Polymorphic crystals, absorption

It was observed that ultrasound stimulation (ultrasonication) also accelerated the crystallization of the more stable polymorphs of CB (111). A fundamental study of the effects of ultrasonication on the polymorphic crystallization of PPP and LLL showed that several factors, such as pressure effect, shear flow, cavitation, and thermal energy caused by absorption of attenuated ultrasound wave, may play concurrent effects of ultrasonication. As a result, there are optimal conditions for temperature and duration of ultrasonication to increase the rate of crystallization and the occurrence of the more stable polymorphs (20). This effect was also observed in CB (111). 

Besides diastereoisomerism, iodinated radiopaques may show polymorphism crystals formed under different conditions show identical infrared absorption bands but with different intensities (262,263). 

A rate enhancement effect due to secondary nucleation has been identified in the solution-mediated transformation of the 7-phase of (i)-glutamic acid to its / -phase [82]. In this study, the kinetics of the polymorphic transition were studied using optical microscopy combined with Fourier transform infrared, Raman, and ultraviolet absorption spectroscopies. The crystallization process of n-hexatriacontane was investigated using micro-IR methodology, where it was confirmed that single... 

Scheme 5 shows a group of alkynylgold(i) complexes for which the studies focused on the UV-VIS electronic absorption and emission properties. Most of these compounds are of the type [(L)AuC=CR], for which the methods of synthesis have been summarized above. The products were found to show phosphorescence in various polymorphs and crystal forms of solvates. Although there are no metallophilic interactions discernible in the crystal between most of the monomers due to the steric effect of the large tertiary phosphines, there is nevertheless strong excitonic coupling based on other weak interactions, which depend on the organization of the molecules in the crystal.105,106... 

Solid state characterization studies of the previously mentioned polymorphic systems [26-34] all utilize IR as a means to differentiate the various crystal modifications. In some cases, the observation of variations in IR absorption intensities has led to conclusions regarding intramolecular hydrogen bonding [26]. For other systems, fairly complete IR spectral band assignment has allowed for determination of structure for the polymorphic system. In one study [29], DSC-IR was used to identify the polymorphs and determine simultaneously the correlation between thermal events and structural changes. 

In contrast to what is known about a-Si, much less is understood about polyamorphism in Ge. The authors of most early experiments reported no direct evidence of LDA-HDA transition in Ge [260-262, 270, 271]. Shimomura et al. [260] observed a stepwise drop of the electronic resistance (at 6 and lOGPa) after compression of an -Ge him. This decrease, however, may have resulted from (partial) recrystallization to a metallic high-pressure polymorph under pressure. Tanaka [270] measured X-ray diffraction patterns and optical absorption spectra of -Ge at pressures up to lOGPa. In this experiment, the sample was indeed partly transformed to the (3-tin crystalline phase ( 25% in volume) at 6 GPa. Imai et al. [262] also observed an amorphous to [3-tin crystal transition. Freund et al. [271], in contrast, have observed no sign of crystallization or transition to an HDA form after compression up to 9 GPa. 

Many drugs can exist in more than one crystalline form, for example chloramphenicol palmitate, cortisone acetate, tetracyclines and sulphathiazole, depending on the conditions (temperature, solvent, time) under which crystallization occurs. This property is referred to as polymorphism and each crystalline form is known as a polymorph. At a given temperature and pressure only one of the crystalline forms is stable and the others are known as metastable forms. A metastable polymorph usually exhibits a greater aqueous solubility and dissolution rate, and thus greater absorption, than the stable polymorph. 

Nature of the drug formulation Drug absorption may be altered by factors unrelated to the chemistry of the drug. For example, particle size, salt form, crystal polymorphism, and the presence of excipients (such as binders and dispersing agents) can influence the ease of dissolution and, therefore, alter the rate of absorption. 

The existence of two polymorphic structures of the dichloro derivative of 6-XXXV (R = Cl) (Bernstein and Izak 1976) provided an opportunity for the direct examination of the relationship between the molecular structure and the electronic spectrum. The two structures are conformational polymorphs, with the metastable very pale yellow triclinic needle form exhibiting a planar molecular conformation a = fi = 0°) (Bernstein and Schmidt 1972) and the stable yellow orthorhombic form (with chunky rhombic crystals) exhibiting a non-planar conformation (a = 25° fi = —25°) (Bernstein and Izak 1976). Assuming that the two crystal structures merely serve to hold the molecule in the two different conformations (i.e. the oriented gas model), the absorption spectra should reflect the difference in conformation that measured on the triclinic structure, with a planar conformation, should closely resemble the spectra of 6-XXXVI and 6-XXXVII, while that for the orthorhombic structure, with the nonplanar molecular conformation, should retain the characteristics of 6-XXXV (R = H) in solution. 

Different physical forms of a drug can affect its absorption. Typically, the crystal or polymorphic form, the state or nature of hydration or solvation, and physical size of drug particles may have considerable impact on the rate and extent of drug absorption. 

Polymorphism is the ability of a chemical species to crystallize in more than one distinct crystal habit. The pharmaceutical applications of polymorphism have been reviewed by several authors. The differences in dissolution rate and solubility that polymorphs can produce may have a dramatic impact on bioavailability when dissolution is the rate-limiting step in the absorption process. 

Photodegradation in the solid state takes place only at the sample surface. The degradation rate is therefore dependent on factors that will influence the depth of light penetration, i.e., change the absorption and reflection at the surface (e.g., particle size, crystal modification, color, thickness of powder bed, and coating of the individual particles or the dosage form). Mefloquine, chloroquine, carbamazepine, and furosemide are examples of drug substances that show different decomposition rates dependent on their polymorphous modiflcation.P ... 

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