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Metastable Form (metastable + form)
Selected AbstractsEnantiotropic phase transition and twinning in 2,2,3,3,4,4-hexafluoropentane-1,5-diolACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2009Jeong-Myeong Ha Four crystal structure determinations of 2,2,3,3,4,4-hexafluoropentane-1,5-diol (HFPD), C5H6F6O2, were conducted on a single specimen by varying the temperature. Two polymorphs of HFPD were found to be enantiotropically related as phases (I) and (II), both in the space group P1. These structures contain closely related R44(20) sheets. A structure determination was completed on form (Ia) at 283,K. Form (Ia) was then supercooled below the phase transition temperature at 279 to 173,K to give form (Ib) for a second structure determination. Metastable form (Ib) was transformed by momentary warming and recooling to give form (II) for a third structure determination at 173,K. Form (II) transformed to form (Ic) upon warming to 283,K. Enantiotropic phase transitions between phases (I) and (II) were confirmed with X-ray powder diffraction and differential scanning calorimetry. Form (Ia) was found as a twin by nonmerohedry by a reflection in (011). This twinning persists in all phases described. Additional twinning was found after the phase (I) to phase (II) transformation. These two additional twin components are related to the first pair by a 180° rotation about the (012) plane. This latter pair of twins persisted as the specimen was warmed back to form (Ic) at 283,K. [source] X-ray crystallographic analysis and differential scanning calorimetry of two polymorphic forms of 2,6-diphenyl-1,4-dithiinHETEROATOM CHEMISTRY, Issue 6 2004Xuehua Piao 2,6-Diphenyl-1,4-dithiin 2 exists in the two polymorphic forms. The dithiin 2L, mp 62,63°C, was assigned as the metastable form, while the dithiin 2H, mp 79,80°C, was assigned as the stable form, mainly on the basis of differential scanning calorimetry (DSC) analysis. X-Ray crystallographic analysis showed that the largest difference in the molecular structure between 2L and 2H is found in the coplanarity between the benzene ring and the double bond part of the dithiin ring; the benzene ring and the double bond are nearly coplanar for 2L, whereas these are far from being coplanar for 2H. © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:424,427, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20035 [source] In situ Raman and optical microscopy of the relaxation behavior of amorphous ices under pressureJOURNAL OF RAMAN SPECTROSCOPY, Issue 6 2010Yukihiro Yoshimura Abstract The transformation of low-density amorphous (LDA) ice produced from high-density amorphous (HDA) ice was studied up to 400 MPa as a function of temperature by in situ Raman spectroscopy and optical microscopy. Changes in these amorphous states of H2O were directly tracked without using emulsions to just above the crystallization temperature Tx. The spectra show significant changes occurring above ,125 K. The results are compared with data reported for the relaxation behavior of HDA, to form what we call relaxed HDA, or rHDA. We find a close connection with expanded HDA (eHDA), which is reported to exist as another metastable form in this P,T region. The observation of this temperature-induced LDA transition under pressure complements the previously observed pressure-induced reversible transition between LDA and HDA at 120,140 K. Copyright © 2009 John Wiley & Sons, Ltd. [source] Single-crystal diffuse scattering studies on polymorphs of molecular crystals.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2009The drug benzocaine (ethyl 4-aminobenzoate), commonly used as a local anaesthetic, is a bimorphic solid at room temperature. Form (I) is monoclinic P21/c, while the metastable form (II) is orthorhombic P212121. Three-dimensional diffuse X-ray scattering data have been collected for the two forms on the 11-ID-B beamline at the Advanced Photon Source (APS). Both forms show strong and highly structured diffuse scattering. The data have been interpreted and analysed using Monte Carlo (MC) modelling on the basis that the scattering is purely thermal in origin and indicates the presence of highly correlated molecular motions. In both forms (I) and (II) broad diffuse streaks are observed in the 0kl section which indicate strong longitudinal displacement correlations between molecules in the ,031, directions, extending over distances of up to 50,Å. Streaks extending between Bragg peaks in the hk0 section normal to [100] correspond to correlated motions of chains of molecules extending along a that are linked by N,H...O=C hydrogen bonds and which occur together as coplanar ribbon pairs. The main difference between the two forms is in the dynamical behaviour of the ribbon pairs and in particular how they are able to slide relative to each other. While for form (I) a model involving harmonic springs is able to describe the motion satisfactorily, as simple excursions away from the average structure, there is evidence in form (II) of anharmonic effects that are precursors of a phase transition to a new low-temperature phase, form (III), that was subsequently found. [source] Polymorphism of 4-bromobenzophenoneACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2007Mikhail A. Strzhemechny A combination of single-crystal and powder X-ray diffractometry was used to study the structure of two polymorphs of 4-bromobenzophenone over the temperature range from 100 to 300,K. One of the polymorphs of the title compound was known previously and its structure has been determined at room temperature [Ebbinghaus et al. (1997). Z. Kristallogr.212, 339,340]. Two crystal growth methods were employed, one of which (a modification of the Bridgman,Stockbarger technique) resulted in single crystals of a previously unknown structure. The basic physical properties of the stable polymorph are: growth method, from 2-propanol solutions or gradient sublimation; space group, monoclinic P21/c; melting point, Tm = 355.2,K; X-ray density (at 100,K), Dx = 1.646,g,cm,3. The same properties of the metastable polymorph (triclinic ) are: growth method, modified Bridgman,Stockbarger method; X-ray density (at 100,K), Dx = 1.645,g,cm,3; Tm = 354,K. Thermograms suggest that the melting of the metastable form is accompanied by at least a partial crystallization presumably into the monoclinic form; the transformation is therefore monotropic. Analysis of short distances in both polymorphs shows that numerous weak hydrogen bonds of the C,H,, type ensure additional stabilization within the respective planes normal to the longest dimension of the molecules. The strong temperature dependence of the lattice constants and of the weak bond distances in the monoclinic form suggest that the weak bond interactions might be responsible for both the large thermal expansion within plane bc and the considerable thermal expansion anisotropy. [source] | ||||||||||