Home  About us  Contact
 

Anisotropic Temperature Factors (anisotropic + temperature_factor)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

Structural basis for the phase transitions of Cs2HgCl4

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2001
Bagautdin Bagautdinov
The a0× b0× 2c0 twofold superstructure of dicaesium mercury tetrachloride, Cs2HgCl4, at T = 120,K has been determined by single-crystal X-ray diffraction using synchrotron radiation. Lattice parameters were found as a = 9.7105,(2), b = 7.4691 (1), c = 26.8992 (4) Å, and , = 90.368,(1)° with the supercell space group P21/c. Refinements on 1828 observed unique reflections converged to R = 0.053 (wR = 0.057) using anisotropic temperature factors for all atoms. This phase is the stable phase of Cs2HgCl4 below 163,K. A quantitative comparison is made of the distortions of the 2c0 superstructure with the undistorted phase that is stable at room temperature, and with the 3c0 and 5a0 superstructures that are stable at temperatures between 163,K and room temperature. The principal difference between the 2c0 superstructure and all other phases of Cs2HgCl4 is that the Cs cations are displaced away from the centers of their coordination polyhedra in the 2c0 superstructure. The structural basis for the driving force of the series of phase transitions in this compound is found in the variations of the environments of Cs atoms and in the variations of the distortions of the HgCl4 tetrahedra. [source]

Structure, phase transitions and ionic conductivity of K3NdSi6O15·xH2O.

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2000
-K3NdSi6O15·2H2O, its polymorphs
Hydrothermally grown crystals of ,-K3NdSi6O15·2H2O, potassium neodymium silicate, have been studied by single-crystal X-ray methods. The compound crystallizes in space group Pbam, contains four formula units per unit cell and has lattice constants a = 16.008,(2), b = 15.004,(2) and c = 7.2794,(7),Å, giving a calculated density of 2.683,Mg,m,3. Refinement was carried out with 2161 independent structure factors to a residual, R(F), of 0.0528 [wR(F2) = 0.1562] using anisotropic temperature factors for all atoms other than those associated with water molecules. The structure is based on highly corrugated (Si2O52,), layers which can be generated by the condensation of xonotlite-like ribbons, which can, in turn, be generated by the condensation of wollastonite-like chains. The silicate layers are connected by Nd octahedra to form a three-dimensional framework. Potassium ions and water molecules are located in interstitial sites within this framework, in particular, within channels that extend along [001]. Aging of as-grown crystals at room temperature for periods of six months or more results in an ordering phenomenon that causes the length of the c axis to double. In addition, two phase transitions were found to occur upon heating. The high-temperature transformations, investigated by differential scanning calorimetry, thermal gravimetric analysis and high-temperature X-ray diffraction, are reversible, suggesting displacive transformations in which the layers remain intact. Conductivity measurements along all three crystallographic axes showed the conductivity to be greatest along [001] and further suggest that the channels present in the room-temperature structure are preserved at high temperatures so as to serve as pathways for easy ion transport. Ion-exchange experiments revealed that silver can readily be incorporated into the structure. [source]

Structure, phase transitions and ionic conductivity of K3NdSi6O15·xH2O.

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2000

Hydrothermally grown crystals of ,-K3NdSi6O15, potassium neodymium silicate, have been studied by single-crystal X-ray methods. Under appropriate conditions, the compound crystallizes in space group Bb21m and has lattice constants a = 14.370,(2), b = 15.518,(2) and c = 14.265,(2),Å. There are 30 atom sites in the asymmetric unit of the basic structure. With eight formula units per unit cell, the calculated density is 2.798,Mg,m,3. Refinement was carried out to a residual, wR(F2), of 0.1177 [R(F) = 0.0416] using anisotropic temperature factors for all atoms. The structure is based on (Si2O52,), layers, connected by Nd polyhedra to form a three-dimensional framework. Potassium ion sites, some of which are only partially occupied, are located within channels that run between the silicate layers. The silica,neodymia framework of ,-K3NdSi6O15, in particular the linkages formed between the silicate layers and Nd polyhedra, bears some similarities to that of the essentially isocompositional phase ,-K3NdSi6O15·2H2O. In both, the silicate layers are corrugated so as to accommodate a simple cubic array of NdO6 octahedra with lattice constant , 7.5,Å. Furthermore, the Si2O5 layers in ,-K3NdSi6O15 are topologically identical to those of the mineral sazhinite, Na2HCeSi6O15. Although ,-K3NdSi6O15 and sazhinite are not isostructural, the structures of each can be described as slight distortions of a high-symmetry parent structure with space group Pbmm. [source]

Refined structure of bovine carboxypeptidase A at 1.25,Å resolution

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 2 2003
Alexandra Kilshtain-Vardi
The crystal structure of the bovine zinc metalloproteinase carboxypeptidase A (CPA) has been refined to 1.25,Å resolution based on room-temperature X-ray synchrotron data. The significantly improved structure of CPA at this resolution (anisotropic temperature factors, R factor = 10.4%, Rfree = 14.5%) allowed the modelling of conformational disorders of side chains, improved the description of the protein solvent network (375 water molecules) and provided a more accurate picture of the interactions between the active-site zinc and its ligands. The calculation of standard uncertainties in individual atom positions of the refined model of CPA allowed the deduction of the protonation state of some key residues in the active site and confirmed that Glu72 and Glu270 are negatively charged in the resting state of the enzyme at pH 7.5. These results were further validated by theoretical calculations that showed significant reduction of the pKa of these side chains relative to solution values. The distance between the zinc-bound solvent molecule and the metal ion is strongly suggestive of a neutral water molecule and not a hydroxide ion in the resting state of the enzyme. These findings could support both the general acid/general base mechanism, as well as the anhydride mechanism suggested for CPA. [source]

Ultrahigh-resolution structure of high-potential iron,sulfur protein from Thermochromatium tepidum

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 7 2002
Lijun Liu
Crystals of the high-potential iron,sulfur protein (HiPIP) from Thermochromatium tepidum diffract X-rays to 0.80,Å using synchrotron radiation at 100,K. The crystal structure of this HiPIP was refined at this ultrahigh resolution with anisotropic temperature factors for all atoms to conventional crystallographic R factors of 0.092 and 0.101 for Fo > 4,(Fo) and all reflections, respectively. The present structure provides a more precise picture than the previous 1.5,Å structure and allows location of the positions of most H atoms. The structure revealed a partly hydrophobic cavity near the main hydrophobic area and a much larger inter-cluster approach distance (23.454,Å, the c constant of the unit cell) in the crystal packing than other types of HiPIPs. The structural features involved in the electron-transfer reaction of HiPIP are discussed. [source]