Crystal structure of the pyrochlore oxide superconductor KOs2O6

We report the single-crystal X-ray analysis of the structure of the pyrochlore oxide superconductor KOs₂O₆. The structure was identified as the β-pyrochlore structure with space group and lattice constant at : the K atom is located at the 8b site, not at the 16d site as in conventional pyrochlore oxides. We found an anomalously large atomic displacement parameter at for the K cation, which suggests that the K cation weakly bound to an oversized Os₁₂O₁₈ cage exhibits intensive rattling, as recently observed for clathrate compounds. The rattling of A cations is a common feature in the series of β-pyrochlore oxide superconductors AOs₂O₆ (A=Cs, Rb and K), and is greatest for the smallest K cation.     

Crystal structures and cation ordering of Sr2AlSbO6 and Sr2CoSbO6

The two double perovskite oxides Sr₂AlSbO₆ and Sr₂CoSbO₆ were prepared and their structures studied with the X-ray powder diffraction method. At room temperature the crystal structure of Sr₂AlSbO₆ is cubic , with . It was found that depending on the preparation conditions, the Al³⁺ and Sb⁵⁺ cations can be either entirely or partially ordered. In the case of the partially ordered Sr₂AlSbO₆ sample, the extension of cation ordering was estimated from the -dependent broadening of the diffraction peaks and the results were interpreted as evidence of the formation of anti-phase domains in the material. Low-temperature Raman spectroscopic measurements demonstrated that the cubic phase of Sr₂AlSbO₆ is stable down to 79 K.The room-temperature crystal structure of Sr₂CoSbO₆ is trigonal (space group with and . At 470 K, however, the material undergoes a continuous phase transition and its structure is converted to cubic (space group . The studied Sr₂CoSbO₆ sample was partially ordered, but unlike Sr₂AlSbO₆, no indication of the formation of anti-phase domains was observed.     

An automatic gravimetric titrator and its use for the potentiometric determination of uranium by the Davies and Gray method

Summary Construction and mode of operation of an automatic gravimetric titrator are described. Center of the system is a Titroprint E 475 which was modified on a few points. The titrant is added by use of two automatically controllable motor burettes as a main volume, and then in steps of 0.01 ml. The losses of weight of the reservoir vessel are correspondingly recorded by an electronic balance. The measured data are obtained on a punched tape and allow evaluations of the analysis as gravimetric and volumetric titration by use of two Basic computer programs. When the system was used for the potentiometric determination of uranium a relative standard deviation of 0.48 (gravimetric and volumetric) and relative deviations from the nominal value of –0.41 (gravimetric) and –0.79 (volumetric) were obtained as mean values for 14 groups of 5–6 samples.

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Ein automatischer gravimetrischer Titrator und seine Anwendung für die potentiometrische Bestimmung von Uran nach Davies und Gray

Zusammenfassung Es werden Aufbau und Betriebsweise eines automatischen gravimetrischen Titrators beschrieben, dessen Hauptkomponente ein in mehreren Punkten modifizierter Titroprint E 475 ist. Das Titrationsmittel wird mit zwei elektronisch steuerbaren Motorbüretten grob- bzw. feindosiert, wobei eine elektronische Waage die entsprechenden Gewichtsverluste des Reservoirgefäßes registriert. Die auf Lochstreifen ausgegebenen Meßdaten lassen Auswertungen der Analyse als gravimetrische und volumetrische Titration über entsprechende Basic-Computerprogramme zu. Bei Benutzung des Systems für die potentiometrische Bestimmung von Uran wurden für Gruppen von 5–6 Proben mittlere relative Standardabweichungen von 0,48 (gravimetrisch und volumetrisch) sowie mittlere relative Abweichungen vom Sollwert von –0,41 (gravimetrisch) bzw. –0,79 (volumetrisch) erhalten.

     

Synthesis, Structure, and Fluorescence of Isomeric Indolizinediones. Carbonyl-Bridged Isodipyrrinones

Summary. In one-pot reactions, pyrrole-- and -aldehydes condense readily with 4-ethyl-3-methyl-3-pyrrolin-2-one to give isodipyrrinone analogs, which undergo intramolecular cyclization when the pyrrolealdehyde possesses an or -CO₂R group. The resulting regioisomeric pyrroloindolizinediones, with structures confirmed by NMR analysis, exhibit strong fluorescence, with quantum yields (_F) as high as 0.91 at _em450–550nm.     

Growth of high quality and large-sized Rb0.3MoO3 single crystals by molten salt electrolysis method

High quality and large-sized Rb_0.3MoO₃ single crystals were synthesized by molten salt electrolysis method. X-ray diffraction (XRD) patterns and rocking curves, as well as the white beam Laue diffraction of X-ray images show the crystals grown by this method have high quality. The lattice constants evaluated from XRD patterns are a₀=1.87 nm, b₀=0.75 nm, c₀=1.00 nm, β=118.83^°. The in situ selected area electron diffraction (SAED) patterns along the , and zone axes at room temperature indicate that the Rb_0.3MoO₃ crystal possess perfect C-centered symmetry. Temperature dependence of the resistivity shows this compound undergoes a metal to semiconductor transition at 183 K.     

Historical Aspects of the Chemical Bond. Chemical Relationality <Emphasis Type="Italic">versus</Emphasis> Physical Objectivity

Summary. Chemists have always defined the properties of materials on the basis of the changes observed when they reacted with other substances. Whereas this approach led chemists to relational concepts such as affinity, acid, and oxidant, physicists made measurements of objects they considered unchanged, determining their mass, charge, dipole moment, etc. In the middle of the 18th century, the Jesuit Josip Ruder Bocovi started thinking about the way in which atoms might be present in crystals, introducing a new concept according to which atoms in condensed phases represented punctual centres of attracting and repelling forces. Josef Loschmidt became aware of these ideas through the philosopher J. F. Herbart, and it is to Loschmidt that we owe the first representation of bonds as atomic spheres penetrating one another. The term quantum chemistry was first used by the physicist Arthur Erich Haas, who was born in Brno. However, Hans Hellmann (1903–1938) was the true founder of quantum chemistry. Hellmann, who was shot in Moscow in 1938, was the first person to realize the quantum-physical effect that leads to the chemical bond. In the 1960s K. Ruedenberg and others took theory a stage further when – thanks to the concept of the localized electron pair – they realized that the different approaches to the phenomenon of the chemical bond taken by chemists and physicists were largely comparable. This made it possible to bring the differing standpoints largely into line with one another.     

Eine praxisorientierte Bestimmung der Nachweisgrenze

Zusammenfassung Am Beispiel der Spurenanalyse von Quecksilber in biologischem Material wird eine Methode zur Bestimmung der Nachweisgrenze vorgeschlagen, die die Nachweisgrenze als geringsten, mit einer bestimmten Präzision meßbaren Gehalt auffaßt und unter Anwendung einfacher statistischer Kalküleals solche ermittelt.

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A practice-orientated determination of the limit of detection

Summary Illustrated by mercury trace analysis in biological material, a method for determining the limit of detection is proposed. It interpretes limit of detection as lowest content measurable with a certain precision and ascertains itas such applying simple statistical calculus.

     

A Spectroscopic Study of Charge Transfer Complexes of Tetraphenylporphyrin

Summary. The fluorescence quenching and complexation behaviour of tetraphenylporphyrin (TPP) with some organic acceptors such as chloranilic acid (CHL), 5,5-dithiobis-2-nitrobenzoic acid (DTNB), or 3,4-dinitrobenzoic acid (DNB) and tetravalent metal ions such as Th⁴⁺ and Zr⁴⁺ have been studied in methanol. The second-order fluorescence quenching rate constant (k_q), the association constant (K), the molar absorption coefficient (), and the thermodynamic parameters of the complexation process (G°, H°, and S°) have been evaluated using different organic solvents.     

Boron-chelate assisted synthesis of new bipyrazole derivatives

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New Cyclic Dimers of Cholic Acid

Summary. Two new cyclic dimers of cholic acid were obtained in the reaction of 3-O-acetyl methyl cholate with oxalyl chloride. The oxalates bound the cholate subunits side-to-side as a result of acylation of 7 and 12 OH groups in the substrate. The selective deprotection of hydroxy groups at C-3 and C-24 proved to be rather difficult and led to various products depending on the reaction conditions.     

Hypericin and Pseudohypericin. Purity Criteria and Quantitative Determination in Extracts of Saint John’s Wort (Hypericum perforatum)

Summary. Hypericin and pseudohypericin in extracts of St. Johns Wort (Hypericum perforatum) are assayed using UV-Vis, measuring the absorbance at 588nm of a methanolic solution of the compounds. The value of the absorption coefficient is difficult to define insofar as none of the routine purity criteria can be applied to these two compounds. Analysis of the entire UV-Vis spectrum from 220 to 740nm would seem to be a reliable way of comparing different samples. A UV-Vis spectrum was recorded for each of the compounds using a Photodiode array detection system. A more accurate way of determining the levels of hypericin and pseudohypericin in extracts of St. Johns Wort using these reference spectra is described.     

Pentafluoroperbenzoic acid as the efficient reagent for Baeyer–Villiger oxidation of cyclic ketones

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A new synthetic route to chiral 3-aryl-5-ethyl-1,4,2-oxazaphosphorines

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Unusual examples of methane hydrate nucleation in bilayer water–oil systems

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Equilibrium Studies of Binary and Ternary Complexes Involving Tricine and Some Selected α-Amino Acids

Summary. The formation equilibria for the binary complexes of Co^II, Ni^II, Cu^II, Zn^II, Cd^II, Mn^II, Pb^II, Th^IV, UO₂^II, and Ce^III with tricine and for the ternary complexes involving some -amino acids (glycine, -alanine, proline, serine, asparagine, and aspartic acid) were investigated using pH-metric technique. The formation of binary and ternary complexes was inferred from the pH-metric titration curves. It was deduced that tricine acts as a primary ligand in the ternary complexes involving the monocarboxylic amino acids (glycine, -alanine, proline, serine, and asparagine), whereas it behaves as a secondary ligand in the ternary systems containing the dicarboxylic aspartic acid. The ternary complex formation was found to take place in a stepwise manner. The stability constants of the complexes formed in aqueous solutions were determined potentiometrically under the experimental conditions (t=25°C, I=0.1moldm^–3 NaNO₃). The order of stability of the ternary complexes in terms of the nature of the amino acids is investigated and discussed. The values of log K for the ternary complexes have been evaluated and discussed. Evaluation of the effects of ionic strength and temperature of the medium on the stability of the ternary system M^II-tricine--alanine (M^II=Co^II, Ni^II, and Cu^II) has been studied. The thermodynamic parameters were calculated and discussed.     

Structural evolution in iron tellurates

Three new members within the iron tellurate family have been synthesized and characterized by single-crystal diffraction. Fe₂Te₃O₉ is orthorhombic, , , , Z=4, space group Pnma, final agreement factors R₁=0.0261(wR₂=0.0688) for 1271 independent reflections. Fe₃Te₄O₁₂ is monoclinic, , , , β=107.950(10)^°, Z=4, space group P2₁/c, final agreement factors R₁=0.0380(wR₂=0.0281) for 3302 independent reflections. FeTe₆O₁₃ is trigonal, , , Z=6, space group , final agreement factors R₁=0.0309(wR₂=0.0641) for 1264 independent reflections. Together with the four already known members of the family, Fe₂TeO₅, Fe₂TeO₆, and Fe₂Te₃O₉ (a dimorphic variant of the afore-mentioned structure with the same chemical formula), and Fe₂Te₄O₁₁, the iron tellurates now span from relatively Fe-rich and Te-poor to relatively Fe-poor to Te-rich compounds. The structural diversity within Fe-Te-O system is discussed in terms of the lone-pair stereochemistry of the Te⁴⁺ anion and the cross-over from Fe³⁺ to mixed-valence Fe³⁺/Fe²⁺ and Fe²⁺ coordination polyhedra compounds.     

Exciton Chirality Method. Oriented Molecules – Anisotropic Phases

Summary. The results of the theory of the anisotropy of optical activity (ACD), especially of the circular dichroism (CD), in view of its application to the exciton chirality method was presented. In order to introduce the phenomenology some typical experimental anisotropy effects of the ACD of a dibenzoate and two taddoles were discussed. The CD and ACD of unbridged and bridged 1,1-binaphthols were described taking into account results of the polarized spectroscopy. Their CD and ACD spectra were decomposed into contributions of their two exciton transitions. It was proven that the electric dipole/electric quadrupole transition moments contribute in same order of magnitude as the electric dipole/magnetic dipole transition moments to the tensor coordinates of the CD tensor for a bridged 1,1-binaphthol. The CD tensor coordinates _ii^* for a light beam propagation along the principal axes of the order tensor of a 1,1-binaphthol of approximately C₂ symmetry are very different in size and also of different sign. The largest tensor coordinate belongs to the direction along the naphthyl–naphthyl bond. The CD along the C₂ symmetry axis is approximately zero. The CD tensor coordinates of different sign along different directions within a molecule can be assigned to different helicities in their molecular structure along these directions. For (R)-1,1-binaphthol skeleton the left handed helix along an axis, which is perpendicular to the naphthyl–naphthyl bond and perpendicular to the C₂ symmetry axis, leads to a positive couplet whereas for the right-handed helix along the naphthyl–naphthyl bond a negative couplet has been found. Thus, the ACD with its determination of the _ii^* allows to observe different helicities along different directions within a molecule. As well for the bridged as the unbridged binaphthols in the spectral region of the exciton bands a third transition not belonging to the exciton band system was detected.     

The distribution of degree of crystallinity—New concept in polymer science

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Ring-opened 4-hydroxy-δ-valerolactone subunit as a key structural fragment of polyesters that degrade without acid formation

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Hybrid membranes based on short side chain perfluorinated sulfonic acid membranes (Inion) and heteropoly acid salts

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