Crystals polymorphism

MOLECULAR SIMULATION METHODS TO COMPUTE INTRINSIC AQUEOUS 

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In some cases there is evidence of multiple solid-solid transitions, either crystal-crystal polymorphism (seen for Cl salts [20]) or, more often, formation of plastic crystal phases - indicated by solid-solid transitions that consume a large fraction of the enthalpy of melting [21], which also results in low-energy melting transitions. The overall enthalpy of the salt can be dispersed into a large number of fluxional modes (vibration and rotation) of the organic cation, rather than into enthalpy of fusion. Thus, energetically, crystallization is often not overly favored. 

Applications The general applications of XRD comprise routine phase identification, quantitative analysis, compositional studies of crystalline solid compounds, texture and residual stress analysis, high-and low-temperature studies, low-angle analysis, films, etc. Single-crystal X-ray diffraction has been used for detailed structural analysis of many pure polymer additives (antioxidants, flame retardants, plasticisers, fillers, pigments and dyes, etc.) and for conformational analysis. A variety of analytical techniques are used to identify and classify different crystal polymorphs, notably XRD, microscopy, DSC, FTIR and NIRS. A comprehensive review of the analytical techniques employed for the analysis of polymorphs has been compiled [324]. The Rietveld method has been used to model a mineral-filled PPS compound [325]. 

Keywords Bundles Chain-folding Chiral crystal polymorphs Mesophases ... 

Crystal polymorphs will usually exhibit different melting points, with the highest melting form being regarded as the most stable. The interconversion of... 

In a manner similar to that just described for differential thermal analysis, DSC can be used to obtain useful and characteristic thermal and melting point data for crystal polymorphs or solvate species. This information is of great importance to the pharmaceutical industry since many compounds can crystallize in more than one structural modification, and the FDA is vitally concerned with this possibility. Although the primary means of polymorph or solvate characterization s centered around x-ray diffraction methodology, in suitable situations thermal analysis can be used to advantage. 

Paul Verwer and Frank J. J. Leusen, Computer Simulation to Predict Possible Crystal Polymorphs. 

Davey, R.J., Blagden, N., Righini, S., Alison, H., Quayle, M.J., Fuller, S., 2001, Crystal Polymorphism as a Probe for melecular Self-Assembly during Nucleation from Solutions The Case of 2,6-Dihydroxybenzoic Acid, Crystal Growth and Design, Vol. l,No. 2, 59-65. 

Crystal polymorphism of bmimCl was reported almost simultaneously by two groups [8,9]. We found by chance that two different types of crystals. Crystal (1) and Crystal (2), formed when liquid bmimCl was cooled down to 18°C and was kept for 48 h [8]. Orthorhombic Crystal (2) dominantly formed but monoclinic Crystal (1) also formed occasionally (see Fig. 2). 

Subsequent to the discovery of the crystal polymorphism of bmimCl, the crystal structures of bmimCl and bmimBr were determined. We determined the crystal structures of bmimCl Crystal (1) and bmimBr at room temperature [11, 12]. Independently, Holbrey et al. [9] reported the crystal structures of bmimCl Crystal (1) and Crystal (2), as well as that of bmimBr at — 100°C. The two sets of structures determined at different temperatures agree well with each other except for the lattice constants that vary with temperature. They also show that the molecular structure of the bmim cation in bmimCl Crystal (2) is different from that in (1) but that it is the same as that in bmimBr, as already indicated by the Raman spectra. In the following, we discuss the crystal structures of bmimCl Crystal (1) and bmimBr as the two representative stmctures at room temperature. 

Borka L. Review on crystal polymorphism of substances in the European Pharmacopeia. Pharm Acta Helv 1991 66 16-22. 

Gas-solid reactions and the relationship with solvation processes and the formation of crystal polymorphs and pseudo-polymorphs [15]. 

Holbrey, J. D., Reichert, W. M., Nieuwenhuyzen, M., Johnston, S., Seddon, K. R., and Rogers, R. D., Crystal polymorphism in l-butyl-3-methylimidazolium halides Supporting ionic liquid formation by inhibition of crystallization, Chem. Commun., 1636-1637, 2003. 

Hayashi, S., Ozawa, R., and Hamaguchi, H., Raman spectra, crystal polymorphism, and structure of a prototype ionic-liquid [bmimJGl, Chem. Lett., 32, 498-499,2003. 

Control of Crystal Polymorphism with the Assistance of "Tailor-Made" Auxiliaries... 

Acceptor species concentrations, equations, 400-401 Acentric materials biomimetic design, 454-455 synthesis approaches, 446 Ar-(2-Acetamido-4-nitrophenyl)pyrrolidene control of crystal polymorphism with assistance of auxiliary, 480-482 packing arrangements, 480,481-482/ Acetylenes, second- and third-order optical nonlinearities, 605-606 N-Acetyltyrosine, phase-matching loci for doubling, 355,356/, t Acid dimers, orientations, 454 Active polymer waveguides, applications, 111... 

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