Layer-type packing structure

The layer-type packing structure has been sometimes observed when the guest molecule is so large that a part of the molecule cannot be accommodated within the CyD cavity. CyD rings are arranged in a plane to make a molecular layer and two adjacent layers are shifted with respect to each other by half a molecule, showing a brick-work pattern (Fig. 7.13C). Both ends of the cavity are open to an inter-molecular space of the adjacent layers. A part of the guest molecule not included in the host cavity protrudes into the intermolecular space and is in contact with host molecules of the adjacent layer. 

The layer-type packing structure is observed in the complexes with para-isomers of disubstituted benzenes, such as p-iodoaniline [109], p-iodophenol [110], p-... 

Heptakis(2,6-di-0-methyl)-j8-CyD includes acetic acid [194], 2-adamantanol [190], n-butylacrylate [191], and isobomylacrylate [191] in its cavity. The complex with carmofur, which is larger than the host cavity, crystallizes in the layer-type packing structure [156]. Compared with the crystal structures of other methylated CyD... 

Fig. 7.17. Ciystal structure of the f-CyD complex with sulfathiazole, showing layer-type packing.

Occurrence of layer types among structures based on anion close packing... 

Side and expanded views of hexagonal and cubic close-packed crystal types. In the hexagonal close-packed structure, spheres on both sides of any plane are in the same positions, and the third layer is directly above the first. In the cubic close-packed structure, layers take up three different positions, and the fourth layer is directly above the first. 

In either of these close-packed structures, each sphere has 12 nearest neighbors 6 in the same plane, 3 in the dimples above, and 3 in the dimples below. The expanded views in Figure 11-30 show the different arrangements of the hexagonal and cubic close-packed crystalline types, hi the hexagonal close-packed structure, notice that the third layer lies directly above the first, the fourth above the second, and so on. The layers can be labeled ABAB. 

Figure 3.20. A lateral view of different stacking sequences of triangular nets. They correspond to some typical close-packed structures. The first layer sequence shown corresponds to a superimposition according to the scheme ABABAB... (equivalent to BCBCBC... or CACACA... descriptions) characteristic of the hexagonal close-packed, Mg-type, structure. With reference to the usual description of its unit cell, the full stacking symbol indicating the element, the relative position of the superimposed layers and their distance is Mg Mg. The other sequences correspond to the schemes ABC.ABC. (Cu, cubic), ABAC.ABAC. (La, hexagonal), ACACBCBAB. (Sm, hexagonal). For Cu the constant ch of the (equivalent, non-conventional) hexagonal cell is shown which may be obtained by a convenient re-description of the standard cubic cell (see 3.6.1.3). ch = cV 3, body diagonal of the cubic cell.

Figure 3.21. The face-centred cubic close-packed structure (Cu type). On the left a block of eight cells is shown (one cell darkened). On the right a section of the structure is presented it corresponds to a plane perpendicular to the cube diagonal. Notice that the plane is the same presented on the left in Fig. 3.19. The sequence of the layers in this structure is shown in Fig. 3.20 in comparison with other close-packed elemental structures.

Another important feature of close-packed structures is the shape and number of the small amounts of space trapped in between the spheres. Two different types of space are contained within a close-packed structure the first we will consider is called an octahedral hole. Figure 1.5(a) shows two close-packed layers again but now with the octahedral holes shaded. Six spheres surround each of these holes three in layer A and three in layer B. The centres of these spheres lay at the corners... 

The simplest of the cubic structures is the primitive cubic structure. This is built by placing square layers like the one shown in Figure 1.1 (a), directly on top of one another. Figure 1.9(a) illustrates this, and you can see in Figure 1.9(b) that each atom sits at the corner of a cube. The coordination number of an atom in this structure is six. The majority of metals have one of the three basic structures hep, cep, or bcc. Polonium alone adopts the primitive structure. The distribution of the packing types among the most stable forms of the metals at 298 K is shown in Figure 1.10. As we noted earlier, a very few metals have a mixed hcp/ccp structure of a more complex type. The structures of the actinides tend to be rather complex and are not included. 

The third layer of spheres can now be placed on top of the second in two ways.These three layers are called A, B and C, the spheres of a layer with the same letter are placed vertically above each other. By placing the third layer vertically above the first and the fourth above the second, a packing ABABA. arises, a so-called hexagonal close-packed structure, abbreviated hep. A second possibility is a packing of the type ABCABC..., a so-called cubic close-packed structure, abbreviated ccp. Both types of packing are shown in figure 4.3. 

The objective is to use simple, clear, and informative notation for designating the close-packing type and layers occupied, including partial occupancy. From this information and knowledge of close-packed structures we can determine coordination numbers and local symmetries. For these purposes there are two parts of the notation the index, and the layers occupied (L) ... 

Figure 6.23 Schematic representation of the packing of cyclodextrin structures, (a) Head-to-head channel type (b) head-to-tail channel type (c) cage type (d) layer type and (e) layer type composed of /TCD dimers. (Reproduced from [24] with permission of Elsevier).

The 17 rare-earth metals are known to adopt five crystalline forms. At room temperature, nine exist in the hexagonal closest packed structure, four in the double c-axis hep (dhep) structure, two in the cubic closest packed structure and one in each of the body-centered cubic packed and rhombic (Sm-type) structures, as listed in Table 18.1.1. This distribution changes with temperature and pressure as many of the elements go through a number of structural phase transitions. All of the crystal structures, with the exception of bep, are closest packed, which can be defined by the stacking sequence of the layers of close-packed atoms, and are labeled in Fig. 18.1.1. 

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