Crystalline state molecular dimensions

There is actually no sharp distinction between the crystalline and amorphous states. Each sample of a pharmaceutical solid or other organic material exhibits an X-ray diffraction pattern of a certain sharpness or diffuseness corresponding to a certain mosaic spread, a certain content of crystal defects, and a certain degree of crystallinity. When comparing the X-ray diffuseness or mosaic spread of finely divided (powdered) solids, the particle size should exceed 1 um or should be held constant. The reason is that the X-ray diffuseness increases with decreasing particle size below about 0.1 J,m until the limit of molecular dimension is reached at 1-0.1 nm (10-1 A), when the concept of the crystal with regular repetition of the unit cell ceases to be appropriate. 

Table 4 presents molecular dimension parameters of 1,2,3-triazole and 2-methyl-4-trinitromethyl-2//-1,2,3-triazole based on electron diffraction (ED) data alone, joint analysis of electron diffraction and microwave spectral data (ED-MW), and ab initio calculations. The corresponding molecular dimensions of 1,2,3-triazoles in the crystalline state obtained by x-ray diffraction (XD) are the average values based on <85ACS(A)259>. Figure 5 shows the bond distances and angles of the crystal structure of 2-phenyl-4-chloro-1,2,3-triazole <84ACS(A)497>. As examples of 4,5-unsubstituted H-... 

In polymer crystallization the challenge is to identify and clarify the transformations by which chain molecules pass from a disordered, molten state to the ordered supra-molecular organization known as the semi-crystalline state. The subject is highly relevant in terms of both basic science and technology it is indeed clear that many modern applications require complete control of the structure and the morphology of polymers from macroscopic dimensions down to below the nanoscale. As a simple example, making the crystallites in a polymer liber equally oriented and reducing the number of chain folds (or hairpins) therein, usually turn out to be very favorable requisites for mechanical performance. 

The liquid crystal state (LCS) shows order in one or two dimensions it lacks the three-dimensional long-range order of the crystalline state. LCS has characteristics intermediate between those of the crystalline and the disordered amorphous states. These phases are called liquid crystals because many of them can flow like ordinary liquids but they display-birefringence and other properties characteristic of crystalline soHds. In liquid crystal phases the molecules can move but the orientational order is conserved in at least ne direction. The LCS can be displayed by small molecules and by polymersj but in both cases a characteristic chemical structure is needed. The existence of the liquid crystal state is related to the molecular asymmetry and the presence of strong anisotropic intermolecular interactions (19-21). Thus, molecules with a rigid rod structure can form highly ordered... 

In solid state, cellulose exists in mostly crystalline state. This state has been the subject of extensive studies for at least 100 years, and several important aspects need to be pointed out at the onset. Crystallinity is neither uniform (options exist) nor static (crystallinity can be lost as well as gained in relation to molecular mobility), nor is it permanent (conditions have been identified under which transitions of order take place). Cellulose crystallinity manifests itself through the existence of distinctive X-ray diffraction patterns. These patterns allow the determination of the overall dimensions of unit cells which are spatial units that represent the... 

One of the most distinct properties of the crystalline state is, therefore, the presence of long-range order, or in other words, a regular and in the simplest case periodic repetition of atoms or molecules in space. In theory, periodic crystals are infinite, but in practice, their periodicity extends over a distance fi om 10 to 10 ° atomic or molecular dimensions, which occurs because any crystal necessarily has a number of defects and may contain impurities without losing its crystallinity. Furthermore, a crystal is always finite regardless of its size. 

Liquid crystal is a term that is now commonly used to describe materials that exhibit partially ordered fluid phases that are intermediate between the three dimensionally ordered crystalline state and the disordered or isotropic fluid state. Phases with positional and/or orientational long-range order in one or two dimensions are termed mesophases. As a consequence of the molecular order, liquid crystal phases are anisotropic, i.e., their properties are a function of direction. 

The early, traditional, and strict definition of zeolites is based on the natural minerals and states that zeolites are crystalline microporous aluminosilicates belonging to the tectosilicate group and comprising within their structures channels and cavities of molecular dimensions. Dissecting this definition we find three important aspects ... 

Q he greatest changes in enthalpy are observed at Tp, but they are lower than would have been expected (on the basis of molecular dimensions) from crystalline-to-mesomorphic transitions. The cholesteric state was observed at the flow temperature under a polarizing microscope. The cholesteric reflections are observed at lower temperat ares than Tp in systems containing polymers or at Tp of the monomer itself. 

This chapter explored the current state of the use of zeolites in biomass conversion strategies. From this, it can be concluded that zeolites, in an extension to their success in petrochemistry, can also play a pivotal role in the conversion of biomass. The success of zeolites in petrochemistry is closely linked to their strong Bronsted acidity in a crystalline matrix with porosity of molecular dimensions which could be one by one easily modified. The examples in this chapter showed that these properties are also very decisive in biomass transformations. 

---