bluuebunny/crossref_metadata_2025_split huggingface dataset:
Isotypism and low-dimensional physical properties of CuAs2/3H2/3O3 (theoparacelsite), the spin-Peierls phase CuGeO3 and CuSiO3
Authors: Otto H. H. , Wolfram H. 
Published: December 2003

Abstract
Theoparacelsite is a recently discovered orthorhombic copper arsenate hydroxide mineral with the ideal chemical formula Cu3As2O7(OH)2, but because of disorder only two smaller formula units CuAs2/3H2/3O3 are established in the orthorhombic unit cell with lattice parameters a = 4.6644(5) Å, b = 8.3212(8) Å and c = 2.9377(3) Å, and space group Pbmm. We show that this rare mineral is iso-typic to the unique inorganic spin-Peierls phase CuGeO3 (TSP
= 14.3 K) and to the recently discovered copper polysilicate, CuSiO3, which is also a quasi-one-dimensional S = 1/2 anti-ferromagnetic chain compound (TN
= 7.9 K). We discuss in detail the previously reported crystallographic data of the phases at room temperature in view of its ability to attain similar low-dimensional physical properties and conclude that the arsenate with its S = 1/2 spin chains of edge-sharing CuO4+2 octahedra may be a further inorganic compound that could show the spin-Peierls effect. Although the Cu-O(2)-Cu angle, which is considered to be important for the magnetic interaction and ground state properties, has been found for the arsenate (98.8°) almost identical to that of the germanate (99.1°), the modified inter-chain coupling of the arsenate as a defect variant of the CuGeO3 structure type may suppress the spin-Peierls effect and favour other states of magnetic order (chiral spin chains, AF order). With respect to the rarity of the mineral some remarks on synthesis routes are made.
Natural and synthetic compounds with kröhnkite-type chains: review and classification
Authors: Fleck Michel , Kolitsch U. , Hertweck B. 
Published: December 2003

Abstract
The crystal structure of kröhnkite [Na2Cu(SO4)2·2H2O], which contains infinite chains composed of CuO6 octahedra corner-linked with SO4 tetrahedra, was originally determined 1952 by Dahlman. Since then, a large number of both minerals (e.g., collinsite, fairfieldite, etc.) and synthetic compounds with closely related struc-tures have been investigated, but the structural relation was not always recognised. In the present review we compare the structures of AnM(XO4)2·2H2O (A = mono- or divalent cation, n = 1, 2; M = di- or trivalent cation; X = penta- or hexavalent cation) compounds with kröhnkite-type infinite chains, and propose a structural classification into six types. The rod group symmetries of the chains are compared. Furthermore, the linkage of these chains to double chains (in krausite-type compounds) and sheets (in yavapaiite-, merwinite-, bafertisite-type and similar structures) is discussed, and the occurrence of octahedral-tetrahedral chains topologically identical to those in kröhnkite (referred to as kröhnkite-type) and similar chains (referred to as kröhnkite-like) in several related compounds is pointed out.
Thermal behaviour of the rigid boron-oxygen groups in the α-Na2B8O13 crystal structure
Authors: Bubnova R. S. , Shepelev Ju. F. , Sennova N. A. , Filatov S. K. 
Published: December 2003

Abstract
The crystal structure of the α-Na2B8O13 stable modification has been refined by single crystal X-ray diffraction at 293, 573 and 773 K. The compound is monoclinic, P21/a space group, unit-cell parameters at 273 K are: a=6.507(3), b=17.796(8), c=8.377(5) Å, β=96.60(4)°, Z=4. The structure contains two interlocking boron-oxygen frameworks, each of them consisting of alternating single and double rings composed of two triangles and a tetrahedron so called triborate and pentaborate groups. Two non-equivalent sodium atoms linking two frameworks are coordinated by 8 oxygens. They are connected into finite chains consisting of four NaO8 polyhedra by sharing oxygen edges. The α-Na2B8O13 thermal expansion investigated by high temperature powder X-ray diffraction has sharply anisotropic character including negative thermal expansion: the coefficients of a thermal expansion tensor in temperature region 273–1000 K are α11 = 55–0.042·(273+t), α22 = 11, α33 = -15 + 0.032·(273+t) (×10–6 K–1), μ = (c^α33) = 42°. That high anisotropy is caused by anisotropic thermal vibrations of the Na atoms. Under heating a high stability of the B-O rigid groups and a weak contraction of some B-O bond lengths were revealed. In response to anisotropic thermal behaviour of Na atoms rigid B-O groups of the anion framework rotate relative to each other as a three-dimensional hinge.
Crystal structure of Pb10O7(OH)2F2(SO4) and crystal chemistry of lead oxysulfate minerals and inorganic compounds
Authors: Krivovichev Sergey V. , Burns Peter C. 
Published: December 2003

Abstract
The crystal structure of Pb10O7(OH)2F2(SO4) (triclinic, P1̅, a = 9.6637(16), b = 10.1922(17), c = 11.1407(18) Å, α = 110.978(3), β = 91.795(3), γ = 107.020(3)°, V = 968.2(3) Å3) has been solved by direct methods and refined to R1 = 0.050 (wR = 0.089). There are ten symmetrically unique Pb2+ cations in the structure all of which have highly asymmetric coordination polyhedra owing to presence of stereoactive 6s
2 lone pairs of electrons. The structure is based upon [O7Pb10] zigzag chains of edge-sharing OPb4 oxocentered tetrahedra. The observed [O7Pb10] chain is the most complex chain of oxocentered tetrahedra known so far and was not observed previously in structures of inorganic compounds. The chains run along [011] and their plane is parallel to (111). The chains form sheets parallel to (111). The SO4) tetrahedra, F– anions and OH groups are between the sheets providing their linkage into three-dimensional structure. A comparison of the structure with other Pb oxysulfates is given in the framework of theory of complexes of anion-centered tetrahedra.
Lattice parameters of La3Ga5SiO14 as a function of hydrostatic pressure
Authors: Werner S. , Maximov B. , Schulz H. , Molchanov V. , Vigdorchik A. , Pisarevskii Yu. 
Published: December 2003

Abstract
Langasite (La3Ga5SiO14) crystallizes in the polar space group P321. Here we report the pressure dependence of the lattice parameters at high hydrostatic pressures up to 10.2 GPa. The results presented in terms of compressibility agree well with results from measurements of the elastic constants. According to a Birch-Murnaghan equation of state, the bulk modulus and its pressure derivative are 134.6(2) and 2.8(1) GPa, respectively. The compression of langasite crystals is anisotropic, the c/a-ratio appears to be a linear function of the pressure applied according to c/a = 0.62460(6) + 0.00055(1)* p/GPa. Altogether four single crystals have been investigated. Two of them showed a splitting of reflections (hhl). This splitting reaches a maximum at approximately 9 GPa and proved to be reversible.
Comparative topological analysis of simple anhydrous borates, carbonates and nitrates
Authors: Blatov V. A. , Zakutkin Yu A. 
Published: December 2003

Abstract
Topological analysis and comparing of the crystal structures of anhydrous inorganic salts containing triangular oxo-anions (borates, carbonates and nitrates) was performed using the program package for multipurpose crystallochemical analysis TOPOS. The examples of topological similarities between these classes of substances and 2920 framework binary compounds AyXz were revealed and discussed. It is shown that in most cases the ionic arrays constructed according to one of close packings or to a b.c.c. motif can be selected in the crystal structures of anhydrous inorganic salts. The conclusions were made about the structure-forming role of atoms and atomic groups in these substances. The crystal structures of anhydrous salts are substantiated to be correctly considered as packings of metal cations and quasi-spherical oxo-anions.
Crystal structures and phase transitions of (NO)2TiCl6
Authors: Henke Henning 
Published: December 2003

Abstract
Solid (NO)2TiCl6 undergoes two phase transitions closely related to the order-disorder behaviour of the NO+ species present in this salt-like compound. Starting from ambient conditions the sequence of space groups is P4/mnc → C2/c → P21/n when the temperature is lowered. At Tc

1 = 199 K the formation of quadruplet domains is observed, while at Tc

2 = 176 K superstructure reflexions appear due to decentring of the unit cell. In the tetragonal high-temperature form the orientational disorder of the nitrosonium groups is fourfold, partial order then starts to develop in the intermediate phase, whereas almost complete order is reached at 123 K in the second monoclinic phase. This two-step symmetry reduction was studied by DSC, powder diffraction, and by single-crystal measurements including complete structure determinations at 291, 203 and 123 K. In P21/n the selection of just one NO+ orientation per site is coupled to a characteristic displacement pattern of the titanium atoms. For electrostatic reasons the latter show a response to short N···Cl contacts by an off-centre shift (ca. 11pm) inside their 'octahedral' chlorine cages. The stereochemical influence of antibonding NO+ orbitals determines the inclination of the cation towards the nearest chlorine neighbours.
A structural study of polymorphism in phenyl salicylate: determination of the crystal structure of a meta-stable phase from X-ray powder diffraction data using a direct space systematic search method
Authors: Hammond R. B. , Jones M. J. , Roberts K. J. , Kutzke H. , Klapper H. 
Published: December 2003

AbstractIn this paper we report the crystal structure solution of a meta-stable phase of phenyl salicylate (salol). This modification, which is obtained from a super-cooled melt, has been observed before, but the structure has not been previously determined. Structure solution was achieved by a procedure combining a computer based systematic search method with a subsequent Rietveld refinement of experimental X-ray powder diffraction data. A clustering procedure to group similar trial structures based upon energy minimisations is discussed. The best trial structure identified by the combination of a systematic search and clustering was refined using the Rietveld method and a final structure with a weighted pattern indexRwpof 0.0513 (excluding background) was obtained. The meta-stable phase of salol crystallises with a monoclinic unit cell with cell parametersa= 17.4047(8) Å,b= 5.9800(5) Å,c= 10.6882(7) Å, and β = 108.988(5)º. The space group isP21/nwith four molecules in the unit cell. A structural comparison of the stable and meta-stable modifications is given.
A new case of polymorphism in platinum(II) complexes: [PtBr2(2,9-dimethyl-1,10-phenanthroline)]
Authors: Capitelli F. , Margiotta N. , Moliterni A. G. G. , Natile G. 
Published: December 2003

Abstract
A novel case of polymorphism in platinum(II) complexes is reported. Single crystals of [PtBr2(2,9-dimethyl-1,10-phenanthroline)] were obtained at room temperature from a dilute dichloromethane/pentane (1:3, v/v) solution of the title compound in the presence of guanosine. The space group resulted to be Pbca [a = 20.4024(3), b = 20.3638(3), and c = 13.9522(2) Å, V = 5796.7(1) Å3, Z = 8]. Crystals of the same compound, obtained in slightly different conditions, had already been investigated by X-ray diffraction and found to have P21/n space group, [a = 12.155(4), b = 7.933(2), and c = 14.522(2) Å, β = 93.74(2)°, V = 1397.3(6) Å3, Z = 4]. Comparison of the two polymorphous forms reveals huge differences in the crystal packing and the important role of the stacking interactions between phenanthroline ligands.
The Absorption Spectrum of C4
Authors: Jungen M. , Xu R. 
Published: December 2003

Abstract
New quantum chemical calculations for the optical valence transitions of C4 have been carried out for both vertical excitation energies up to 6 eV and oscillator strengths. For some of the transitions, including the only one predicted to be in the visible spectrum, we found strong dependence of the transition moment on the molecular geometry, leading to inaccurate predictions for the transition probabilities if a local value is used instead of an appropriate average. It is proposed to calibrate the absorbance of C4 using an electronic excitation in the near infrared.
Analysis of the Quality of Kohn–Sham Orbitals for Subsequent MRSD-CI Calculations of Excitation Energies
Authors: Hupp T. , Engels B. , Della Sala F. , Görling A. 
Published: December 2003

Abstract
In the present work we study the properties of Kohn–Sham orbitals used in subsequent CI methods for the computation of electronic excitation energies in terms of orbital relaxation, static and dynamical correlation effects. As model systems, we use the water and the ethene molecule. As sets of orbitals, we use orbitals from an effective exact exchange Kohn–Sham method, the localized Hartree–Fock (LHF) method, as well as BLYP and HF orbitals. Our study shows that the use of LHF orbitals leads to negligible static correlation effects and thus facilitates assignments of excitation spectra, but it also shows that the remaining correlation contributions have to be taken into account carefully since error cancellation is only found in some fortitious cases. The comparison of LHF and BLYP orbitals shows that the BLYP orbitals are less suited for the description of the cationic system and as a consequence for the computation of Rydberg states.
Linear Centrosymmetric SiC5Si: Floppy or Not?
Authors: Botschwina P. 
Published: December 2003

Abstract
Linear SiC5Si has been studied by the coupled cluster variant CCSD(T) and a large basis set of 393 contracted Gaussian-type orbitals. The full cubic force field was calculated, from which a variety of spectroscopic constants was obtained. The calculations indicate that linear SiC5Si behaves like a fairly normal semirigid molecule. In particular, excitation of any of its vibrations by one vibrational quantum leads to changes in the rotational constant not exceeding 0.3%.
Quantum Chemical Calculation of Raman Intensities for Large Molecules: The Photoisomerization of [{Fe‘S4’(PR3)}2(N2H2)] (‘S4’2− = 1,2-bis(2-Mercaptophenylthio)-Ethane(2−))
Authors: Reiher M. , Neugebauer J. , Hess B. A. 
Published: December 2003

Abstract
Since Raman spectra of isolated large molecules are hardly accessible by present day quantum chemical methods, we demonstrate how Raman intensities for selected vibrational normal modes can be calculated by extension of a semi-numerical, very flexible approach to the calculation of vibrational spectra [see J. Neugebauer et al., J. Comp. Chem. 23 (2002) 895–910]. The methodology is applied to a Ci
-symmetric dinuclear diazene iron–sulfur complex with relevance to the biological nitrogen fixation problem. The accuracy of the vibrational spectra allows us to use the spectra for structure determination and assignment of bands to isomeric structures, which were conjectured to be generated by a photoisomerization process. We can in turn support the photoisomerization hypothesis.
Synthesis and Antibacterial Activity of 1β‐Methyl‐2‐(5‐substituted oxadiazolo pyrrolidin‐3‐yl‐thio)carbapenem Derivatives
Authors: Oh Chang‐Hyun , Dong Hyun‐Gu , Lee Joo‐Shin , Lee Su‐Chul , Hong Joon Hee , Cho Jung‐Hyuck 
Published: December 2003

AbstractSynthesis of a new series of 1β‐methylcarbapenems with a substituted oxadiazolopyrrolidine moiety is described. Their in vitro antibacterial activities against both Gram‐positive and Gram‐negative bacteria were tested and the effect of the substituent on the oxadiazole ring investigated. In particular, compounds 13a and 13c with ester and carbamoyl substituted oxadiazole moieties showed the most potent antibacterial activity.
1, 2‐Dihydroisoquinoline‐N‐Acetic Acid Derivatives as New Carriers for Specific Brain Delivery I: Synthesis and Estimation of Oxidation Kinetics Using Multivariate Calibration Method
Authors: Mahmoud Sahar , Aboul‐Fadl Tarek , Sheha Mahmoud , Farag Hassan , Mouhamed Abdel‐Maaboud I. 
Published: December 2003

AbstractIn order to overcome the slow oxidation of 1, 2‐dihydro‐N‐alkylisoquinoline, 1, 2‐dihydroisoquinoline‐N‐acetic acid derivatives (3b—d) were designed and synthesized as new chemical delivery systems (CDS) for the brain. Molecular orbital calculations for the suggested derivatives revealed that these carriers are stable against oxidation. However, hydrolysis to their corresponding anions will accelerate the rate of oxidation, and accordingly increase the efficiency of brain specific delivery. A multivariate calibration method for in vitro determination of the oxidation rate for the suggested brain specific chemical delivery system is described. The method is based on measurement of individual rates of oxidation of prepared 1, 2‐dihydroisoquinolines, using silver ions to provide the corresponding quaternary forms. Components of binary mixtures formed through the oxidation step (composed of the dihydro‐compound and its quaternary form), showed a considerable degree of spectral overlapping — more than 90% in all cases. Resolution of these binary mixtures under investigation has been accomplished mainly using classical least squares analysis. Kinetic oxidation data of the tested compounds revealed that these new CDSs have a reasonable oxidation rate for efficient brain delivery.
The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis
Authors: Rubio Mercedes , Merchán Manuela , Pou‐Amérigo Rosendo , Ortí Enrique 
Published: December 2003

AbstractThe low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 11Bu and 21Bu, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 23Ag and 23Bu, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.
In‐Plane Structures of Synthetic Oligolactose Lipid Monolayers—Impact of Saccharide Chain Length
Authors: Tanaka Motomu , Schneider Matthias F. , Brezesinski Gerald 
Published: December 2003

AbstractGrazing incidence X‐ray diffraction (GIXD) was used to investigate the in‐plane structure of monolayers of synthetic glycolipids with oligolactose head groups (Lac N, with N=number of lactose units) at the air/water interface. The Lac 1 monolayer exhibits three sharp diffraction peaks. One peak can be deconvoluted into two diffraction peaks, which suggests that alkyl chains in the monolayer form an orthorhombic lattice. On the other hand, the other two peaks are related to bulk crystalline materials residing at the interface. Another weak peak from the head group correlation, either in a monolayer or in bulk crystals, can also be observed in some experiments. The scattering patterns of the Lac 2 monolayer indicate that the alkyl chains order in an orthorhombic lattice, where a shift in the hydrophilic/hydrophobic balance seems to fluidize the film. Alkyl chains in the Lac 3 monolayer also seem to assume an orthorhombic lattice; however, a weak diffraction peak from the correlation between trilactose head groups can also be observed. The estimated lattice dimensions are compared systematically to those of bulk dispersions, as well as to the viscoelastic properties of the monolayer. The obtained results strongly suggest that the in‐plane structure of synthetic glycolipid monolayers can provide a well‐defined basis to understand the impact of the chemical structure on the cooperativity and function of the glycocalix of cellular surfaces.
Trends in Microfluidics with Complex Fluids
Authors: Pfohl Thomas , Mugele Frieder , Seemann Ralf , Herminghaus Stephan 
Published: December 2003

AbstractThe rapid developments in biotechnology create a great demand for fluid handling systems on the nano‐ and picoliter scale. The characterization of minute quantities of DNA or protein samples requires highly integrated, automated, and miniaturized “total analysis systems” (μ‐TAS). The small scales necessitate new concepts for devices both from a technological and from a fundamental physical point of view. Here, we describe recent trends in both areas. New technologies include soft lithography, chemical, and topographical structuring of surfaces in order to define pathways for liquids, as well as electrowetting for manipulation purposes. Fundamentally, the interplay between geometric confinement and the size of biological macromolecules gives rise to complex dynamic behavior. The combination of both fluorescence imaging and scattering techniques allows for detailed insight into the dynamics of individual molecules and into their self‐assembly into supramolecular aggregates.
Generation of Iron Atoms in the Gas Phase by Microwave‐Induced Plasma Afterglows
Authors: Vinckier Chris , Cappan Kevin 
Published: December 2003

AbstractA fast‐flow reactor technique is described by which Fe atoms can be produced in the gas phase in the afterglow of microwave‐induced plasmas in hydrogen/argon and hydrogen/helium mixtures. When the iron salt FeCl3(s) was brought into the gas phase by thermal sublimation at temperatures between 360 and 405 K, it was partly converted to Fe atoms by reaction of the gaseous compounds FexCl3x(g) with hydrogen atoms. The Fe atoms were detected by atomic absorption spectroscopy (AAS). It was shown that sublimation of the salt is the rate‐determining step of the overall plasma‐afterglow atomisation process. Experimental conditions for the generation of Fe atoms suited to kinetic studies start at a temperature of 303 K. In the downstream region the concentration of Fe atoms decays due to diffusion to the reactor wall. Binary diffusion coefficients DFe/Ar and DFe/He of 231.5±6.6 and 370.0±15.5 cm2 s−1 Torr at 303 K, respectively, were determined.
Single‐Walled Carbon Nanotube Based Molecular Switch Tunnel Junctions
Authors: Diehl Michael R. , Steuerman David W. , Tseng Hsian‐Rong , Vignon Scott A. , Star Alexander , Celestre Paul C. , Stoddart J. Fraser , Heath James R. 
Published: December 2003

AbstractThis article describes two‐terminal molecular switch tunnel junctions (MSTJs) which incorporate a semiconducting, single‐walled carbon nanotube (SWNT) as the bottom electrode. The nanotube interacts noncovalently with a monolayer of bistable, nondegenerate [2]catenane tetracations, self‐organized by their supporting amphiphilic dimyristoylphosphatidyl anions which shield the mechanically switchable tetracations from a two‐micrometer wide metallic top electrode. The resulting 0.002 μm2 area tunnel junction addresses a nanometer wide row of ≈2000 molecules. Active and remnant current–voltage measurements demonstrated that these devices can be reconfigurably switched and repeatedly cycled between high and low current states under ambient conditions. Control compounds, including a degenerate [2]catenane, were explored in support of the mechanical origin of the switching signature. These SWNT‐based MSTJs operate like previously reported silicon‐based MSTJs, but differently from similar devices incorporating bottom metal electrodes. The relevance of these results with respect to the choice of electrode materials for molecular electronics devices is discussed.