Polymorphism single-crystal studies

The Ca-Cu system has been reexamined using thermal analysis and x-ray diffraction methods an independent study of the CaCuj-Cu section has also been completed. The resultant phase diagram, although similar to that in ref. 3 at the Cu-rich end, differs markedly for Ca-rich alloys. Supporting evidence for the modifications has been obtained from the Ca-Mg-Cu ternaiy system. Three intermediate compounds are formed in the system CaCuj (950 C) melts congruently, whereas CajCu (488 C) and CaCu (567°C) are formed in peritectic reactions. Single-crystal x-ray diffraction studies verify the stoichiometry of CajCu and examine the polymorphism of CaCu. ... 

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]. 

A single-to-single crystal phase transition was found to take place at 333 K in a new polymorph of ort/zo-ethoxy-trans-cinnamic acid [77]. In this study, the structures of the title compound obtained at two temperatures above the transition point were determined in addition to the structures of the stabilized forms existing at lower temperatures. It was found that the phase transition involved a cooperative conformational transformation coupled with a shift in layers of the constituent molecules. 

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... 

The structural changes that accompanied the [2 + 2] photodimerization of the metastable a -polymorph of ort/zo-ethoxy-tranx-cinnamic acid have been studied [93]. In this study, the photochemical reaction was carried out at 293 K, and observed in situ by single-crystal X-ray diffraction. In the structure of the title compound, the three molecules in the asymmetric unit are arranged to form two potential reaction sites, but only one of these was found to be photoreactive. Since only two out of three molecules in the asymmetric unit take place in the photodimerization reaction, the crystal of the final product contains an ordered arrangement of the photodimer and the unreacted monomer. 

Analogous to the DuP 747 study, complete crystallographic information was not possible on the fosinopril sodium polymorphic system [25], Two known polymorphs (A and B) were studied via a multidisciplinary approach (XRD, IR, NMR, and thermal analysis). Complementary spectral data from IR and solid state 13C NMR revealed that the environment of the acetal sidechain of fosinopril sodium differed in the two forms. In addition, possible cis-trans isomerization about the CgN peptide bond may exist. These conformational differences are postulated as the origin of the observed polymorphism in fosinopril sodium in the absence of the crystallographic data for form B (single crystals not available). 

Most polymers do not form crystals suitable for single crystal X-ray diffraction, so powder or film methods are usually employed. X-ray and LJV data recorded at various temperatures provide the detailed information required to correlate conformational and electronic properties, since the former is sensitive to the inter- and intrachain packing, and the latter is sensitive to the conformation. DSC provides further evidence for any phase transitions. Detailed studies have been performed by Winokur and West,260 261 who reported a comparison of the polymorphism, structure, and chromism in poly(di- -octylsilylene), (Si- -Oct2), 89, and poly(di- -dccylsilylcnc)(Si- -Dcc2) , 90. These investigations will be described in detail for the useful insights into polysilane structures that they afford. 

The TiC>2 samples, mounted in a helium cryostat (10-300 K), are lmm-thick (001) and (110)-cut single crystals to study the electronic response with the (THz) electric field perpendicular ( L) and parallel (//) to the c axis, respectively. Rutile is the most common and stable TiC>2 polymorph and has a tetragonal structure with a=4.6A and c=2.9A. The band gap of rutile is 2.9 eV at room temperature, increasing slightly at lower temperatures. 

Huttermann J, Ward JF, Myers LS Jr (1971) Electron spin resonance studies of free radicals in irradiated single crystals of 5-methylcytosine. Int J Radiat PhysChem 3 117-129 Huttermann J, Ohlmann J, Schaefer A, Gatzweiler W (1991) The polymorphism of a cytosine anion studied by electron paramagnetic resonance spectroscopy. Int J Radiat Biol 59 1297-1311 Hwang CT, Stumpf CL, Yu Y-Q, Kentamaa HI (1999) Intrinsic acidity and redox properties of the adenine radical cation. Int J Mass Spectrom 182/183 253-259 Ide H, Otsuki N, Nishimoto S, Kagiya T (1985) Photoreduction of thymine glycol sensitized by aromatic amines in aqueous solution. J Chem Soc Perkin Trans 2 1387-1392 Idris Ali KM, Scholes G (1980) Analysis of radiolysis products of aqueous uracil + N20 solutions. J Chem Soc Faraday Trans 176 449-456... 

Similar studies employing polarized specular reflection spectroscopy on single crystals have been carried out on a polymorphic cyanine (Sano and Tanaka 1986) dye and a polymorphic acridine (Mizuguchi et al. 1994) dye. As many as seven polymorphic oriented layers of a polymethine dye have been prepared and characterized, the different polymorphs serving as different models for states of aggregation (Dahne and Biller 1998a,( ). 

Kobayashi, M., Matsumoto, Y, Ishida, A., Ute, K. and Hatada, K. (1994). Polymorphic structures and molecular vibrations of linear oligomers of polyoxymethylene studied by polarized infrared and Raman spectra measured on single crystals. Spectrochim. Acta A, 50, 1605-17. [128]... 

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