

Molecular as well as ionic substances can form hydrates, but of an entirely different nature. In these crystals, sometimes referred to as clathrates, a molecule (such as CFI4, CHCI3) is quite literally trapped in an ice-like cage of water molecules. Perhaps the best-known molecular hydrate is that of chlorine, which has the approximate composition Cl2- 7.3H20. This compound was discovered by the great... 

The importance of carefully considering anomalies when studying the behavior of solute-solvent interactions has been stressed. For aqueous solutions, many anomalous results presented in the literature suggest the existence of some type of discreteness in water structure. Discreteness is consistent with a view of water structure providing distinct sites such as those found in the models of water, implying a broken down ice lattice structure or clathrate hydrate cage-like structures. 

Despite the formation of clathrate-like clusters and complete 512 cages during these simulations, the increased ordering observed from the radial distribution functions and local phase assignments resulted in the authors concluding that their simulation results are consistent with a local order model of nucleation, and therefore do not support the labile cluster model. 

Hydrates with related clathrate-like structures are formed by tetramethyl and tetraethyl ammonium salts. The tetramethyl and tetraethyl ammonium salts form hydrates in which the cations occupy voids in hydrogen-bonded structures formed by the anions and the water molecules. Unlike the gas hydrates, the alkylamine hydrates, and the butyl and isoamyl ammonium salt hydrates discussed in the next section, these cage-like structures do not feature the pentagonal dodecahedron as a common structural component. 

The encapsulation of methane into a cavity is clearly favorable. A small drop in the potential with a concomitant increase of signifies the initialization of nucleation at 1.3 ps. The growth of the crystalline clathrate commences at ca. 23.0 ps. The cage-like structures formed around 1.3 ps are very mobile and they continuously break and form new H-bond connections (Figure 25). A well-formed and sustainable clathrate-like crystal was observed only at time longer than 3.0 ps. 

In contrast to carbon, which forms structures derived from both sp2 and sp3 bonds, silicon is unable to form sp2 related structures. Since one out of four sp3 bonds of a given atom is pointing out of the cage, the most stable fullerene-like structure in this case is a network of connected cages. This kind of network is realized in alkali metal doped silicon clathrate (19), which were identified to have a connected fullerene-like structure (20). In these compounds, Si polyhe-dra of 12 five-fold rings and 2 or 4 more six-fold rings share faces, and form a network of hollow cage structures, which can accommodate endohedral metal atoms. Recently, the clathrate compound (Na,Ba), has been synthesized and demonstrated a transition into a superconductor at 4 K (21). The electronic structure of these compounds is drastically different from that of sp3 Si solid (22). 

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 - ... 

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