The MCD spectrum of matrix-isolated osmium tetroxide

The MCD spectrum of OsO₄ in argon and nitrogen matrices at ≈ 20 K has been measured and is remarkably similar to the first band of MnO₄⁻ in KIO₄ at 4.2 K. From this it is concluded that there are two electronic origins in the region 30 000–39 000 cm⁻¹ and the spectrum of OsO₄ is reassigned accordingly.     

Crystal structure and magnetic properties of Sr4Mn2NiO9

The crystal structure of Sr₄Mn₂NiO₉ has been refined on single crystal. This phase belongs to the series A_1+x(A^′_xB_1–x)O₃ (x=1/3) related to the 2H-hexagonal perovskite. The structure contains transition metals in chains of oxide polyhedra (trigonal prisms and octahedra); neighboring chains are separated from each other by the Sr atoms. The sequence of the face sharing polyhedra along the chains is two octahedra + one trigonal prism. Mn occupies the octahedra and Ni is disordered in the trigonal prism with ≈80% in the pseudo square faces of the prism and ≈20% at the centre. This result has been confirmed by XANES experiments at Mn K and Ni K edges, respectively. Sr₄Mn₂NiO₉ is antiferromagnetic with a Néel temperature at T=3 K. The Curie constant measured at high temperature is in good agreement with ≈80% of the Ni²⁺ ions in the spin state configuration S=0.     

Equilibrium constant for the reaction Xe + 2Ar XeAr + Ar in the temperature range 150–300 k and the dissociation energy of XeAr

The equilibrium Xe + 2Ar has been investigated in the temperature range 150–300 K using a selected ion flow tube appratus. From the temperature variation of the equilibrium constant the standrad enthalpy change for the reaction is determined to be −25 ± 5 kj mol⁻¹ and the dissociation energy of XeAr⁺ is estimated to be 24 ±5 kj mol⁻¹ (0.25 ± 0.05 eV). At ≈ 150 K the approach to equilibrium is consistent with a rate coefficent of (5 ± 3) × 10⁻²¹ cm ⁶ s⁻¹ for the forward three-body association reaction.     

Oxygen reduction in acid media at the amorphous Mo–Os–Se carbonyl cluster coated glassy carbon electrodes

An amorphous Mo–Os–Se carbonyl cluster compound has been synthesized in 1,2-dichlorobenzene (b.p.≈180°C) to be tested as an electrocatalyst for molecular oxygen reduction in 0.5 M H₂SO₄. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) performed for the powder supported on pyrolytic carbon show a distribution of nanometer-scale amorphous particles with agglomerations in cluster forms. The catalytic activity was studied by the rotating disc electrode technique. Kinetic studies show a first-order reaction with a Tafel slope of −0.118 V dec⁻¹ and dα/dT=1.55×10⁻³ K⁻¹. In the temperature range 298–343 K, an activation energy of 32 kJ mol⁻¹ was determined.     

Photoelectrochemical behavior of coupled SnO2|CdSe nanocrystalline semiconductor films

A photoelectrochemical cell with a coupled SnO₂|CdSe nanocrystalline semiconductor electrode has been prepared by sequential deposition of SnO₂ and CdSe films onto an optically transparent electrode (OTE), and its photoelectrochemical behavior has been studied. The results show that the coupling of CdSe with SnO₂ leads to an improvement in the performance of OTE|SnO₂|CdSe over OTE|CdSe cells in terms of increased incident photon-to-current conversion efficiency, increased stability and smaller reversal of current. The favorable positioning of the energy bands of SnO₂ and CdSe is responsible for the above observations. Various photoelectrochemical parameters of the OTE|SnO₂|CdSe cell obtained for an incident light power of 0.31 mW cm⁻² at 470nm, are as follows: I_sc ≈ 25–30 μA cm⁻², V_oc ≈ 0.5–0.6 V, ƒƒ = 0.47 and a power conversion efficiency of about 2.25%.     

Magnetic and EPR Studies of α-, β-, and γ-Fe2WO6 Phases at Low Temperatures

The polymorphic modifications α-, β-, and γ-Fe₂WO₆ of the iron tungstate system were studied by means of magnetic susceptibility and EPR measurements at low temperatures. Both methods revealed a significant paramagnetic contribution, probably resulting from local distortions of the antiferromagnetic bulk structure induced by a disturbed cation ordering or the presence of Fe²⁺ ions. The magnetic susceptibility revealed a peak at 260 K for all samples which can be related with an AF phase transition. The EPR spectra comprised the contribution of various isolated paramagnetic iron centers, one arising from high-spin Fe³⁺ ions in rhombic crystal field symmetry with E/D ≈ 1/3 and D ≈ 0.22 cm^-1, an anisotropic EPR signal consistent with an S= 3/2 ground state with large zero-field splitting, and a dominant component in the g ≈ 2 region presumably arising from an S = 1/2; spin state. The latter spectra were tentatively attributed to the formation of multi-iron clusters, one of them invoking the presence of Fe²⁺ ions as well. For the βFe₂WO₆ phase an additional EPR spectrum was observed, which probably results from high-spin Fe³⁺ ions in a weak crystal field.     

The impedance behavior of the cell Au/HClO4 · 5.5 H2O/Au in the temperature range 4.2–300 K

The impedance of the cell Au/HClO₄-5.5 H₂O/Au was investigated in the frequency range 1 to 10⁵ Hz between 4.2 and 300 K. The analysis of the data enables an evaluation of important electrolyte properties such as conductivity and dielectric constant in a wide range of temperatures, predominantly in the solid state of the electrolyte HClO₄-5.5 H₂O (T_f = 228 K). The double layer capacity of the gold electrodes was also determined; it shows a qualitatively similar result compared with previous measurements. In the solid state, the ionic conductivity exhibits two distinct activation energies of 0.37 and 0.54 eV corresponding to the two phases present in HClO₄-5.5 H₂O above and below 170 K. Below 120 K the activation energy becomes very small and tends to zero around 80 K indicating possible tunneling processes in the rigid H₂O structure. At about the same temperature the dielectric constant reaches its low temperature limit with a value _∞ ≈ 11 which is considerably higher than the value of pure ice of _∞ ≈ 3.     

Conductometric Determination of the Stability Constants of the Inclusion Complexes of Alkali Cations with [22-DD] Diaza Crown Ether, [211] and [221] Cryptands in Acetonitrile

An equilibrium study concerning the association of Na⁺, K⁺, Rb⁺ and Cs⁺ with 4, 7, 13, 18-tetraoxa-1,10-diazabicyclo [8, 5, 5]-eicosane [211], 4, 7, 13, 16, 21-pentaoxa-1, 10-diazabicyclo [8, 8, 5]-tricosane [221] and 4, 13-didecyl-1, 7, 10, 16-tetraoxa-4, 13-diazacyclooctadecane [22-DD] in acetonitrile has been carried out at 25 °C by using a conductometric technique. The observed molar conductivity, Λ, of a test solution was found to decrease significantly for mole ratios less than 1:1 upon the addition of the complexing ligand. A model based on 1:1 stoichiometry has been used to analyze the conductivity data. The data have been fitted according to a non-linear least-squares analysis that provides the stability constant, K, and the molar conductivity, Λ_c, for each cation – ligand inclusion complex. The binding sequences were found to follow the order: Na⁺ > K⁺ > Rb⁺ ≫ Cs⁺ (K ≈ 0) for [211], Na⁺ > K⁺ > Rb⁺ > Cs⁺ for [221] and K⁺ > Na⁺ > Rb⁺ > Cs⁺ for [22-DD] complexes. Trends in ionic conductivities of complexed ions are also discussed.     

Schwingungsspektren und normalkoordinatenanalyse von CF3,-verbindungen : XXVI. Molekülstrukturen und schwingungsspektren von α-CF3HgN3 und CF3HgNCO

The crystal structures of the isostructural compounds α-CF₃HgN₃ (I) and CF₃HgNCO (II) were determined by X-ray methods. Centrosymmetric dimers are formed with weak Hg---n′ bonds of 2.74(2) Å in I and 2.88(2) Å in II. The Hg---N distance and Hg---N---N angle are 2.02(2) Å and 127(2)° in the monomeric fragment of I, the corresponding values in II are 2.03(2) Å and 137(2)°. Raman and, in part, IR spectra of both compounds in the solid state were recorded and assigned for Cs and C_2h symmetry of the monomers and dimers, respectively. A normal coordinate analysis proved that the stretching vibrations of the (HgN)₂ four-membered ring appeared at ≈400 and ≈75 cm^-1; the force constants amounted to ≈2.0 for the short and to ≈0.25 X 10² N/m for the long HgN bonds.     

An atomic boron laser - pumping by incomplete autoionization, or ion-electron recombination

Pulsed lasing was obtained from discharges in mixtures of He (≈ 10 torr) and a number of boron containing species (≈ 25 mtorr). Discharge conditions for optimizing the output are briefly described. Laser emission appears at 2777 ± 1.6 cm⁻¹, with 20–50 μs delay after passage of the discharge pulse (≈ 1 μs half width). The emitters are boron atoms. Among the term values for B I and B II only one pair of levels for B I have a separation close enough to the observed frequency to be considered. One possible mechanism is based on excitation to a metastable level above the first ionization limit. A distant collision may induce transition to captured-electron states at the limit (incomplete autoionization), which constitute the upper lasing levels; lasing them occurs by induced transitions to the lower level, 7s, ²S_1/2, which is rapidly depleted. Another possibility is the capture of an electron by B⁺ into a very large Rydberg orbit, at the ionization limit, which are the upper lasing states.     

Further evidence for formation of xenon dihydride from neutral hydrogen atoms: a comparison of ESR and IR spectroscopic results

A comparative study of the decay of hydrogen atoms and formation of xenon dihydride upon annealing electron-irradiated xenon/hydrocarbon mixtures revealed a remarkable correlation between these two processes. The addition of an electron scavenger had a similar effect on the yield of trapped H atoms resulting from irradiation at 15 K and the formation of XeH₂ upon subsequent annealing. The results suggest that ≈70% of the initially trapped H atoms react at 40 K to yield XeH₂ under the experimental conditions. The contribution of hydrogen abstraction from alkane molecules is less important and the recovery of the parent alkane molecule due to H^·+R^· combination is negligible.     

Triterpene glycosides ofAstragalus and their genins. XLVIII. The structures of cycloalpigenin B and cycloalpioside B

The structures of the new cycloartane methylsteroid cycloalpigenin B and its glycoside cycloalpioside B, isolated fromAstragalus alopecurus Pall. (Leguminosae) have been determined on the basis of chemical transformations with the assistance of¹H and¹³C NMR spectroscopy and 2D NMR¹H-¹H and¹H-¹³C correlations of chemical signals and IR, CD, and electron-impact mass spectrometry. Cycloalpigenin B is 20R,24S-epoxycycloartane-3,12,16,25-tetraol. A transition from cycloalpigenin B to cycloalpigenin A has been achieved. Cycloalpioside B is 20R,24S-epoxycycloartane-3,12,16,25-tetraol 3-O--D-xylopyranoside.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, FAX (3712) 89 14 75. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 515–521, July–August, 1994.     

Structure and Magnetism of Pr1−xSrxFeO3−δ

Crystal structure, redox, and magnetic properties for the Pr_1−xSr_xFeO_3−δ solid-solution phase have been studied. Oxidized samples (prepared in air at 900°C) crystallize in the GdFeO₃-type structure for 0≤x≤0.80, and probably in the Sr₈Fe₈O₂₃-type (unpublished) structure for x=0.90. Reduced samples (containing virtually only Fe³⁺) crystallize as the perovskite aristotype for x=0.50 and 0.67 with randomly distributed vacancies. The Fe⁴⁺ content increases linearly in the oxidized samples up to x≈0.70, whereupon it stabilizes at around 55%. Antiferromagnetic ordering of the G type is observed for oxidized samples (0≤x≤0.90) which show decreasing Néel temperature and ordered magnetic moment with increasing x, while the Néel temperature is nearly constant at 700 K for reduced samples. Electronic transitions for iron from an average-valence state via charge-separated to disproportionated states are proposed from anomalies in magnetic susceptibility curves in the temperature ranges 500–600 K and 150–185 K.     

Ionic Transport in Dilute Solid Polymer Electrolytes with Amorphous Structure

The ionic structure and transport properties of amorphous solid polymer electrolytes in the system copolymer of acrylonitrile and butadiene (40 : 60)—lithium hexafluoroarsenate (SPE) is studied in the region of small salt concentrations (up to 0.37 mol dm⁻³) at 298–368 K. In conditions studied, LiAsF₆ is dissociated predominantly to ions. Macroscopic models of ion transport are used to analyze the results of measurements of transport characteristics of SPE. Transport of anions free of the polymer matrix is realized activationlessly and resembles the Stokes drift in viscous media. Transport of cations solvated by electron-donating groups of the polymer turns possible only at temperatures in excess of a critical value (T _crit ≈ 333 K), when the statistical mean of molecules in the first coordination sphere of the lithium cation becomes less than four (which is the coordination number for solvation) and requires the overcoming of an energy barrier of ∼6 kJ mol⁻¹. Below T _crit, the SPE are unipolar anionic conductors.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 537–545.Original Russian Text Copyright © 2005 by Bushkova, Sofronova, Lirova, Zhukovskii.     

Structure Determination of Sr1.25Bi0.75O3 and Sr0.4K0.6BiO3 as a Function of Temperature from Synchrotron X-Ray Powder Diffraction Data

The structures of Sr_1.25Bi_0.75O₃ and superconducting Sr_0.4K_0.6BiO₃ have been determined from synchrotron X-ray powder diffraction data between 4 K and the decomposition temperature, at 973 and 573 K, respectively. The symmetry remains monoclinic (a≈ a_p, b≈ a_p, c≈2a_p, β≈90°, P2₁/n space group with two Bi sites allowing charge localization) for the undoped compound, and tetragonal (a≈ a_p, c≈2a_p, I4/mcm space group with a unique Bi site implying a charge delocalization) for the K-doped phase, over the whole temperature range. In both cases the distortion from cubic symmetry decreases as temperature increases. Above 400 K, Sr_0.4K_0.6BiO₃ progressively loses oxygen until it reaches the Sr_0.4K_0.6BiO_2.5 stoichiometry, after which it decomposes.     

Copper(II) complexes with derivatives of salen and tetrahydrosalen: a spectroscopic, electrochemical and structural study

Salen type complexes, CuL, the corresponding tetrahydrosalen type complexes, Cu[H₄]L, and N,N′-dimethylated tetrahydrosalen type complexes, Cu[H₂Me₂]L, were investigated using cyclic voltammetry, and electronic and ESR spectroscopy. In addition, the analogous copper(II) complexes with a derivative of the tetradentate ligand ‘salphen’ [salphen=H₂salphen=N,N′-disalicylidene-1,2-diaminobenzene] were studied. Solutions of CuL, Cu[H₄]L and Cu[H₂Me₂]L are air-stable at ambient temperature, except for the complex Cu(^tBu, Me)[H₄]salphen [H₂(^tBu, Me)[H₄]salphen=N,N′-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-1,2-diaminobenzene]. Cu(^tBu, Me)[H₄]salphen interacts with dioxygen and the ligand is oxidatively dehydrogenated (–CH₂–NH–→–C=N–) to form Cu(^tBu, Me)[H₂]salphen and finally, in the presence of base, Cu(^tBu, Me)salphen. X-ray structure analysis of Cu(^tBu, Me)[H₂Me₂]salen confirms a slightly tetrahedrally distorted planar geometry of the CuN₂O₂ coordination core. The complexes were subjected to spectrophotometric titration with pyridine, to determine the equilibrium constants for adduct formation. It was found that the metal center in the complexes studied is only of weak Lewis acidity. In dichlormethane, the oxidation Cu(II)/Cu(III) is quasireversible for the CuL type complexes, but irreversible for the Cu[H₄]L and Cu[H₂Me₂]L type. A poorly defined wave was observed for the irreversible reduction Cu(II)/Cu(I) at potentials less than −1.0 V. The ESR spectra of CuL at both 77 K and room temperature reveal that very well resolved lines can be attributed to the interaction of an unpaired electron spin with the copper nuclear spin, ¹⁴N donor nuclei and to a distant interaction with two equivalent protons [A^Cu(iso)≈253 MHz, A^N(iso)≈43 MHz, A^N(iso)≈20 MHz]. These protons are attached to the carbon atoms adjacent to the ¹⁴N nuclei. In contrast to CuL, the number of lines in the spectra of the complexes Cu[H₄]L and Cu[H₂Me₂]L is greatly reduced. At room temperature, only a quintet with a considerably smaller nitrogen shf splitting constant [A^N(iso)≈27 MHz] is observed. Both factors, planarity and conjugation, are thus essential for the observation of distant hydrogen shf splitting in CuL. Due to the C=N bond hydrogenation, the coordination polyhedra of the complexes Cu[H₄]L and Cu[H₂Me₂]L is more flexible and more sensitive to ligand modification than that of CuL. The electron-withdrawing effect of the phenyl ring of the phenylenediamine bridge is reflected in a reduction of the copper hyperfine coupling constants in Cu(^tBu, Me)[H₄]salphen and Cu(^tBu, Me)[H₂Me₂]salphen complexes [A^Cu(iso)≈215 MHz].     

Crystal optics for hard-X-ray spectroscopy of highly charged ions

A twin crystal-spectrometer assembly, operated in the focusing compensated asymmetric Laue geometry has been developed for accurate spectroscopy of fast highly charged heavy ions in the hard-X-ray region. Coupled to the focusing crystal optics is a specially developed two-dimensional position-sensitive X-ray detector which is necessary for retaining spectral resolution also for fast moving sources. We summarize the crystal optics and demonstrate the usefulness of the instrument for spectroscopy of both stationary and fast moving X-ray sources. Results are reported from several tests employing a ¹⁶⁹Yb gamma-ray source and the Lyman radiation of one-electron Pb⁸¹⁺ ions traveling at a velocity corresponding to β ≡ v/c ≈ 0.59. The features of the instrument presented may be useful in many applications where it appears difficult to make the leap from conventional X-ray energy measurements to wavelength-dispersive spectroscopy based on crystal optics.     

Bemerkungen zur flammenphotometrischen Bestimmung von Spurenelementen

Zusammenfassung Es wurde der Einfluß von organischen Lösungsmitteln auf die flammenphotometrische Emission untersucht und festgestellt, daß neben der spezifischen Oberfläche auch die Verdampfungswärme eine Rolle spielt.Eine Methode unter Benutzung von 3 Wellenlängen wurde zur Bestimmung von 10^–4–10^–5 % K bzw. Na in Na- bzw. Li-Salzen ausgearbeitet.

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Summary The effect of organic solvents on the emission in flame photometry has been investigated and has been found to depend on the specific surface of the atomized substance and also on the heat of evaporation.A new method has been worked out for the determination of 10^–4 to 10^–5 % K in sodium salts and K and Na in lithium salts, respectively. It is based on the evaluation of three different wave-lengths.

     

The defect solid solution Na7/8(Fe7/8+xTi9/8−2xSbx)O4 (0 ≤ x ≤ 1/3): Evidence of Na mobility in the tunnels of a quadruple rutile-chain structure

A new defect solid solution, the series Na_7/8(Fe^III_7/8+xTi^IV_9/8−2xSb^V_x)O₄, was synthesized. Its homogeneity range is rather wide: 0 <- x ≤ 0.33. The incorporation of Sb^V gives rise to a progressive increase of the parameters of the orthorhombic unit cell. X-ray powder structure calculations point to a partial occupancy of the large double tunnels in a quadruple rutile-chain structure. A significant ordering of cations over the octahedral framework is observed, owing to a Ti^IV---Sb^V segregation. Electrical measurements emphasize a cationic conductivity, mainly related to a 1D motion of Na^I cations. A transition from a low activation energy process—E_A ≤ 0.20 eV—to a high activation energy one—E_A ≈ 0.75 eV—systematically occurs at T ≈ 440°C, independent of the Sb^V concentration. A possible skew motion from a half tunnel to another one is proposed as a tentative explanation of the high-temperature conductivity mechanism.     

Selective separation of gold from iron ore samples using ion exchange resin

Gold in iron ore samples is separated from iron (major matrix cation), antimony and vanadium using anion exchange resin in (0.2 M) HBr, potassium peroxodisulfate and acetone:water:nitric acid media. The exchangeable anion Cl⁻ of the ion exchanger Dowex 1X 4 is replaced by Br⁻ using (6 M) HBr solution. Certified reference material DGP-M1, spiked ferric magnetic oxide, gold radioactive tracer ¹⁹⁸Au and gold standard solutions are used to study the adsorption efficiency and the yield recovery of tetrabromoaurate AuBr₄⁻ from the resin. Ten eluents have been tried to elute gold from the column, and it has been found that a 10 ml potassium peroxodisulfate and 240 ml acetone:water:nitric acid [125:5:5] solution fulfills the objective. The set up of the separation procedure allows quantitative adsorption of gold by the resin, while the major matrix cation (Fe) and others (Cd, Ag, Cu, V, Sb, Ti) have been passed through the column with the feeding solution (0.2 M) HBr. The resin selectivity coefficient (K) of separating Au from Fe has been found to be K_Fe^Au≈6.4×10¹¹. The eluted Au is treated with K₂S₂O₈ and H₂O₂ for spectrophotometric determination as rhodamine-B complex at 555.6 nm. The linearity, detection limit, precision, and accuracy of the determination method have been found to be up to 2.0 μg g⁻¹, 0.018 μg g⁻¹, 0.009 μg g⁻¹ and 3%, respectively.