Alpha-crystalline form

Figure 19.2 Molecular helix of alpha crystalline form ofisotactic polypropylene.

In contrast to 1, the related pure host 7 may be obtained in crystalline form 68). The crystal structure of 7 is built via helical chains of alternating intra- and inter-molecular H-bonding through the carboxyl functions. This structure supplies the information that the carboxyl groups are therefore already positioned in an appropriate way to facilitate analogous H-bonding in the known inclusions of 7. As discussed later (Sect. 4.2.2), these are exclusively salt-type associates and as such, intimately interact with the carboxyl groups. Hence one may infer that displacement of the carboxyl functions from position 2 in 1 to position 8 in 7 reduces the ability of inclusion formation. Similar reasons such as the solid-solubility differences observed in the classical naphthalene/chloronaphthalene systems (alpha- vs. beta-substituted derivatives, cf. Ref. 28 may also be applied here. 

The beta crystalline form of isotactic polypropylene differs from the alpha form by having a lower crystalline density and lower melting point. The beta form is metastable to the alpha form and will rearrange to the alpha structure when heated to approximately 100 °C or placed under strain. Beta lamellae form parallel stacks as shown in Fig. 19.4. 

White phosphorus has a white waxy appearance that turns slightly yellow with age and impurities. There are two allotropic forms of white phosphorus. The alpha (a) form has a cubic crystal structure, and the beta (P) form has a hexagonal crystalline structure. White phosphorus is extremely reactive and will spontaneously burst into flame when exposed to air at a temperature of about 35°C. It must be kept under water. But this property of spontaneous combustion has made it useful for military applications. 

Colorless or light yellow metal at ordinary temperatures it occurs in hexagonal close-packed crystalline form, known as alpha-gadolinium alpha form transforms to a body-centered cubic allotropic form, beta-gadolinium upon heating at 1,262°C density 7.90 g/cm melting point 1,313°C vaporizes at 3,266°C vapor pressure 9.0 torr at 1,800°C (calculated) electrical resistivity 134.0 microhm-cm at 25°C Poisson ratio 0.259 modulus of elasticity 8.15x106 psi thermal neutron absorption cross section 46,000 barns insoluble in water dissolves in acid (reacts). 

Black tetragonal crystal exhibits two allotropic modifications—a stable alpha phase, occurring in tetragonal crystalline form (as hausmannite) and an unstable beta modification density 4.85 g/cm Moh s hardness 5.5 melts at 1,567°C insoluble in water soluble in hydrochloric acid. 

Two crystalline forms exist (1) alpha allotrope a simple cubic low temperature form density 9.196 g/cm, and (2) beta modification a rhombohedral high temperature form density 9.398 g/cm ... 

Crystalline selenium exhibits two monochnic forms an alpha form constituting dark red transparent crystals, density 4.50 g/cm. The alpha form converts to a metastable beta form of hexagonal crystal structure when heated to about 170°C. Both the crystalline forms are insoluble in water soluble in sulfuric and nitric acids very slightly soluble in carbon disulfide. Also, both the crystalline forms convert into gray metallic modification on heating. 

Zinc sulfide is white to gray-white or pale yellow powder. It exists in two crystalline forms, an alpha (wurtzite) and a beta (sphalerite). The wurtzite form has hexagonal crystal structure refractive index 2.356 density 3.98 g/cm3 melts at 1,700°C practically insoluble in water, about 6.9 mg/L insoluble in alkalis soluble in mineral acids. The sphalerite form arranges in cubic crystalline state refractive index 2.368 density 4.102 g/cm changes to alpha form at 1,020°C practically insoluble in water, 6.5 mg/L soluble in mineral... 

Cocoa butter NF is defined as the fat obtained from the seed of Theobroma Cacao Linne (Family Sterculiaceae) (44). Cocoa butter softens at 30°C and melts at 34°C. It contains four different forms alpha, beta, beta prime, and gamma with melting points of 22°C, 34°C to 35°C, 28°C and 18°C, respectively. The beta form is the most stable and is desired for suppositories. The biggest challenge with the polymorphism of cocoa butter is the impact of the manufacturing process on the characteristics of the suppository itself. When cocoa butter is hastily melted at a temperature greatly exceeding the minimum required temperature and then quickly chilled, the result is metastable crystalline form (a crystals), which may not even... 

Lactose may be obtained in two crystalline forms a-lactose and P-lactose (in addition to amorphous forms). The alpha form is obtained when water is incorporated into the lattice structure during crystallization (usually by supersaturation below 93.5°C) (5). Alternatively, the beta form does not contain water and exists as a non-hygroscopic and anhydrous form. Amorphous lactose is formed when either the crystallization is rapid or sufficient transient energy is introduced into the crystalline forms (74), i.e., spray drying (75), micronization and milling (76), freeze-drying, and anti-solvent crystallization (77). 

In further work, Strepkov obtained99 labiose in crystalline form, of m.p. 126-8°, [c ]d + 136.7° (in water, air-dry basis). Labiose is completely hydrolyzed by invertase at 47-8° in 20 hours, and by 1% hydrochloric acid at 68-70° in 10 to 12 minutes, to give one molecular proportion of D-galactose and two molecular proportions of D-fructose. Labiose is not hydrolyzed by almond emulsin, and it is fermented but little by pressed yeast. Strepkov concluded that labiose is related to the alpha series ( ) and that, probably, the D-galactose unit is in the center of the molecule. 

PVDF exhibits a complex crystalline polymorphism, which cannot be found in other known synthetic polymers. There are a total of four distinct crystalline forms alpha, beta, gamma, and delta. These are present in different proportions in the material, depending on a variety of factors that affect the development of the crys-... 

The physiological properties of SF matrices strongly depend on its molecular conformation and surface texture. SF exhibits at least three crystalline forms silk I, silk II, and alpha-helix. All three conformations can be formed from the appropriate preparation conditions and each is interchangeable under certain conditions. Crystallization of SF involved the conformational transition that can be easily induced by simple physical (thermal), mechanical or chemical treatments. The most common method to convert the distorted conformation (random coil or silk I) of SF into the more stable P-sheet (silk II) conformation is to treat the SF film with an organic solvent. It is well known that organic... 

PVDF exhibits a complex crystalline polymorphism, which cannot be found in other known synthetic polymers. There are a total of four distinct crystalline forms alpha, beta, gamma, and delta. These are present in different proportions in the material, depending on a variety of factors that affect the development of the crystalline structure, such as pressure, intensity of the electric field, controlled melt crystallization, precipitation from different solvents, or seeding crystallization (e.g., surfactants). The alpha and beta forms are most common in practical situations. Generally, the alpha form is generated in normal melt processing the beta form develops under mechanical deformation of melt-fabricated specimens. The gamma form arises under special circumstances, and the delta form is obtained by distortion of... 

Crystalline Form The crystalline form of the ortho nitrotoluene has evidently not been thoroughly investigated, since the various chemists do not agree upon this point. This is possibly due to the presence of both the alpha and the beta forms in the mixture. 

Koehler, J. E., Saenger, W. and van Gunsteren, W. F. (1988). Conformational differences between alpha-cyclodextrin in aqueous solution and in crystalline form. A molecular dynamics study. J Mol Biol, 203, 241-250. 

TiClj is a highly colored solid that exists in several crystalline forms, designated as alpha (a), beta (p), gamma (y) and delta (S). The a, y and S forms have layered crystal structures and are violet. The P form has a linear structure and is brown. 

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