Polymorphic compounds

The majority of unipolar ionic conductors identified to date are polymorphic compounds with several phase transitions, where the phases have different ionic conductivities owing to modifications in the substructure of the mobile ions [28], One of the first studied cationic conductors was a-Agl [21]. Silver iodide exhibits different polymorphic structures. Agl has a low-temperature phase, that is, [3-Agl, which crystallizes in the hexagonal wurtzite structure type, and a high-temperature cubic phase, a-Agl, which shows a cubic CsCl structure type [20,22] (see Section 2.4.5). 

Scheme 3 Relative percentages of organic, organometallic and coordination polymorphic compounds present in the Cambridge Structural Database (CSD, version October 2001) [20a],...

For polymorphic compounds, such as sulfa drugs, talc excipients induced polymorphic transformation of sulfamethoxazole during the process of microencapsulation by spray-drying,... 

In the presence of polymorphs, developing a proper crystaUization process to control the desired crystal form is a real challenge. A good understanding of solid-liquid equilibrium behaviors under different conditions—for example, the temperature or solvent mixmre— is a must. Seeding and control of supersaUiration are two critical, if not indispensable, requirements. Example 7-5 shows an example of developing a crystallization process for a polymorphic compound. 

Categories that cause problems for this definition of chemical substance include (1) enantiomers (species containing equal amounts of two optical isomers, like I- and d-tartaric acid) (2) azeotropic mixtures (3) dissociative compounds in equilibrium (4) certain types of mixed crystals or other polymorphic compounds (e.g,d- and /-camphoroxime) (5) synthetic polymers (6) many biochemical compounds (7) systems that are not in "pure" thermodynamic equilibrium and (8) isotopes. In each case, pragmatic decisions have to be made, as the notion of pure substance cannot be essen-tialized. There are no competing definitions of "pure substance" that can avoid the need for "inspired adhoccery" to deal with difficult cases. 

Do Polymorphic Compounds Make Good Co-crystal I izing Agents [107] ... 

Three of the four polymorphic compounds examined in this study [107] readily form co-crystals e.g. 2-hexeneoic acid isonicotinamide, 2-amino-5-nitropyrimi-dine 2-amino-3-nitropyridine, and 3-dimethylaminobenzoic acid 4-chlorobenza-mide. The structural behavior of 1-3 supports the suggestion that polymorphs make good co-crystallizing agents (provided that solubility differences are not too large). However, no co-crystals with maleic hydrazide were obtained, despite the fact that it exists in three different polymorphs. 

Infrared, and more recently Raman spectroscopy, have been widely used for the qualitative and quantitative characterization of polymorphic compounds of pharmaceutical interest (for the sake of brevity, the term polymorphism will encompass polymorphs, pseudo-polymorphs, hydrates, and solvates). Since solid-state vibrational spectroscopy can be used to probe the nature of polymorphism on the molecular level, these methods are particularly useful in instances where full crystallographic characterization of polymorphism was not found to be possible. 

This volume contains 23 chapters organized in five sections. More than a third of the cluq)ters deal with the crystallization of polymorphic compounds that can crystallize in more than one form and the crystallization of chiral compounds. This emphasis is a reflection of die current int st in these two fields. The other four areas are basic studies, smdies related to industrial crystallizers and processes, and two areas involving crystallization of particular organic and inorganic compounds. A diif nt organization of the book would have shown that 10 of the papers deal with aspects of crystallization of proteins, amino acids, and pharmaceuticals, a sign of the increased activity in this area of crystallization. 

Liquid crystals (LCs) are critically important soft materials in which molecules are less strictly confined than in the crystalline state. Although no effective design principle for polymorphic compounds has been established, LC molecules can be designed by a suitable combination of rigid mesogenic unit(s) and soft segment(s). 

L-glutamic acid is a mono tropic polymorphic compound. Transformation between the a- and p-polymorphs is solution-mediated. The a-poly-morph of L-glutamic acid has a granular consistency whilst the p-form has a needle-like flaky structure. ... 

Polymorphic compounds change their structure depending on temperature and pressure. Detailed analysis of these parameter dependencies allows study of the nature of the phase transition. A special type of polymorphism is polytypism, in which identical sheets are stacked together in different sequences. 

Phase transition is a fundamental defining characteristic of this approach to pure substances. Hence substances that exist in one phase only, easily decompose, only occur in solution, etc. can only be included by analogy. Timmermans [1928, 23-53] lists the following potentially difficult cases for the molar approach of substance definition as summarised in this section azeotropic mixtures, dissociative compounds in equilibrium, enantiomers and racemates ( 18), certain types of mixed crystals or other polymorphic compounds, polymers ( 16), many biochemical compounds, and systems that are not in thermodynamic equilibrium. 

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