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Reversible Phase Transformation (reversible + phase_transformation)
Selected AbstractsThermally Induced Reversible Phase Transformations Accompanied by Emission Switching Between Different Colors of Two Aromatic-Amine CompoundsADVANCED MATERIALS, Issue 31 2009Yunfeng Zhao Thermally driven reversible emission switching between different colors and solid-phase transformation are investigated for two aromatic-amine compounds bearing trifluoromethyl (CF3) groups. The phase-dependent emission properties are attributed to the different molecular packing properties and changeable molecular geometry for different solid phases (see figure). [source] Two new paracetamol/dioxane solvates,a system exhibiting a reversible solid-state phase transformationJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 10 2003Ranko M. Vrcelj Abstract This work reports on the crystal structures of two dioxane solvates of paracetamol that are true polymorphs. The high temperature phase is an orthorhombic form, space group Pbca, Z,=,8, a,=,12.6078(3) Å, b,=,12.1129(2) Å, c,=,13.4138(3) Å, V,=,2048.52(7) Å3, (at 295 K) and the low temperature form is monoclinic, space group P21/c, Z,=,4, a,=,12.325(6) Å, b,=,11.965(4) Å, c,=,13.384(6) Å, ,,=,92.01°, V,=,1972.6(14)Å3 (at 123 K). The structures of these polymorphs are described as is the interrelationship between the two structures. In addition to the structural interrelationship, it is shown that the two forms undergo a reversible phase transformation. Desolvation of either form generates the stable monoclinic phase of paracetamol. © 2003 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 92:2069,2073, 2003 [source] A 13C solid-state NMR analysis of steroid compoundsMAGNETIC RESONANCE IN CHEMISTRY, Issue 8 2008Jen-Hsien Yang Abstract 13C CP/MAS solid-state NMR spectroscopy has been utilized to analyze six steroid compounds, namely testosterone (Tes), hydrocortisone (Cor), trans -dehydroandrosterone (Adr), prednisolone (Prd), prednisone (Pre) and estradiol (Est). Among them, Tes displays a doublet pattern for all residues, whereas Prd, Pre and Est, exhibit exclusively singlets. For Cor and Adr, the 13C spectra contain both doublet and singlet patterns. The 13C doublet signal, with splittings of 0.2,1.5 ppm, are ascribed to local differences in the ring conformations associated with polymorphism. We have assigned all of the 13C resonances to the different residues in these steroid compounds on the basis of solution NMR data. The C-7, C-8, C-10, C-15 and C-16 residues of Tes, Cor and Adr consistently give rise to singlets or doublets with splittings of less than 0.5 ppm, indicating similar local conformations. Accompanying hydration and dehydration processes, a reversible phase transformation between ,- and ,-crystal forms has been observed in Tes, corresponding to singlet and doublet13C patterns, respectively. To further characterize the ring conformations in the ,-form, we have successfully extracted chemical shift tensor elements for the 13C doublets. It is demonstrated that 13C solid-state NMR spectroscopy provides a reliable and sensitive means of characterizing polymorphism in steroids. Copyright © 2008 John Wiley & Sons, Ltd. [source] Phase Transformations in the High-Temperature Form of Pure and TiO2 -Stabilized Ta2O5JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2007Geoff L. Brennecka The high-temperature forms of undoped tantalum pentoxide (H-100Ta2O5) and TiO2,modified Ta2O5 (H-92Ta2O5,8TiO2) were investigated by in situ synchrotron X-ray diffraction and Raman scattering measurements. Two unquenchable and reversible phase transformations were observed in pure H-Ta2O5, while only one was detected for TiO2 -stabilized H-Ta2O5. Diffraction studies were consistent with displasive transformations, but hot-stage Raman spectroscopy indicated the existence of transient intermediate forms during the transformations. Use of complementary techniques enabled the reinterpretation of phase transformations in light of a newly proposed crystal structure model for H-Ta2O5,1 and emphasized the structural contributions of the oxygen sublattice. [source] | |||||||||||||