Assembled structures crystal states

The structure of SbX3 molecules is trigonal bipyramidal in the vapour phase (Table 20) and this symmetry is retained in the crystal state with some variations in bond lengths. In the case of the iodide the d(Sb—I) is 2.12 A (I—Sb—199°) the crystal consists of hexagonal close packed assemblies of iodine atoms with antimony in the octahedral interstices, but off-centre, giving two different d(Sb—I) distances of 2.87 and 3.32 A. 

In examining a crystalline structure as revealed by diffraction experiments it is all too easy to view the crystal as a static entity and focus on what may be broadly termed attractive intermolecular interactions (dipole-dipole, hydrogen bonds, van der Waals etc., as detailed in Section 1.8) and neglect the actual mechanism by which a crystal is formed, i.e. the mechanism by which these interactions act to assemble the crystal from a non-equilibrium state in a super-saturated solution. However, it is very often nucleation phenomena that are ultimately responsible for the observed crystal structure and hence we were careful to draw a distinction between solution self-assembly and crystallisation at the beginning of this chapter. For example paracetamol, when crystallised from acetone solution gives the stable monoclinic crystal form I, but crystallisation from a molten sample in the absence of solvent... 

We conclude this survey with extended systems, where the assembled structures show the translational symmetry characteristic of the crystalline state. The packing of chiral units into a crystal lattice will inevitably involve some type of diastereo-selectivity, either homochiral or heterochiral, although this is fiequently not discussed in crystal stucture reports. If the associations are all homochiral, then an enantiomerically pure crystal will be obtained, and a solution of the racemate will yield a racemic mixture (see section 3). If, on the other hand, heterochiral association (often related by a centre of inversion or glide plane) is favoured, a racemic compound will crystallize. The double helicate [ 02(51)2] " crystallizes with a homochiral association of complexes along the helieal axis (Figure 38). The homochiral columns are then arranged in pseudohexagonal arrays with the ehirality... 

Our group investigated the host-guest ability of H5.31 in the crystal state. The units in H5.31 are arranged in an up-to-down manner by intra-molecular hydrogen bonds. The stabilized hexagonal structure assembled into infinite one-dimensional channels in the solid state (Figure 5.30 detail in Chapter 6). ... 

Investigating the assembled structures of macrocyclic compounds in the solid state is very important because such studies reveal the optical properties, mechanical and thermal stabilities in macrocyclic-based bulk materials, and the complexation ability of macrocyclic compounds. Assembled structures of macrocyclic compounds, such as cyclodextrins (CDs), calix[ ]arenes and cucutbit[ ]urils in the crystal state have... 

In this chapter, first, the assembled structures of the cyclic pentamers, pillar[5]arenes, and cyclic hexamers, pillar[6]arenes, in the crystal state are discussed. They have been most widely used because they can be obtained in relatively good yields. Pillar[5]- and pillar[6]arenes possess clear cylindrical pillar-shaped structures when compared with other macrocyclic compounds. Therefore, it is envisioned that the assembled structures of pillar[n]arenes in the crystal state should be easily analyzed and this pre-organized conformation will facilitate the formation of herringbone, one-dimensional channel and slipped-stacked pillar[w]arenes. It has been revealed that the assembled structures of pillar[n]arenes largely depend on the ring size, substituents and solvents used to obtain the single crystal. The assembled structures of pil-lar[ ]arenes contribute to the gas and organic vapor adsorption properties. 

Second, we describe crystal state-assembled structures of pillar[5]arenes, pillar[6]arenes, and larger pillar[n]arene homologs (n = 8, 9, 10). Furthermore, the bulk materials formed using pillar[ ]arenes are discussed. 

Huang et al. also reported the assembled structures of an amphiphilic pillar[5]arene having all five ester groups on the same side (6.20) in the crystal state. The amphiphilic pillar[5]arene also formed onedimensional channels with a rotation angle of 36° (Figure 6.12). The trend is the same as for the non-symmetrical pillar[5]arenes having all five alkoxy groups on the same side, and for a pillar[5]arene with 10 ester moieties. 

In the crystal state, both 2b Cl and 3b - Cl formed columnar structures with counter cations, with the TPA+ salt of 2b - Cl showing alternately stacking cationic and anionic components. The TBA+ salt of 3b - Cl formed a columnar assembly comprising a pair of 3b Cl and a pair of TBA+ in a row (Figure 4.3b and c). ... 

The diffractions assignable to the repeat distances of charge-by-charge assemblies could not all be observed clearly, suggesting that fairly disordered structures were produced. On consideration of the crystal-state assembled mode of la ArC>C02 TPA+, the anion modules may be located at distorted angles to the receptor planes and may control the assembled structures predominantly through van der Waals interactions between the aliphatic chains. 

In their crystalline state, pillar[5]arenes mainly form three different kinds of assembled structure, including 3D structures (e.g., herringbone), ID channels and slipped-stack structures. The ring size, nature of the substituents on the rims of the pillar[ ]arene structure and type of solvent used for the crystallization aU play a critical role in dictating the assembled structures of pillar[/i]arenes. 

Within a particle-based model, there is no well-defined reference state for the self-assembled structure. However, one can try to relate the seF-assembled structure to a disordered melt (or a different self-assembled morphology) via a reversible path and calculate the change of the free energy by thermodynamic integration. Typically, transitions between disordered and ordered morphologies or between different self-assembled structures are of first order. Thus, in an analogy to crystallization of hard condensed matter, there is no path in the space of physical intensive variables - for example, temperature, incompatibility, or composition - that reversibly cormects disordered and ordered structures. 

Crystallography is a very broad science, stretching from crystal-structure determination to crystal physics (especially the systematic study and mathematical analysis of anisotropy), crystal chemistry and the geometrical study of phase transitions in the solid state, and stretching to the prediction of crystal structures from first principles this last is very active nowadays and is entirely dependent on recent advances in the electron theory of solids. There is also a flourishing field of applied crystallography, encompassing such skills as the determination of preferred orientations, alias textures, in polycrystalline assemblies. It would be fair to say that... 

Metallurgists originally, and now materials scientists (as well as solid-state chemists) have used erystallographic methods, certainly, for the determination of the structures of intermetallic compounds, but also for such subsidiary parepistemes as the study of the orientation relationships involved in phase transformations, and the study of preferred orientations, alias texture (statistically preferential alignment of the crystal axes of the individual grains in a polycrystalline assembly) however, those who pursue such concerns are not members of the aristocracy The study of texture both by X-ray diffraction and by computer simulation has become a huge sub-subsidiary field, very recently marked by the publication of a major book (Kocks el al. 1998). 

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