Carbon clathrates

Other complex low-density carbon forms that are known to form including onions, nanotubes, foams and schwarzites [114, 115]. One interesting possible route to formation of carbon clathrates is the 3D-polymerization of fullerene samples, and one proposed 3D-form of a clathrate material (but not Type I or Type 11 materials described to date) has been observed experimentally [116]. 

Molecular complexation or clathration was observed somewhat earlier using similar compounds referred to by MacNicol and Wilson as hexa-hosts ". These compounds formed clathrates with carbon tetrachloride, toluene, dioxane, bromoform and a num-... 

One important difference between the present and the previous case should be noted. For the hydroquinone clathrates, where the wall of a cavity consists of 12 OH groups, 6 adjacent carbon atoms, and 6 CH groups in ortho position to the OH groups, it seemed best to consider the product z qjk) as one unknown. For hydrates one may not do this the walls of both types of cavities consist exclusively of tetrahedrally-coordinated water molecules. Hence, one should use the same value of (,eg/k) —characteristic for a water molecule in a hydrate lattice—for both types of cavities and multi-... 

Carbon dioxide, clathrate in hydro-quinone, 7, 11 as a "hilfsgas, 18 hydrate, 3... 

Carbon tetrachloride-hydrogen sulfide-water ternary system, 49, 51, 52 Carboniuin ion polymerization, 158 Carboxylic groups initiator, 174 Catalyst clathrates equilibrium, 35 Cell partition function, in calculation of thermodynamic quantities of clathrates, 26... 

Hindered rotation, 33, 34 internal, 367 Homopolymer, 168, 183 Hot bands, 374 Hot lattice, 4, 11, 21 Hydrates, 7, 9, 21, 31, 41 crystallization, 44 Hydrochloric acid clathrates, 2 in hydroquinone, 7 Hydrogen, bound, 4, 175 bromine hydrate, 35 4- carbon dioxide system, 110 4 carbon monoxide system, 96, 108 chloride hydrate, 35 clathrates, 2 chloride, 30... 

Hydrogen molecule, carbon oxide intramolecular energy, 110 clathrates, 12, 20 correlated wave function, 300... 

Metallic solutions, 120 Methacrylonitrile, 155 Methane, -f carbon dioxide system, 97 in clathrates, 30, 41 hydrate, 33, 34, 47... 

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. 

Radon forms a series of clathrate compounds (inclusion compounds) similar to those of argon, krypton, and xenon. These can be prepared by mixing trace amounts of radon with macro amounts of host substances and allowing the mixtures to crystallize. No chemical bonds are formed the radon is merely trapped in the lattice of surrounding atoms it therefore escapes when the host crystal melts or dissolves. Compounds prepared in this manner include radon hydrate, Rn 6H20 (Nikitin, 1936) radon-phenol clathrate, Rn 3C H 0H (Nikitin and Kovalskaya, 1952) radon-p-chlorophenol clathrate, Rn 3p-ClC H 0H (Nikitin and Ioffe, 1952) and radon-p-cresol clathrate, Rn bp-CH C H OH (Trofimov and Kazankin, 1966). Radon has also been reported to co-crystallize with sulfur dioxide, carbon dioxide, hydrogen chloride, and hydrogen sulfide (Nikitin, 1939). 

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). 

Clathrate hydrates Solid cages of water that form around small gas molecules such as methane, hydrogen, or carbon dioxide under conditions of high pressure and low temperature such as found on the deep sea floor and within the sediments. 

Figure 3.49 summarizes the oxygen isotope curve for the last 65 Ma. The most pronounced warming trend is expressed by a 1.5%o decrease in 8 0 and occurred early in the Cenozoic from 59 to 52 Ma, with a peak in Early Eocene. Coinciding with this event is a brief negative carbon isotope excursion, explained as a massive release of methane into the atmosphere (Norris and Rohl 1999). These authors used high resolution analysis of sedimentary cores to show that two thirds of the carbon shift occurred just in a few thousand years, indicating a catastrophic release of carbon from methane clathrates into the ocean and atmosphere. 

Kang, S.-P. Lee, H. (2001). Enthalpies of dissociation of clathrate hydrates of carbon dioxide, nitrogen, (carbon dioxide + nitrogen), and (carbon dioxide + nitrogen + tetrahydrofuran). J. Chem. Thermodynamics, 33 (5), 513-521. 

Linga, P. Kumar, R. Englezos P. (2007b). The clathrate hydrate process for post and pre combustion capture of carbon dioxide. J Hazardous Materials, revised submitted May 10, 2007... 

Uchida, T. Ebinuma, T. Kawabata, J. Narita, H. (1999b). Microscopic observations of formation processes of clathrate-hydrate films at an interface between water and carbon dioxide. J. Crystal Growth, 204 (3), 348-356. 

The induction of unconsciousness may be the result of exposure to excessive concentrations of toxic solvents such as carbon tetrachloride or vinyl chloride, as occasionally occurs in industrial situations (solvent narcosis). Also, volatile and nonvolatile anesthetic drugs such as halothane and thiopental, respectively, cause the same physiological effect. The mechanism(s) underlying anesthesia is not fully understood, although various theories have been proposed. Many of these have centered on the correlation between certain physicochemical properties and anesthetic potency. Thus, the oil/water partition coefficient, the ability to reduce surface tension, and the ability to induce the formation of clathrate compounds with water are all correlated with anesthetic potency. It seems that each of these characteristics are all connected to hydrophobicity, and so the site of action may be a hydrophobic region in a membrane or protein. Thus, again, physicochemical properties determine biological activity. 

Thus the addition of n-pentane to mixtures of p-xylene and m-xyiene permits complete separation of the xylenes which form a binary eutectic with 11.8% para. Without the n-pentane, much para is lost in the eutectic, and none of the meta is recoverable in pure form. A detailed description of this process is given by Dale (1981), who calls it extractive crystallization. Other separation processes depend on the formation of high melting molecular compounds or clathrates with one of the constituents of the mixture. One example is carbon tetrachloride that forms a compound with p-xylene and alters the equilibrium so that its separation from m-xylene is... 

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