Layered clathrate structures

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)... 

SOME SYMMETRY ASPECTS OF LAYERED-CLATHRATE STRUCTURES FORMED BY Ni (NCS)2(4-Methylpyridine) ... 

Fig. 11.1 (a) Arrangement of guest methylcellosolve molecules in layered clathrate of the host Ni(NCS)2(4-methylpyridine)4. (b) Mutual orientation of the guest molecules in the layered structure displayed in (a)... 

Fig. 11.8 Two 1-bromonaphthalene molecules in the layered clathrate isostructural with the one depicted in Fig. 11.2. The two non-related by crystal symmetry molecules as obtained by structure refinement and the respective orientational disordering modes which produce apparent molecular deformations (Notations as in Fig. 11.3)...

A novel class of crystalline, microporous aluminophosphate phases has been discovered. It represents the first class of molecular sieves with framework oxide compositions free of silica. The new class of materials encompasses some fourteen reported three-dimensional microporous framework structures, and six two-dimensional layer-type structures. The three-dimensional structures include structural analogues of the zeolites sodalite and erionite-offre-tite. The novel phases can be synthesized hydro-thermally in the presence of organic amines and quaternary ammonium templates. The template is entrapped or clathrated within the crystallizing aluminophosphate network. After thermal decomposition of the template the three-dimensional molecular sieves have the general composition of Al303 1.0 ... 

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. 

Fig. 5 The Hoffman clathrate, structurally characterized by X-ray crystallography. Nickel centers separated by cyanide linkers form extended two-dimensional frameworks. These layers are linked together by hydrogen-bonded ammonia ligands, trapping benzene molecules within the three-dimensional framework. Reprinted from [9] by permission from Macmillan, copyright (1949)...

Later observations of Irish scientists [74-75] have shown that in the surface layers of a foam or in a foam in narrow cylinders or in the narrow space between two plates, Kelvin s polyhedra are observed as well as fragments of clathrate structures, consisting of 6 tetrakaidecahedra and 2 dodecahedra [75]. 

Data on the separation of thietane by gas chromatography have been reported along with data on other sulfur compounds. Thin-layer chromatographic separation of thietane from 1,2-dithiolane and 1,2.3-trithiane works well on silica gel. Thietane forms a hydrate (dec 11.7°) with water. Its clathrate structure was investigated by use of the nmr line shapes of the deuterium oxide hydrate. ... 

Monomolecular surface films change the structure of the uppermost water layer within a thickness of around some micrometers to possibly some hundred micrometers (Huhnerfuss and Alpers 1983, Huhnerfuss 1986). For example, ice-like clathrate structures are induced by OLA films in a water layer of d < 190 pm. Furthermore, the surface potential of pure water of about - 180 mV becomes positive and may approach values of > 400 mV (Gericke and Huhnerfuss 1989), and the dilational surface viscosity is drastically increased (Huhnerfuss 1985). The relaxation time for disturbances of the surface film order TCOmp attains values of around 10 to 20 min, which are about 1013 times larger than the relaxation time xs of the water molecules (Huhnerfuss and Alpers 1983). 

Figure 7.12 (a) Part of a planar layer of TMA hydrate showing the rectangular channel and (b) the hexagonal clathrate structure that TMA forms when in the presence of large, templating guests. 

The atomic positions of the host Ni(NCS)2(4-MePy), molecules were easily found and refined. The molecules adopt a four-blade propeller shape and are asymmetric the conformations being close to those found in previously studied layered clathrates (ref. 2 and 3) the packing of the molecules in the crystal structures being essentially the same as well. However, determination of positions of the guest molecules presented difficulties. In particular ... 

Urea is well-known to form clathrate structures (Section 4.3.1) however, it is also able to form layered structures in a tape motif. The urea functionalities are able to align in such a way that each carbonyl receives two NH - O hydrogen bonds from the molecule next to it (Figure 4.18(d)). This results in the formation of R (6) rings running throughout the structure. 

In many layer structures, such as clay minerals, the extent of lattice adjustment on entry of guest molecules is intermediate between the behaviors of zeolites and of clathrates. The layers remain intact, but the distance between them changes substantially (8). For water-free smectite crystals the d(001) distance is 9.4 A. The van der Waals diameter of a water molecule is 2.8 A so that, in batavite, for example, the water layer in the Na form is about 14.8 — 9.4 = 5.4 A thick, corresponding with two monolayers. 

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