Cage forms

Liu K, Brown M G and Saykally R J 1997 Terahertz laser vibration rotation tunneling spectroscopy and dipole moment of a cage form of the water hexamer J. Phys. Chem. A 101 8995-9010... 

HELICALLY COILED AND TOROIDAL CAGE FORMS OF GRAPHITIC CARBON... 

Helically coiled and toroidal cage forms of graphitic carbon... 

Let us consider a clathrate crystal consisting of a cage-forming substance Q and a number of encaged compounds ( solutes ) A, B,. . ., M. The substance Q has two forms a stable modification, which under given conditions may be either crystalline (a) or liquid (L), and a metastable modification (ft) enclosing cavities of different types 1,. . ., n which acts as host lattice ( solvent ) in the clathrate. The number of cavities of type i per molecule of Q is denoted by vt. For hydroquinone v — for gas hydrates of Structure I 1/23 and v2 = 3/23, for those of Structure II vx = 2/17 and v2 = 1/17. 

Sulfurous acid is an equilibrium mixture of two molecules (12a and 12b) in the former, it resembles phosphorous acid, with one of the H atoms attached directly to the S atom. These molecules are also in equilibrium with molecules of S02, each of which is surrounded by a cage of water molecules. The evidence for this equilibrium is that crystals of composition S02-aH20, with x about 7, are obtained when the solution is cooled. Such substances, in which a molecule occupies a cage formed by other molecules, are called clathrates. Methane, carbon dioxide, and the noble gases also form clathrates with water. 

In solids, atoms rattle around — vibrate—in the cages formed by the surrounding atoms. In liquids, atoms or molecules move past one another continuously, like minnows in a stream endlessly changing positions. In gases, atoms or molecules are free to move over large distances. Figure 2 is a schematic illustration of motion of a monatomic substance in these three phases. 

Gas hydrates are non-stoichiometric crystals formed by the enclosure of molecules like methane, carbon dioxide and hydrogen sulfide inside cages formed by hydrogen-bonded water molecules. There are more than 100 compounds (guests) that can combine with water (host) and form hydrates. Formation of gas hydrates is a problem in oil and gas operations because it causes plugging of the pipelines and other facilities. On the other hand natural methane hydrate exists in vast quantities in the earth s crust and is regarded as a future energy resource. 

The linking pattern of two zeolites is shown in Fig. 16.24. They have the /I-cage as one of their building blocks, that is, a truncated octahedron, a polyhedron with 24 vertices and 14 faces. In the synthetic zeolite A (Linde A) the /3-cages form a cubic primitive lattice, and are joined by cubes. j3-Cages distributed in the same manner as the atoms in diamond and linked by hexagonal prisms make up the structure of faujasite (zeolite X). 

A model for the diffusion of light ions in structured liquids has been suggested recently by Schmidt.61 The elementary act of diffusion is considered in this model to be the transfer of the ion from one cage formed by solvent molecules to a neighboring one. 

Figure 11.6 Views of perovskite crystal structure. Top—conventional cubic unit cell white circles = oxygen black circle = transition metal gray circles = alkali or alkaline earth metal. Bottom—extended unit cell to show the cage formed by the oxygen octa-hedra. Adapted from Bragg et al. (1965).

For mixed lanthanide-transition metal clusters, Yukawa et al. have synthesized an octahedral [SmNi6] cluster by the reaction of Sm3+ and [Ni(pro)2] in nonaque-ous medium [66-68]. The six [Ni(pro)2] ligands use 12 carboxylate oxygen atoms to coordinate to the Sm3+ ion, which is located at the center of an octahedral cage formed by six nickel atoms. The coordination polyhedron of the central Sm3+ ion may be best described as an icosahedron. The [SmNir, core is stable in solution but the crystal is unstable in air. The cyclic voltammogram shows one reduction step from Sm3+ to Sm2+ and six oxidation steps due to the Ni2+ ions. Later, similar [LaNis] and CjdNif> clusters were also prepared. 

Both these cages form complexes with most of the alkali and alkaline earth ions. In particular, the caesium complex of (240) is especially stable compared to other complexes of this ion. 

A further complication in the action of the solvent is connected with the possibility that the original ions will recombine and reverse the ionization reaction even before they can escape from the cage formed by the surrounding solvent molecules. This return of the original departing group to its parent molecule is called internal return or the... 

Stabilization of activated oxidoreductases on time scales of months to years has historically been challenging, and the lack of success in this regard has limited the industrial implementation of redox enzymes to applications that do not require long lifetimes. However, as mentioned in the Introduction, some possibility of improved stability has arisen from immobilization of enzymes in hydrophilic cages formed by silica sol—gels and aerogels, primarily for sensor applications.The tradeoff of this approach is expected to be a lowering of current density because... 

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