Enhancement on the hydrothermal stability of ZSM-5 zeolites by the cooperation effect of exchanged lanthanum and phosphoric species

It was explored by density functional calculations that exchanged La or P species exert great influence on the local Al sites as well as on the adjacent exchanged species. In partially exchanged La- or P/H-ZSM-5 zeolite, some of the Al sites will fall off from the zeolite framework even more easily than in H-form ZSM-5, consistent with our XRF experiments. However, when exchanged by both La and P species, Al at either of the two exchanged sites shows better stability compared to H-from. zeolite. La and P species will interact strongly with each other, as evidenced by the charge donation process and the shortening of P-O-1 bond length. It was just the cooperation of La and P species that enabled RSCC catalysts worked normally under severe conditions. (C) 2004 Elsevier B.V. All rights reserved.

band Structure calculations on orthorhombic bulk and nanocrystalline SrY2O4

The electronic structure of SrY2O4 is calculated by using a density functional method, and the exchange and correlation have been treated by using a the generalized gradient approximation (GGA) within the scheme due to Perdew, Burke, and Ernzerhof (PBE). SrY2O4 is predicted to be a direct-gap material because the top of the valence band and the bottom of the conduction band are along the same direction at G. The bond length and the bond covalency are also calculated by using a chemical bond method.

Magnetic structure and orbital ordering in tetragonal and monoclinic KCrF3 from first-principles calculations

KCrF3 has been systematically investigated by using the full-potential linearized augmented plane wave plus local orbital method within the generalized gradient approximation and the local spin density approximation plus the on-site Coulomb repulsion approach. The total energies for ferromagnetic and three different antiferromagnetic configurations are calculated in the high-temperature tetragonal and low-temperature monoclinic phases, respectively.

Effect of polydispersity on the self-assembly structure of diblock copolymers under various confined states: A Monte Carlo study

We studied the self-assembly of polydisperse diblock copolymers under various confined states by Monte Carlo simulation. When the copolymers were confined within two parallel walls, it was found that the ordered strip structures appeared alternately with the increase in wall width. Moreover, the wall width at which the ordered structure appeared tended to increase with an increase in the polydispersity index (PDI). On the other hand, the simulation results showed that the copolymers were likely to form ordered concentric strip structures when they were confined within a circle wall.

Charge, orbital, and magnetic ordering in YBaFe2O5 from first-principles calculations

First principles calculations using the augmented plane wave plus local orbitals method, as implemented in the WIEN2k code, have been used to investigate the electronic and magnetic properties of YBaFe2O5, especially as regards the charge-orbital ordering. Although the total 3d charge disproportion is rather small, an orbital order parameter defined as the difference between t(2g) orbital occupations of Fe2+ and Fe3+ cations is large (0,73) and gives unambiguous evidence for charge and orbital ordering: Strong hybridization between O 2p and Fe e(g) states results in the nearly complete loss of the separation between the total charges at the Fe2+ and Fe3+ atoms.

Magnetic Coupling between Copper(II) Ions Mediated by Hydrogen-Bonded (Neutral) Water Molecules

A new hydrogen-bonded dinuclear copper(II) coordination compound has been synthesized from the Schiff-base ligand 6-(pyridine-2-ylhydrazonomethyl)phenol (Hphp). The molecular structure of [Cu-2(php)(2)(H2O2)(2)(ClO4)](ClO4)- (H2O) (1), determined by single-crystal X-ray diffraction, reveals the presence of two copper(II) centers held together by means of two strong hydrogen bonds, with O center dot O contacts of only 2.60-2.68 angstrom. Temperature-dependent magnetic susceptibility measurements down to 3 K show that the two metal ions are antiferromagnetically coupled (J = -19.8(2) cm(-1)). This exchange is most likely through two hydrogen-bonding pathways, where a coordinated water on the first Cu, donates a H bond to the O atoms of the coordinated php at the other Cu. This strong O center dot H (water) bonding interaction has been clearly evidenced by theoretical calculations. In the relatively few related cases from the literature, this exchange path, mediated by a (neutral) coordinated water molecule, was not recognized.

Crystal structures and elastic properties of superhard IrN2 and IrN3 from first principles

First principles calculations were performed to investigate the structural, elastic, and electronic properties of IrN2 for various space groups: cubic Fm-3m and Pa-3, hexagonal P3(2)21, tetragonal P4(2)/mnm, orthorhombic Pmmn, Pnnm, and Pnn2, and monoclinic P2(1)/c. Our calculation indicates that the P2(1)/c phase with arsenopyrite-type structure is energetically more stable than the other phases. It is semiconducting (the remaining phases are metallic) and contains diatomic N-N with the bond distance of 1.414 A. These characters are consistent with the experimental facts that IrN2 is in lower symmetry and nonmetallic. Our conclusion is also in agreement with the recent theoretical studies that the most stable phase of IrN2 is monoclinic P2(1)/c. The calculated bulk modulus of 373 GPa is also the highest among the considered space groups. It matches the recent theoretical values of 357 GPa within 4.3% and of 402 GPa within 7.8%, but smaller than the experimental value of 428 GPa by 14.7%. Chemical bonding and potential displacive phase transitions are discussed for IrN2. For IrN3, cubic skutterudite structure (Im-3) was assumed.

Structures and elastic properties of OsN2 investigated via first-principles density functional calculations

The structure, elastic, and electronic properties of OsN2 at various space groups: cubic Fm-3m, Pa-3, and orthorhombic Pnnm were studied by first-principles calculations based on density functional theory. Our calculation indicates that the structure in orthorhombic Pnnm phase is energetically more stable compared with cubic systems. It is metallic, mechanically stable and contains diatomic N-N units with the bond distance 1.418 A. These characters are consistent with experimental facts that OsN2 is orthorhombic and metallic. The calculated bulk modulus 394 GPa is also the highest among the considered space groups, slightly larger than previous value 358 GPa. The calculated elastic anisotropic factors and directional bulk modulus showed that OsN2 possess high elastic anisotropy.

First-principles investigation on the elastic, magnetic and electronic properties of MFe3N (M = Fe, Ru, Os)

The elastic, magnetic and electronic properties of MFe3N (M = Fe, Ru, Os) are investigated via first-principles calculations. The calculated results are in agreement with the experimental and other theoretical data. The high ratios of bulk modulus to shear modulus 2.7, 2.0, and 1.8 for gamma'-Fe4N, RuFe3N, and OsFe3N, respectively, indicate that they have good ductility. gamma'-Fe4N possesses the largest B/C-44 (3.41) ratio, which suggests that it is much prone to shearing. The net magnetic moment per formula unit decreases from 9.90 for gamma'-Fe4N, 7.66 for RuFe3N, to 6.80 mu(B) for OsFe3N.

Trends in elasticity and electronic structure of 5d transition metal diborides: first-principles calculations

We investigate the cohesive energy, heat of formation, elastic constant and electronic band structure of transition metal diborides TMB2 (TM = Hf, Ta, W, Re, Os and Ir, Pt) in the Pmmn space group using the ab initio pseudopotential total energy method. Our calculations indicate that there is a relationship between elastic constant and valence electron concentration (VEC): the bulk modulus and shear modulus achieve their maximum when the VEC is in the range of 6.8-7.2. In addition, trends in the elastic constant are well explained in terms of electronic band structure analysis, e.g., occupation of valence electrons in states near the Fermi level, which determines the cohesive energy and elastic properties. The maximum in bulk modulus and shear modulus is attributed to the nearly complete filling of TM d-B p bonding states without filling the antibonding states. On the basis of the observed relationship, we predict that alloying W and Re in the orthorhombic structure OsB2 might be harder than alloying the Ir element. Indeed, the further calculations confirmed this expectation.

Structure and identity of 4,4 '-thiobisbenzenethiol self-assembled monolayers

Self-assembled monolayers (SAMs) of 4,4'-thiobisbenzenethiol (TBBT) can be formed on Au surface spontaneously. The structural characteristics and adsorption behavior of TBBT SAMs on Au have been investigated by surface enhanced Raman scattering (SERS), electrochemical cyclic voltammetry (CV), ac impedance spectroscopy (EIS), and atomic force microscopy (AFM). It is demonstrated that TBBT adsorbed on Au by losing a H atom, forming one Au-S bond, and the other mercapto group is free at the surface of the monolayer owing to the presence of the nu(S-H) at 2513 cm(-1) and the delta(C-S-H) at 910 cm(-1) in SERS. The enhancement of the vibration of C-S (1064 cm(-1)), the aromatic C-H vibration (3044 cm(-1)), and the absence of the vibration of S-S illustrate TBBT adsorbed on Au forming a monolayer with one benzene ring tilted with respect to the Au surface. The interpretation of the observed frequencies is aided by ab initio molecular orbital (MO) calculations at the HF/6-31G* level of theory. Electrochemical CV and EIS indicate TBBT monolayers can passivate the Au effectively for its low ratio of pinhole defects (theta = 99.6%). AFM studies give details about the surface morphology. The applications of TBBT SAMs have been extensively investigated by exposure of Cu2+ ion to TBBT SAMs on Au and covalent adsorption of metal nanoparticles.

Conformational and orientational analyses of 2,2 '-bithiophene under interaction of external electric field constructed by point charges

The fully relaxed single-bond torsional potentials and orientation-related rotational potentials of 2,2'-bithiophene (BT) under the interaction of an external electric field (EF) constructed by point charges have been evaluated with semi-empirical AMI and PM3 calculations. The torsional potentials are sensitive to both EF strength and direction. While the EF is parallel to the molecular long axis, the torsional barrier around C-x-C-x' bond obviously rises with increasing the EF strength, whereas the relative energies of syn and anti minima show a slight change. The interaction between the EF and the induced dipole moment has been proposed to elucidate this observation. On the other hand, the relative energy difference between the syn and anti minima shows an obvious change, while the EF is perpendicular to the molecular long axis. This feature has been ascribed to the interaction between the EF and the permanent dipole moment of BT. Furthermore, conformational and orientational analyses in two dimensions have been carried out by changing the torsional and rotational angles in the different EF. The conformation and orientation of a gas-phase BT in the EF are governed by both the above factors.

X-ray crystal structure and molecular mechanics calculations of (2,6-iso-dipropyl-phenylamide) dimethyl (tetra-methylcyclopenta-dienyl) silane titanium dichloride

Crystal and molecular structure of (2.6-dipropylphenylamide) dimethyl (tetra-methyl cyclopentadienyl) silane titanium dichloride (I) was fully characterized by X-ray diffraction. The crystal is obtained from a mixture of ether/hexane as orthorhombic. with a = 12.658 (3) Angstrom. b = 16.62 (3) Angstrom. c = 11.760 (2) Angstrom. V = 2474.2 (9) Angstrom(3). Z = 4, space group Pnma. R = 0.0399; Componud I compose of the pi-bounded ring with its dimethylsilyl-dipropyl phenyl amido group and the two terminal chloride atoms coordinated to central metal to form a so-called constrained geometry catalyst (CGC) structure. The result of molecular mechanics (MM) calculations on compound I shows that bond lengths and bond angles from the MM calculation are comparable to the data obtained from the X-ray diffraction study. The relation of the structure of CGCs and their catalytic activity by MM calculations is also discussed.