Crystal structure packing

Figure 31. Crystal structure packing of H2[pzW-Me2)sK -60 (100c). [Adapted from (39).]...

Fig. 9 Molecular structures of TPC (Du) and MDABCO (Di ) [154], Crystal structure packing in (MDABCO ) TPC (C6o ) (a) TPC molecules form a hexagonal hollow and the MDABCO cation fits into the hollow the Ceo" molecules are docked into the hollow in TPC layer in a key-keyhole relationship [top view (h) and side view (c)] to form Di DnC6o" Colors C, dark yellow. H, pale blue and N, dark blue. Calculated Fermi surface at 160 K in (d) layer A and (e) layer B. (f) Projection of the (MDABCO )-TPC layer on the Cgo layer A (red color MDABCO )...

V. W. Manner, B. C. Tappan, B. L. Scott, D. N. Preston, G. W. Brown, Crystal Structure, Packing Analysis, and Structural-Sensitivity Correlations of Erythritol Tetranitrate, Cryst. Growth Des. 2014, 14, 6154-6160. 

Disorder (atomic and molecular) Lack of regularity. In crystal structures it implies that there is not exact register of the contents of one unit cell with those from all others. The atoms or molecules in the crystal structure pack randomly (nonperiodically) in alternative ways in different unit cells. Such disorder may cause some diffuse scattering around intense Bragg reflections. See also the definition in Chapter 2. 

Figure 18 CSD information that can contribute to the three main stages in crystai structure prediction (a) the generation of 3D molecular models, (b) the generation of crystal structure packings, and (c) the final ranking of crystal structures.

Fig. 2.23 a The crystal structure packing in 4,5,6,7-tetrafluoro-l/7-benziiiudazole-2(3H)-one displaying short C-F--F-C contact. Crystal packing in b Form I and c Form II of 4,5,6,7-tetrafluoro-I-methyl-lH-benzimidazole-2(3H)-one, displaying differences in nature of C-F - F-C interactions in these [93]... 

This structure is called close packed because the number of atoms per unit volume is quite large compared with other simple crystal structures. 

Cate J FI, Gooding A R, Podell E, Zhou K, Golden B L, Kundrot C E, Cech T R and Doudna J A 1996 Crystal structure of a group i ribozyme domain principles of RNA packing Science 273 1678-85... 

The parameters in the original parameterization are adjusted in order to reproduce the correct results. These results are generally molecular geometries and energy differences. They may be obtained from various types of experimental results or ah initio calculations. The sources of these correct results can also be a source of error. Ah initio results are only correct to some degree of accuracy. Likewise, crystal structures are influenced by crystal-packing forces. 

In compound materials - in the ceramic sodium chloride, for instance - there are two (sometimes more) species of atoms, packed together. The crystal structures of such compounds can still be simple. Figure 5.8(a) shows that the ceramics NaCl, KCl and MgO, for example, also form a cubic structure. Naturally, when two species of atoms are not in the ratio 1 1, as in compounds like the nuclear fuel UO2 (a ceramic too) the structure is more complicated (it is shown in Fig. 5.8(b)), although this, too, has a cubic unit cell. 

We begin by looking at the smallest scale of controllable structural feature - the way in which the atoms in the metals are packed together to give either a crystalline or a glassy (amorphous) structure. Table 2.2 lists the crystal structures of the pure metals at room temperature. In nearly every case the metal atoms pack into the simple crystal structures of face-centred cubic (f.c.c.), body-centred cubic (b.c.c.) or close-packed hexagonal (c.p.h.). 

WA Lim, A Hodel, RT Sauer, FM Richards. The crystal structure of a mutant protein with altered but improved hydrophobic core packing. Proc Natl Acad Sci USA 91 423-427, 1994. PB Harbury, B Tidor, PS Kim. Repacking proteins cores with backbone freedom Structure prediction for coiled coils. Pi oc Natl Acad Sci USA 92 8408-8412, 1995. 

On silicon carbide, it is easier to see and measure step heights than in crystals like beryl, because SiC has polytypes, first discovered by the German crystallog-rapher Baumhauer (1912). The crystal structure is built up of a succession of close-packed layers of identical structure, but stacked on top of each other in alternative ways (Figure 3.24). The simplest kind of SiC simply repeats steps ABCABC, etc., and the step height corresponds to three layers only. Many other stacking sequences... 

Cate, J. H., et al., 1996. Crystal structure of a group I ribozyme domain Principles of RNA packing. Science 273 1678. 

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