Clathrate structures

FIGURE 2.5 Formation of a clathrate structure by water molecules surrouudiug a hydrophobic solute. 

Clathrate structures have been recently obtained also for s-PS [8] and s-PPMS [10]. In particular for s-PS, the treatment of amorphous samples, as well as of crystalline samples in the a or in the y form, produces clathrate structures including helices having s (2/1)2 symmetries, which present similar diffraction patterns, independently of the considered solvent. The treatment of samples of s-PPMS with suitable solvents also produces clathrate structures including s(2/l)2 helices however, the large differences in the X-ray diffraction patterns suggest different modes of packing, depending on the included solvent. 

Another interesting case is the much higher solvent resistance of the P crystalline form of s-PS, with respect to the other ones. In fact, it has been found that the sorption of solvents (which are suitable to produce transformations from the a or the y form toward clathrate structures) occurs only in the amorphous phase, for the case of P form samples [122-124]. Sorption kinetic curves of liquid methylene chloride in s-PS samples in the a and p form are, for instance, compared in Fig. 21 [124]. 

Fig. 18. Stereoview of the channel-type clathrate structures formed by host 12 with benzene (shown schematically) or o-xylene...

Applying this to our electrochemical system could qualitatively explain some of the observed effects. Assuming that there is only a weak interaction between metal and the perchloric acid hydrate, protons being part of the clathrate structure may not be in a favorable position for a charge transfer reaction at the interface. This could result in small pre-exponential factors. The electrode potential, however, may... 

The earlier rather complicated evidence for clathrate structures enforced by hydrophobic pairs (from EPR lineshape phenomena for paramagnetic hydrophobic solutes, (23)) and for two states of the hydrophobic bond (from thermodynamic excess functions (2k,25)) is provided with a detailed background by these important theoretical developments. 

The spontaneous separation of oil and water, a familiar observation in everyday life, is due to the energetically unfavorable formation of clathrate structures. When a mixture of water and oil is firmly shaken, lots of tiny oil drops form to begin with, but these quickly coalesce spontaneously to form larger drops—the two phases separate. A larger drop has a smaller surface area than several small drops with the same volume. Separation therefore reduces the area of surface contact between the water and the oil, and consequently also the extent of clathrate formation. The AS for this process... 

Chemically speaking, SOAz (I) and SOAz (II) are strictly identical and pure their actions on animal tumors are also identical. Thus, we would expect that the difference in their melting points to be due to some structural peculiarities, either with regard to their space group (if the two kinds of crystals do not contain any insertion of solvent) or possibly by some inclusion of solvent in the unit cell (clathrate structure) as in the case of MYKO 63 when crystallized from C Hg or CCI4 (see above). 

Cadmium cyanide, CdCCN), is analogous to Si02 with respect to the AB2 composition, the tetrahedral confiugration of A, the bridging behavior of B between a pair of A atoms, and the ability to build a three-dimensional framework in which cavities of molecular scale are formed. Cadmium caynide itself crystallizes in a cubic system of the anticuprite type, in which two identical fr-cristobalite-like frameworks interpenetrate each other without any cross-connection the cavity formed in one framework is filled by the other. When we replace one of the frameworks by appropriate guest molecules such as those of CCl, CCl CH, etc., we may obtain a novel clathrate structure with an adamantane-like cavity, as shown in Fig. 1 [1], Our results including those recently obtained are summarized in Table 1. 

Sorensen, C.M., Clathrate Structures and the Anomalies of Supercooled Water, in Proc. 12th Int. Conf. on Prop, of Water and Steam, Orlando, FL, September (1994). 

In a review of the thermodynamics of water, Franks and Reid (1973) showed that the optimum molecular size range for maximum solubility was similar to hydrate stability. Franks and Reid noted, this is not intended to imply that long-lived clathrate structures exist in solution—only that the stabilization of the water structure by the apolar solutes resembles the stabilization of water in a clathrate lattice. Glew (1962) noted that, within experimental error, the heat of solution for ten hydrate formers (including methane, ethane, propane, and hydrogen sulfide) was the same as the heat of hydrate formation from gas and ice, thereby suggesting the coordination of the aqueous solute with surrounding water molecules. 

Fig. 17. Model of the clathrate structure of the water in clathrates I. The centre of the hole is occupied by hydrophobic guest molecules in gas-hydrates (model of the iceberg formation in aqueous solution )...

Wei S, Shi Z, Castleman A W. 1991. Mixed cluster ions as a structure probe experimental evidence for clathrate structure of (H2O)20 FT. J Chem Phys 94 3268-3273. 

Each sublattice of this ellipsoidal clathrate structure contains both diol 10 enantiomers. However, interpenetration creates fourfold screw axes of both handedness. Only one diol enantiomer participates in the 4i axes, and only the other... 

The ellipsoidal clathrate structure contains both enantiomers, whereas the helical tubulate structure contains only one. It follows, therefore, that a sample of... 

Fig. 11.5 Two orientations of the iso-amyl group in the clathrate structure shown in Fig. 11.4. Dark bonds correspond to the more populated orientation (70%), the other part has 30% of site occupancy. Two projections are given perpendicular to the N-C-C fragment and parallel to it [4],...

J. Lipkowski et al., Some symmetry aspects of layered-clathrate structures formed by Ni(NCS)2(4-methylpyridine)4. J. Incl. Phenom. 2, 327-332 (1984)... 

If apolar hydration is characterized by the conditions that AG° > 0, TAS < 0 and AH < 0, then a process which minimizes exposure of apolar groups to water should be a thermodynamically favoured process. Then if two apolar groups of either the same or different molecules come together in water, AS for this process will be positive because some of the structured water is released into the bulk solvent. Such association is called hydrophobic, hydrophobic bonding or hydrophobic interaction (Kauzmann, 1959). The term bond is probably inappropriate because the association is due to entropy rather than to enthalpy effects, a consequence of the disruption of the clathrate structure around the apolar solute (Jolicoeur and Friedman, 1974). Despite the general acceptance of the concept of hydrophobic association, there are different approaches to the problem of understanding this phenomenon. 

Clathrates are crystalline inclusion compounds formed by the physical reaction between host molecules and low molecular weight gases as guests. In the clathrate structure there are two different types of cages which can entrap guest molecules into the network of host molecules. The three-dimensional clathrate structures can be determined by X-ray diffraction method. 

Water clusters containing simple ions are another area of current experimental and theoretical interest. Accordingly, they are also the subject of EA studies. Chaudhury et al. [113] have used EA methods on empirical potentials to obtain optimized structures of halide ions in water clusters, which they then subjected to AMI calculations for simulation of spectra. EA applications to alkali cations in TIP4P water clusters [114,115] have led to explanations of experimental mass-spectroscopic signatures of these systems, in particular the lack of magic numbers for the sodium case and some of the typical magic numbers of the potassium and cesium cases, and the role of dodecahedral clathrate structures in these species. 

A consideration of thermodynamic properties of the aqueous solution of rare gases and hydrocarbons led to the iceberg model for water structure around nonpolar molecules [139], which later had to be abandoned (see Part IV, Chap. 23.4). The gas hydrate clathrate structures described in Part IV, Chap. 21 provided... 

Here the layers contain only water molecules which form antidromic pentagons, quadrilaterals and homodromic hexagons (Fig. 21.10). Clathrate or semi-clathrate structures have been postulated for choline chloride hydrate, (H3Q3N+CH2CH2OH 2H20 CP, on the basis of similarities in the solid-state infrared spectra [162], but this has not been confirmed by crystal structure analysis. 

Do clathrate structures persist in their aqueous solutions. The increase in viscosity of the solutions prior to the formation of crystals suggests that this is so. 

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