Clathrate method

Other experimental methods for obtaining chemical shift anisotropies are the spin-lattice relaxation, molecular beam, and clathrate methods. The spin-lattice relaxation time, T, measures how soon a nuclear spin or assemblage of spins returns to its equilibrium position by releasing eneigy to the total system. Any mechanism that provides a fluctuating magnetic field can, in principle, contain the proper Fourier... 

Werner complexes can be used to form clathrates with the Cg aromatic isomers (35—42). The aromatic compounds are released upon heating. Since the uptake and release characteristics of the four Cg aromatic isomers are each different, this method has been suggested as a means of separating the isomers. 

Several alternative methods have been considered in order to increase the energy density of natural gas and facilitate its use as a road vehicle fuel. It can be dissolved in organic solvents, contained in a molecular cage (clathrate), and it may be adsorbed in a porous medium. The use of solvents has been tested experimentally but there has been little improvement so far over the methane density obtained by simple compression. Clathrates of methane and water, (methane hydrates) have been widely investigated but seem to offer little advantage over ANG [4]. Theoretical comparison of these storage techniques has been made by Dignam [5]. In practical terms, ANG has shown the most promise so far of these three alternatives to CNG and LNG. 

In Part III heterogeneous equilibria involving clathrates are discussed from the experimental point of view. In particular a method is presented for the reversible investigation of the equilibrium between clathrate and gas, circumventing the hysteresis effects. The phase diagrams of a number of binary and ternary systems are considered in some detail, since controversial statements have appeared in the literature on this subject. 

To conclude this section it may be worthwhile to remark that, although the L-J-D method is generally applied to a face-centered cubic lattice (z = 12), it is equally valid for the cavities in a clathrate (z = 20—28), as long as one restricts oneself to first-neighbor interactions. 

Nuclear magnetic resonance spectroscopy of the solutes in clathrates and low temperature specific heat measurements are thought to be particularly promising methods for providing more detailed information on the rotational freedom of the solute molecules and their interaction with the host lattice. The absence of electron paramagnetic resonance of the oxygen molecule in a hydroquinone clathrate has already been explained on the basis of weak orientational effects by Meyer, O Brien, and van Vleck.18... 

Fig. 5. Determination of the composition of the equilibrium clathrate of hydroquinone and methanol at 25°C with the aid of -propanol as an auxiliary solvent. The figure at the bottom illustrates the method used for determining the position of the side CA of the three-phase area A CD.

E. D. Sloan, Jr. Method for controlling clathrate hydrates in fluid systems. Patent WO 9412761,1994. 

Lokshin, K.A. Zhao, Y. (2006). Fast synthesis method and phase diagram of hydrogen clathrate hydrate. App. Phys. Lett., 88, 131909. 

Certain differences in numerical data reported in various publications can be justified to some extent by differences in the analytical methods used. On the other hand, the values given by Lhoste seem to show that the sulphuric acid in nitrocotton could be combined with nitrocellulose molecules less firmly than in an ester group. For instance, it is not out of the question that sulphuric add may be bound with nitrocellulose as an inclusion (clathrate) compound. (Attention was also drawn to this point by Miles [15] in his monograph on cellulose nitrate.)... 

Anderson, B., Molecular Modeling of Clathrate-Hydrates via Ab-Initio, Cell Potential and Dynamic Methods, Massachusetts Institute of Technology, MA (2005). 

In a thorough review of calorimetric studies of clathrates and inclusion compounds, Parsonage and Staveley (1984) presented no direct calorimetric methods used for natural gas hydrate measurements. Instead, the heat of dissociation has been indirectly determined via the Clapeyron equation by differentiation of three-phase equilibrium pressure-temperature data. This technique is presented in detail in Section 4.6.1. 

It is clear from the above that molecular-level methods are required to determine the hydrate structure. Furthermore, these methods have identified several phenomena that shift the paradigm on our understanding of clathrate hydrates, including ... 

The solid inclusion (clathrate) complex of oxygen with hydroquinone had been prepared twice previously by a published method which involved saturating a solution of hydroquinone in propanol in an autoclave at 70°C with oxygen at 20—150 bar,... 

In addition to the use of existing hydrates, it has also been suggested that clathrate hydrates could find applications in storage and separation technology. Bardhun showed in 1962 the viable application of hydrates in desalination of sea water, while hydrates have also been investigated as a method for the storage and transport of methane without the use of high-pressure containers. 

The majority of the interest in liquid clathrates has centred around their potential applications in separation science, particularly in the separation of closely related species such as benzene and toluene, or xylene isomers (cf. zeolite-based methods, Section 9.2). Separation occurs as a result of the greater ability of one organic solvent over another to stabilise the liquid clathrate phase. Hence, in a... 

Jerry L. Atwood is Curator s Professor of Chemistry at the University of Missouri-Columbia. His research has focused on supramolecular chemistry. His research group has synthesized and examined a broad array of host-guest chemical systems (e.g., liquid clathrates, macromolecular hosts). A principal method for characterization of these systems has been single crystal X-ray structure determination [41-43],... 

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