Performances crystal structures

Some examples from our recent study of opiate analgesic structures (8,24) serve to illustrate further the types of information which a combined approach can provide. The approach used in this study was 1) accumulate accurate crystal structure results on representative compounds by literature search and performing crystal structure determinations, then 2) develop and verify molecular mechanics potential parameters for use with analgesics, and 3) perform strain energy calculations to find active conformations by comparing different chemical structural types which act at a common receptor. The object of the study was to 1) better define the three-dimensional requirements of opiate analgesics, and 2) better understand the opiate receptor itself by indirectly investigating it. 

C. Rietveld performs crystal structure refinement and quantitative phase analysis using powder diffraction data and the Rietveld method. Combining refinement programs and advanced modeling tools allows a faster route to determining the structures of both inorganic and molecular crystals. An effective way to know the atomic structure is by means of diffraction techniques using neutrons from nuclear reactors and particle accelerators or X-rays from... 

Ozawa et al. have performed crystal structural analyses on these dinuclear copper complexes in the excited state while irradiating the crystal with CW laser [6]. In their experiment, the interatomic distance between the two copper atoms is almost the same, whereas that between the two bromine atoms is shortened in the excited state. The structural change is reported to be about 10 %. In the excited state, the electron in the copper 3d orbital slightly moved outside from the Cu2Br2 unit plane due to MLCT. Thus, the intermolecular distance between the two bromine atoms is shortened due to the expansion of the two copper atomic orbitals. Luminescence can be explained by the electron in the ligand molecular orbital dropping into the copper 3d and bromine 4p orbitals. 

The lattice energies of the single-molecule crystals and co-crystal are assumed to be those for the most stable forms of each. To predict the stability of a given co-crystal it is therefore necessary to perform crystal structure prediction on each molecule independently and on the co-crystal itself. If it is necessary to predict which stoichiometry of co-crystal is the most stable, it is also necessary to perform crystal structure prediction on crystals with each possible stoichiometry. Since the cost and difficulty of a crystal structure prediction calculation increases considerably with the number of independent molecules in the asymmetric unit, this becomes a very hard problem which few have ventured to tackle. The role of crystal structure prediction is to identify the lowest energy structures that are possible for both co-crystal and its single-molecule crystalline components. The prediction of stoichiometry for a co-crystal requires consideration of all dissociation processes available for a given number of molecules in the asymmetric unit, for example ... 

For less charged anions such as halides, anion-duplex complexes are expected to be more challenging to create because host-guest interactions involving halides are weaker than those with sulfate and phosphate. Martin and Gale reported chloride and fluoride complexes, albeit only in the solid state (Figure 9.16) solution phase studies were not performed. Crystal structures of 18 with bromide were also reported. ... 

Among the variety of nitrogen-containing fulvalenes emerging from types 7-14, X-ray structural determinations have been performed on about 20 representative examples. Tire first crystal structure determination was carried out by application of the folding-molecule method on 3,3 -diphenyl-l,l -bi-isoindolylidene 64 (R = FI) (71CB3108). Tire dimeric isoindolenine system... 

MW and MWD are very significant parameters in determining the end use performance of polymers. However, difficulty arises in ascertaining the structural properties relationship, especially for the crystalline polymers, due to the interdependent variables, i.e., crystallinity, orientation, crystal structure, processing conditions, etc., which are influenced by MW and MWD of the material. The presence of chain branches and their distribution in PE cause further complications in establishing this correlation. 

We have performed full-potential calculations on TisSia in its proposed stable crystal structure. The enthalpy of formation obtained from these calculations agrees well with the value deduced from experiment. Due to the low crystal symmetry, the possibility of a more complex bonding character arises. The charge density in this phase differs considerably from that in the hypothetical unstable structure, so two-electron bonds can be excluded in this phase. We have also showed that the opening of a quasigap in the Si DOS has its origin in the Ti-Si interaction. 

According to investigations performed by Pakhomov and Kaidalova [204], the crystal structure of NF NbC consists of infinite chains made up of distorted octahedral ions (NbOF4 ) linked by oxygen atoms. Ammonium cations, NH/, occupy the spaces between the chains, as shown in Fig. 30. The packing of the structural units in the NH4NbOF4 crystal can be described as a CsCl type structure in which CF ions are replaced by Nb02F4 complexes and Cs" ions are replaced by ammonium ions. 

Precipitated K—salt crystals are carefully filtrated and washed so as to separate them from the mother solution. Drying of filtrated K-salt is also a very delicate and important process that must be performed under conditions that avoid hydrolysis of the material. Potassium heptafluorotantalate is sensitive to water, basic compounds and alcohols, especially at elevated temperatures. The main product of K-salt hydrolysis is Marignac s salt. For a long time it was believed that the composition of Marignac s salt is K/Ta Fg. However, X-ray crystal structure analysis and precise chemical analysis of the... 

For a long time the structural classification of the mineral todorokite was uncertain, until Turner and Buseck [4] could demonstrate by HRTEM investigations that the crystal structure of that mineral consists of triple chains of edge-sharing octahedra, which form [3 x 3] tunnels by further corner-sharing. These tunnels are partially filled by Mg2+, Ca2+, Na+, K+, and water (according to the chemical analysis of natural todorokites). In 1988 Post and Bish could perform a Rietveld structure determination from XRD data taken for a sample of natural todorokite [25], This diffraction study confirmed the results of Turner and Buseck. The cations... 

X-ray absorption spectroscopy has been performed on the isolated Rieske protein from bovine heart mitochondrial bc complex 69) as well as on the Rieske-type cluster in Burkholderia cepacia phthalate dioxygenase (PDO) (72). The analysis performed by Powers et al. 69) was significantly hampered by the fact that the presence of two histidine ligands was not fully recognized therefore, only the results obtained with the dioxygenase where the mononuclear iron has been depleted will be considered here. Table VII gives a comparison of the distances obtained from the fit of the EXAFS spectra assuming an idealized Rieske model and of the distances in the crystal structures... 

The crystal structures of four chlorinated derivatives of di-benzo-p-dioxin have been determined by x-ray diffraction from diffractometer data (MoKa radiation). The compounds, their formulae, cell dimensions, space groups, the number of molecules per unit cell, the crystallographic B.-factors, and the number of observed reflections are given. The dioxin crystal structures were performed to provide absolute standards for assignment of isomeric structures and have been of considerable practical use in combination with x-ray powder diffraction analysis. 

Studies performed on CdS [282, 283] have revealed the importance of the microstructure, i.e., crystal structure, crystallite size, and geometrical surface area, in both the control of band structure and the concentration and mobility of charges, in relation to the photocatalytic performance of the photocatalyst. It has been shown also that the solubility product of CdS colloids prepared from acetate buffer aqueous solutions of suitable precursors increases from 7.2x 10 for large particles to about 10 for small (< 2.5 nm) particle colloids, this increase invoking a positive shift on the cathodic corrosion potential [284]. 

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