Influence of chemical exchange on the temperature dependence of Eu(fod)3-induced shifts

Temperature dependences of the paramagnetic shifts induced by Eu(fod)₃ in ¹H NMR spectra of ethylene oxide in carbon disulphide solution are obtained in the temperature range from +40 to ? 100°C at 100 MHz and from +30 to ?60°C at 60 MHz. The influence of chemical exchange leads to a decrease of the observed paramagnetic shifts with decreasing temperature. It is shown that a modified Swift and Connick equation can be used to describe the observed dependences. Upper limits of the mean lifetimes of the Eu(fod)₃-ethylene oxide adduct are τ_p < 1·7 × 10^?8 s at 14 °C and τ_p < 1 × 10^?8 s at 20 °C, respectively. The corresponding activation energy is equal to V_a = 13·7 kcal/mol.     

Reactions of the HO2 radical with OH,H, Fe2+ and Cu2+ at elevated temperatures

The temperature dependence of the rate constant for the reactions of HO₂ with OH, H, Fe²⁺ and Cu²⁺ has been determined using pulse radiolysis technique. The following rate constants, k (dm³ mol⁻¹ s⁻¹) at 20°C and activation energies, E_a (kJ mol⁻¹) have been found. The reaction with OH was studied in the temperature range 20–296°C (k=7.0×10⁹, E_a=7.4) and the reaction with H in the temperature range 5–149°C (k=8.5×10⁹, E_a=17.5). The reaction with Fe²⁺ was studied in the temperature range 16–118°C (k=7.9×10⁵, E_a=36.8) and the reaction with Cu²⁺ in the temperature range 17–211°C (k=1.1×10⁸, E_a=14.9).     

Effects of Pb2+ ions concentration on the structure and PL intensity of Pb-doped ZnAl2O4 nanocrystals synthesized using sol–gel process

Undoped and Pb²⁺-doped ultrafine cubic zinc aluminate (ZnAl₂O₄) hosts were successfully prepared at a relatively low temperature (~80 °C) using the sol–gel method. The concentration of Pb²⁺ was varied from 0 to 5 mol%. The TGA showed that the minimum annealing temperature required to obtain single phase ZnAl₂O₄ must be above 400 °C. The XRD data revealed that all the annealed samples were single phase crystalline structures and the estimated crystallites size were in the range of 21–30 nm in diameter. The FTIR results suggest that heat-treating can destroy some of the bonds. The surface morphology of the phosphors was influenced by the Pb²⁺ mol%. Undoped and Pb²⁺-doped ZnAl₂O₄ nanoparticles exhibit the violet emission at slightly different positions. The slight peak shifts suggests the possibilities that the luminescence centre can either be due to the defects level in the host or Pb²⁺ ions. The emission peaks at 390 and 399 nm are ascribed to the typical UV transitions ³P_0,1 → ¹S₀ in Pb²⁺ ion. At the higher Pb²⁺ mol%, the luminescence quenching behaviour occurs, which suggests that doping with Pb²⁺ ions is accompanied by the introduction of new defect sites that enhance non-radiative recombination of the excited electrons.     

(NH4)3[M2NCl10] (M = Nb,Ta): Synthese,Kristallstruktur und Phasenumwandlung

(NH₄)₃[M₂NCl₁₀] (M = Nb, Ta): Synthesis, Crystal Structure, and Phase Transition The nitrido complexes (NH₄)₃[Nb₂NCl₁₀], and (NH₄)₃[Ta₂NCl₁₀] are obtained in form of moisture-sensitive, tetragonal crystals by the reaction of the corresponding pentachlorides with NH₄Cl at 400 °C in sealed glass ampoules. Both compounds crystallize isotypically in two modifications, a low temperature form with the space group P4/mnc and a high temperature form with space group I4/mmm. In case of (NH₄)₃[Ta₂NCl₁₀] a continuous phase transition occurs between –70 °C and +60 °C. For the niobium compound this phase transition is not yet fully completed at 90 °C. The structure of (NH₄)₃[Nb₂NCl₁₀] was determined at several temperatures between –65 °C und +90 °C to carefully follow the continuous phase transition. For (NH₄)₃[Ta₂NCl₁₀] the structure of the low temperature form was determined at –70 °C, and of the high temperature form at +60 °C. The closely related crystal structures of the two modifications contain NH₄⁺ cations and [M₂NCl₁₀]^3– anions. The anions with the symmetry D_4h are characterized by a symmetrical nitrido bridge M=N=M with distances Nb–N = 184.5(1) pm at –65 °C or 183.8(2) pm at 90 °C, and Ta–N = 184.86(5) pm at –70 °C or 184.57(5) pm at 60 °C.     

Die Polytherme des Quinären Systems Na+, K+, Mg2+/Cl−, SO//H2O im Bereich von +25° bis −10°C

Polythermal Curves of the Quinary System Na⁺, K⁺, Mg²⁺/Cl^?, SO//H₂O in Range between +25°C and ?10°C Proceeding from the 0°C, ?5°C and ?10°C isothermal curves of the quinary system Na⁺, K⁺, Mg²⁺/C1^?, SO//H₂O with saturation at NaCl, KCl, and carnallite, respectively, the polythermal curve is represented between 25°C and ?10°C. Within the new defined range of the polythermal curve the invariant five-salt-paragenesis NaCI, KCI, Glauber's salt (Na₂SO₄ · 10 H₂O), bitter salt (MgSO₄ · 7 H₂O), Schoenite (K₂SO₄ · MgSO₄ · 6 H₂O) can be found at ?7,2°C. It represents also the lowest temperature of formation of Schoenite in this system. It was necessary, moreover, to reconsider further univariant and invariant equilibrium solutions in the range between 25° and 0°C.     

Amperometric determination of xanthine in tea,coffee, and fish meat with graphite rod bound xanthine oxidase

A method is described for construction of an amperometric xanthine biosensor based on graphite rod modified through adsorption of xanthine oxidase. Enzymatically produced H₂O₂ from xanthine was split into 2H⁺ + O₂ + 2^e− at 0.6 V and the current was measured, which was directly proportional to xanthine concentration ranging from 1 ° 10⁻⁷ to 6 ° 10⁻⁷ M with a detection limit of 1 ° 10⁻⁷ M. The biosensor exhibited optimum response within 35 sec at pH 7.0 and 35°C. It was employed for determination of xanthine in tea leaves (0.9 ° 10⁻⁵−2.5 ° 10⁻⁵ mmol/g), coffee powder (3.2 μmol/g) and fish meat (90 mmol/g). The content of xanthine in fish meat increased 6.5 times with its storage at room temperature during 15 days. The enzyme electrode could be reused 200 times during the span of 30 days, when stored in reaction buffer at 4°C.     

Paramagnetic ion binding to amino acids,structural and dynamical information on a Mn(II) complex of L-proline from 13C relaxation data

The ¹³C relaxation times (T₁ and T₂) and isotropic contact shift (Δω) of 1.28 molar aqueous solutions of L-Proline at pH = 11 (or pD = 11.4) containing 10^?4 - 10^?5 M manganese perchlorate are measured at 62.86 MHz over a temperature range of 28–80°C. Under these conditions, the Mn²⁺ cation is bound to three L-Proline molecules in their dibasic form, and a fast exchange is occurring between bound and bulk L-Proline molecules. The longitudinal relaxation of carbons α, β, γ, δ of L-Proline molecules in this complex is shown to be purely dipolar, and is controlled by the rotational reorientation of the complex. The transverse relaxation of bound L-Proline molecules is mainly scalar and is controlled by the electronic relaxation. Overall relaxation rates and paramagnetic shifts also depend on the ligand exchange rate k_M (from bound to free sites) at lower temperatures. The measurement of these quantities allow us to determine (i) the structure of the complex: the Mn(II) cation may be positioned with respect to each proline ligand, the sites of coordination are the unchanged nitrogen and one carboxylic atom, the distance to the Mn²⁺ cation are respectively 2.08 and 1.97 Å; (ii) Hyperfine coupling constants: A= + 0.16; 0.08; 0.25 and 0.22 MHz for carbons α, β, γ, δ, respectively. (iii) Electronic relaxation parameters: assuming that T_1e ( = 2.18 x 10^?8 s at 25°C) is controlled by the modulation of the quadratic crystalline zero-field splitting interaction allows us to estimate the trace of the corresponding tensor: Δ = 0.0305 cm^?1, and a correlation time τν(25°C) = 1.32 ps for the impact of solvent molecules against the Mn²⁺-L-Proline complex (iv) Kinetic parameters for ligand exchange: k_M(25°C) = 7.41 x 10⁴s^?1; ΔH³ = 15.6 kcal.mol.^?1; ΔS³ = 16.1 e.u.     

Nano particles of iron oxides in SiO<Subscript>2</Subscript> glass prepared by ion implantation

Quartz (SiO₂) glass was implanted with 5 × 10^{16 57}Fe ions/cm² at a substrate temperature of 500 °C, and annealed at temperatures between 700 and 950 °C. The implanted and annealed plates were characterized by conversion electron Mössbauer spectroscopy (CEMS), and measured by a Kerr effect magnetometer or a vibration sample magnetometer. Kerr effect measurement of as-implanted SiO₂ glass showed ferromagnetism at room temperature. CEM spectrum of the as-implanted glass consisted of magnetic relaxation peaks of finely dispersed metallic Fe species, and paramagnetic doublets of Fe³⁺ and Fe²⁺ species. The sample heated at 700 °C contained large grains of metallic Fe and a lot of oxidation products of Fe²⁺ species. After oxidation at temperatures higher than 800 °C, the samples showed also ferromagnetism, which was attributed mainly to ferromagnetic ε-Fe₂O₃ precipitated in SiO₂ matrix. Small amounts of α-Fe₂O₃ were produced at 950 °C. The results suggest that ion implantation and oxidation make a transparent ferromagnetic glass possible.     

Mechanical and dielectric absorption of poly(vinyl chloride) and some related polymers

The mechanical and dielectric low temperature absorptions of poly(vinyl chloride) (PVC) and several modified PVC's have been studied over the temperature range from ?60 to +60°C. with some tests extending to ?150°C. and others to +170°C. The results indicate that the low-temperature absorption near ?50°C (β₂ absorption) decreases in intensity with chlorination, while the absorption at a higher temperature near 0°C (β₁ absorption) decreases in intensity with hydrogenation. The apparent activation energies of the β₁ and β₂ absorptions were calculated to be 16 kcal/mole and 10.7 kcal/mole, respectively. Besides, the β₂ absorption markedly decreases in intensity with addition of plasticizer, while the intensity of β₁ absorption is not much affected by increasing plasticizer content. From these results, the β₁ and β₂ processes are concluded to be the results of molecular motion in crystalline and amorphous region in PVC, respectively. For samples of reduced Cl content, another low-temperature absorption was located near ?120°C (γ absorption) and attributed to the presence of short sequences of ethylene units. It has also been observed that the temperature location of the high temperature absorption near 100°C (α absorption) shifts linearly to higher temperature with increasing chlorine content and to lower temperature with increasing hydrogen content.     

Coordination compounds of europium(III), terbium(III), dysprosium(III), samarium(III), and gadolinium(III) with 2-acetylbenzoic acid

LnAcbenz₃ · 3H₂O complexes of Eu³⁺, Tb³⁺, Dy³⁺, Sm³⁺, and Gd³⁺ with 2-acetylbenzoic acid (HAcbenz) have been synthesized. The complexes have been studied by thermogravimetry and infrared and luminescence spectroscopy. According to IR spectroscopy data, the complexation of Acbenz^? with lanthanide ions occurs due to the bidentate coordination of carboxyl groups. According to thermal analysis, the complexes are dehydrated at a temperature above 140°C, and their thermodestruction begins at a temperature above 250°C. From the luminescence spectra measured at 77 and 300 K, it has been established that the integral luminescence intensity of EuAcbenz₃ · 3H₂O and TbAcbenz₃ ° 3H₂O is, respectively, 10 and 19 times higher than for tris-benzoates of the same metals. TbAcbenz₃ ° 3H₂O, the most intensively luminescing complex, is recommended for use as a promising luminescent material.     

Indiumwolframat,In2(WO4)3 – ein In3+ leitender Festkörperelektrolyt

Indium Tungstate, In₂(WO₄)₃ – an In³⁺ Conducting Solid Electrolyte Polycrystalline In₂(WO₄)₃ has been electrochemically characterized and unambiguously identified as an In³⁺ conducting solid electrolyte. By heating, indium tungstate undergoes a phase transition between 250 °C and 260 °C transforming from a monoclinic to an orthorhombic phase for which the conduction properties have been determined. The adopted crystal structure in this high temperature region corresponds to the Sc₂(WO₄)₃ type structure. The electrical conductivity was investigated by impedance spectroscopy in the temperature range 300–700 °C and amounts to about 3.7 · 10^–5 Scm^–1 at 600 °C with a corresponding activation energy of 59.5 kJ/mol. Polarization measurements indicated an exclusive current transport by ionic charge carriers with a transference number of about 0.99. In dc electrolysis experiments, the trivalent In³⁺ cations were undoubtedly identified as mobile species. A current transport by oxide anions was not observed.     

Dilatometric constant for polymerization of α-methylstyrene

By measurement of the specific volume of solutions of poly-α-methylstyrene in α-methylstyrene monomer at 25°C, the dilatometric constant was found to be K_D = (0.002007 ± 0.000030)%^?1. Estimation of the temperature dependence resulted in the equation (K_D)_t = 1.81 × 10^?3 + 7.82 + 10^?6 t, where t denotes temperature in °C.     

CO oxidation with oxygen of the catalyst and gas-phase oxygen over (0.5−15)%СоО/СеО<Subscript>2</Subscript>

The CO adsorption species on Co₃O₄ and (0.5-15%)CoO/CeO₂ catalysts have been investigated by temperature-programmed desorption and IR spectroscopy. At 20°C, the largest amount of CO is adsorbed on the 5%CoO/CeO₂ sample to form, on Co_m²⁺O_n²⁺ clusters, hydrogen-containing, bidentate, and monodentate carbonate complexes, whose decomposition is accompanied by CO₂ desorption at 300 and 450°C (1.1 × 10²⁰ g^–1). The formation of the carbonates is accompanied by the formation of Co⁺ cations and Co⁰, on which carbonyls form. The latter decompose at 20, 90, and 170°C to release CO (2.7 × 10¹⁹ g^–1). Part of the carbonyls oxidizes to CO₂ upon oxygen adsorption, and the CO₂ undergoes desorption at 20°C. Adsorbed oxygen decreases the decomposition temperature of the H-containing and bidentate carbonates from 300 to 100-170°C and maintains the sample in the oxidized state, which is active in subsequent CO adsorption and oxidation. CO oxidation by oxygen of the catalyst diminishes the activity of the sample in these processes and increases the decomposition temperature of the carbonate complexes. Taking into account the properties of the adsorption complexes, we concluded that the H-containing and bidentate carbonates are involved in CO oxidation by oxygen of the catalyst at ~170°C under isothermal conditions. The rate limiting step is the decomposition of the carbonates, a process whose activation energy is 65-74 kJ/mol.     

Ion Association of Cadmium Acetate from Conductivity Measurements

Conductivity of cadmium acetate over the concentration range of 10^?4 to 10^?3M was measured at 25°C. The approximate dissociation constant of CdAc₂ in dilute aqueous solution was estimated from the relation, α=[Λ?36.5+39√C(1+2α)]/[57.9?93√C(1+2α)]. The limiting value of log₁₀K for the association constant of CdAc⁺ ion was evaluated to be 1.75 at 25°C.     

Electrode performance and X-ray absorption spectroscopy of La0.8Sr0.2Mn1−x Cu x O3 (0 ≤ x ≤ 0.2)–GDC composite cathodes for SOFCs

Electrochemical properties of composite cathodes consisting of La_0.8Sr_0.2Mn_1?x Cu _x O₃ (LSMCu, 0?≤?x?≤?0.2) and Ce_0.8Gd_0.2O_2?x (GDC) were determined by impedance spectroscopy, and conduction mechanism for the composite cathodes was investigated by a near-edge X-ray absorption fine-structure analysis (NEXAFS). LSMCu–GDC cathodes showed lower polarization resistance (R _p) than LSM–GDC up to 750 °C, whereas they exhibited better performance at higher temperature (≥800 °C). The best performance was achieved with the LSMCu10–GDC cathode: 0.27 and 0.08?Ω cm² at 800 °C and 850 °C, respectively. NEXAFS and refinement results confirmed that Cu doping caused the oxidation of Mn³⁺ to Mn⁴⁺ and lattice contraction. This additional Mn⁴⁺ can lead to the formation of oxygen vacancies when Mn⁴⁺ is converted to Mn³⁺ at relatively high temperatures (above 600 °C). This in turn contributes to improved oxygen ion transport in LSM. The LSMCu–GDC composite cathode can thus be considered a suitable potential cathode for SOFC applications.     

Synthesis and characterization of Co2+: MgAl2O4 nanocrystal

Nanocrystalline Co²⁺-doped magnesium aluminate spinel (MgAl₂O₄) has been synthesized for the first time from aqueous solution of metal nitrates containing citric acid as chelating agent by a sol-gel method. The gel was heat-treated at temperatures ranging from 710°C to 1000°C. The heated powder samples were characterized by X-ray diffraction analysis (XRD), transmission electron microscope (TEM), infrared (IR) and absorption spectroscopy. The results showed that the homogeneous nanocrystalline Co²⁺: MgAl₂O₄ could be obtained at the low temperature of 710°C. The optimal temperature is about 900°C and the average size of the powder grains is 50 nm or so. In the absorption spectrum, a broad absorption band from 1200 nm to 1600 nm was found, which indicated the existence of Co²⁺ in the tetrahedral sites because of the ⁴A₂(⁴F) → ⁴T(⁴F) transition of Co²⁺.     

Insertion of iron oxide in calciumsilicate gel-derived materials

Samples of the composition of 10Fe₂O₃·10CaO·80SiO₂ were prepared by the sol-gel method and heat-treated in different atmospheres. They were investigated by X-ray diffraction, scanning electron microscopy and Mössbauer spectroscopy. In the heat-treated samples in air iron is present up to 1000 °C in form of hematite and as Fe³⁺ in the tetrahedral sites. A wide range of hematite particle sizes was observed, the average size increased with heating temperature. At 1000 °C wollastonite was observed, at 1200 °C tridymite was formed and all the iron was incorporated in hematite. A heat-treatment at 500 °C under reducing conditions led to poorly crystallized maghemite and at 700 °C to metallic iron and fayalite formation.     

Li6+2x[B10Se18]Sex (x ≈ 2), ein ionenleitendes Doppelsalz

Li_6+2x[B₁₀Se₁₈]Se_x (x ≈ 2), an Ion‐conducting Double Salt Li_6+2x[B₁₀Se₁₈]Se_x (x ≈ 2) was prepared in a solid state reaction from lithium selenide, amorphous boron and selenium in evacuated carbon coated silica tubes at a temperature of 800 °C. Subsequent cooling from 600 °C to 300 °C gave amber colored crystals with the following lattice parameters: space group I2/a (at 173 K); a = 17.411(1) Å, b = 21.900(1) Å, c = 17.820(1) Å, β = 101.6(1)°. The crystal structure contains a well‐defined polymeric selenoborate network of composition ([B₁₀Se₁₆Se_4/2]^6?)_n consisting of a system of edge‐sharing [B₁₀Se₁₆Se_4/2] adamantanoid macro‐tetrahedra forming large channels in which a strongly disorderd system of partial occupied Li⁺ cations and additional disordered Se^2? anions is observed. The crystal structure of the novel selenoborate is isotypic to Li_6+2x[B₁₀S₁₈]S_x (x ≈ 2) [1]. X‐ray and ⁷Li magic‐angle spinning NMR data suggest that the site occupancies of the three crystallographically distinct lithium ions exhibit a significant temperature dependence. The lithium ion mobility has been characterized by detailed temperature dependent NMR lineshape and spin‐lattice relaxation measurements.     

Purification and Biochemical Characterization of a Highly Thermostable Xylanase from <Emphasis Type="Italic">Actinomadura</Emphasis> sp. Strain Cpt20 Isolated from Poultry Compost

An extracellular thermostable xylanase from a newly isolated thermophilic Actinomadura sp. strain Cpt20 was purified and characterized. Based on matrix-assisted laser desorption–ionization time-of-flight mass spectrometry analysis, the purified enzyme is a monomer with a molecular mass of 20,110.13 Da. The 19 residue N-terminal sequence of the enzyme showed 84% homology with those of actinomycete endoxylanases. The optimum pH and temperature values for xylanase activity were pH 10 and 80 °C, respectively. This xylanase was stable within a pH range of 5–10 and up to a temperature of 90 °C. It showed high thermostability at 60 °C for 5 days and half-life times at 90 °C and 100 °C were 2 and 1 h, respectively. The xylanase was specific for xylans, showing higher specific activity on soluble oat-spelt xylan followed by beechwood xylan. This enzyme obeyed the Michaelis–Menten kinetics, with the K _m and k _cat values being 1.55 mg soluble oat-spelt xylan/ml and 388 min⁻¹, respectively. While the xylanase from Actinomadura sp. Cpt20 was activated by Mn²⁺, Ca²⁺, and Cu²⁺, it was, strongly inhibited by Hg²⁺, Zn²⁺, and Ba²⁺. These properties make this enzyme a potential candidate for future use in biotechnological applications particularly in the pulp and paper industry.     

Mechanistic studies of methane partial oxidation to syngas over LiNiLaOx/Al2O3 catalyst

Reduction of vanadium-titanium oxide catalysts with hydrogen in the temperature range of 150–450°C results in the increase of the content of V⁴⁺ ions in substitution positions of TiO₂ with the anatase structure. The temperature increase up to 250°C results in the growth of the spectral intensity of V⁴⁺ associates in substitution positions of anatase. At higher treatment temperatures their intensity decreases due to the formation of VO₂ fragments in anatase. At 400°C and higher temperatures a solid solution of V⁴⁺ ions in rutile is formed.