Single-crystal structure analyses phases

Powder X-ray diffraction has verified that the isomerization occurs via a crys-tal-to-crystal reaction process, and that the diffraction profiles of the crystals after photoirradiation consist of overlapped patterns of diffraction due to the crystals of 10 and 11. This indicates that the crystal domains of each isomer exist simultaneously in the crystals accompanied by crystal phase separation during the photoisomerization. Single crystal structure analysis has disclosed that the crystals of 11 as the photoproduct have a symmetry different from that of the starting crystals of 10 (Fig. 14). 

The last method for producing standard patterns for phases not in the PDF is more involved. In many instances single crystals of unknown phases can be removed from reaction mixtures. If this is the case, a full three dimensional crystal structure analysis will yield the positions of all atoms in the structure. Once the crystal structure is known, it can be used to calculate the X-ray powder diffraction pattern for the phase. This powder diffraction information can then be used with confidence as a standard powder pattern. 

Y. Song, P.Y. Zavalij, M. Suzuki, and M.S. Whittingham, New iron(III) phosphate phases Crystal structure, electrochemical and magnetic properties, Inorg. Chem. 41, 5778 (2002). This example is also an excellent illustration of a case where the physical state of a material precludes single crystal diffraction analysis, and a powder diffraction experiment becomes the only option for a solution of its crystal structure. 

Calteridol calcium is used as a chelating excipient in a parenteral formulation and was chosen as an example for two reasons. It contains water associated with a metal cation and thus represents a Class 3 hydrate. In addition, water is also contained in channels, as with the second example. What is unique about the calteridol system is that a single crystal structure has been solved for the tetra-decahydrate (reactant) and the tetra-dihydrate (product) using the same crystal. This procedure permits an interesting look not only into the structural differences but also into the energetic differences of the water environment through thermal analysis. A similar study was performed on the dihydrate and trihydrate phases of di-sodium adenosine 5 -triphosphate [25]. 

From the chemical point of view it seems to be of particular interest that further (metastable) tellurium subhalides can be obtained by hydrothermal syntheses in acid solutions. In the hydrothermal work on the crystal growth of arsenic telluro-iodides a pure binary tellurium subiodide with the composition Tcsl was obtained as well as a ternary phase with only small amounts of arsenic (AsT Ij The composition Tejl was checked by chemical as well as energy-dispersive X-ray analysis. Crystals of the new tellurium subiodide are shown in Fig. 43 a crystal structure analysis failed because the quality of the crystals was not suitable for single crystal investigation. During annealing of the subiodide a stable phase combination a-Tel -H Te was formed which indicated the metastable character of TesL... 

Barbera et al. prepared a series of thallium(I) complexes, in which the position and number of side chains were changed systematically ((58) M = Tl). A minimum of four chains was required to induce mesomorphism in these systems (Table 37), and on the basis of X-ray diffraction, the monotropic mesophases were identified as Colii. The formation of the columns was due to the association of these half-disk compounds into dimers, and to their subsequent stacking. The dimeric nature of these thallium(I) complexes was confirmed by the analysis of the single-crystal structure of the non-mesomorphic dimethoxy analog. In the crystalline phase, the co-planar... 

The most spectacular successes of X-ray methods, however, are in molecular and crystal structure analysis. Examples are the structures of insulin [130], hemoglobin (131], and vitamin B 2 [132]. Today, single-crystal X-ray structure analyses of relatively complex compounds (up to ca. 40-200 nonhydrogen atoms) are performed with computer-controlled diffractometers whose computers can be used simultaneously to solve the phase and structure problem within a few days or just a few hours.. The method has gained parity in investigation time with spectroscopic methods... 

Thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD) of la showed that two third of acetone molecules can be removed at lower temperature of 70 °C and induced rearrangement of host to lb form. Whereas the remaining one third of acetone molecules can be removed at 225 °C to yield a pure apohost phase Ic. Thus, lb exhibits surprisingly remarkable thermal stability for a molecular assembly. Barbour and coworkers obtained single crystal of apohost phase (Ic) by sublimation at 270 °C and the follow-up single crystal diffractirMi studies reveal that the structure of Ic is similar to that of lb. Since the crystals were grown rmder vacuum, the discrete void spaces in Ic remain empty [6]. 

Tho-e are a few problems often encountered in crystal structure analysis of mesogens. First, it is sometimes very difficult to obtain single crystals suitable for crystal structure anafysis, because the anisotropy of the molecules results in very thin plate- or needle-like crystals. Secondly, incompleteness of crystals such as disorder and large thermal motions of the molecules as well as structure modulation are occasionally found in crystals due to rather weak dispersive intermolecular forces. S(Mne of the problems are solved by using strong X-ray sources such as synchrotron radiation and/or very sensitive area detectors at low temperatures [2]. More seriously, relationships between the crystal and mesophase structures are not straightforward due to the first order phase transition with... 

Zareba et al. [165] described the crystal structure of the chiral 4-(l-methyl-heptyloxycarbonyl)-phenyl 4-heptyloxytolane-4 -carboxylate (C7-tolane) which shows monotropic antiferroelectric and ferroelectric phases. The single-crystal X-ray analysis of this compound shows that the crystal has a smectic-like layer structure composed of largely bent molecules where the chain of the chiral group is almost perpendicular (86°) to the core moiety. Within the layers, the molecules are tilted. The central tolane group of the molecule is roughly planar. 

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