Synthesis and structure of bilayered manganites based on Ca3Mn2O7 with the simultaneous substitution of calcium by lanthanum and strontium

A tetragonal (space group I4/mmm) solid solution La_2-2x (Ca_1-y Sr_y)_{1 + 2x} Mn₂O₇ based on the Raddlesden-Popper phase (n = 2), which is formed by the simultaneous substitution of calcium in Ca₃Mn₂O₇ by strontium and lanthanum, is synthesized by high-temperature annealing of La₂O₃, Mn₂O₃, CaCO₃, and SrCO₃ mixtures (1500°C, air). The concentration area of the solid solution in the scheme is a pentagon, whose corners correspond to the manganites Ca₃Mn₂O₇, Ca_0.75Sr_2.25Mn₂O₇, La_0.2Sr_2.8Mn₂O₇, La_1.6Sr_1.4Mn₂O₇, and LaCa₂Mn₂O₇.     

Tandem oxidation processes for the regioselective preparation of 5-substituted and 6-substituted 1,2,4-triazines

α-Hydroxyketones undergo MnO₂-mediated oxidation, followed by in situ trapping with 2-pyridylamidrazone, to give 3-pyridyl-5-substituted 1,2,4-triazines in a one-pot procedure, which avoids the need to isolate the reactive α-ketoaldehyde intermediates. By modifying this procedure to allow condensation prior to oxidation, the corresponding 6-substituted 1,2,4-triazines were obtained. The preparation of a novel unsymmetrical 2,2′-bipyridine using one of the pyridyl 1,2,4-triazines prepared herein is also described.     

New sesquiterpenoid and triterpenoids from the fruits of Rhizophora mucronata

A new sesquiterpene (1) and two new pentacyclic triterpenoid esters (2, 3) together with three known compounds (4-6) were isolated from the fruits of Rhizophora mucronata. Their structures were elucidated by analysis of their spectroscopic data. The new compounds were characterized as 3-hydroxy-3,7,11-trimethyl-9-oxododeca-1,10-diene (mucronatone, 1), 3beta-E-caffeoyltaraxerol (2) and 3beta-Z-caffeoyltaraxerol (3).     

Phase formation in the Ag2O-ZnO (CdO)-V2O5 systems

The phase composition of the Ag₂O-ZnO(CdO)-V₂O₅ systems has been studied. Two new orthovanadates have been synthesized: AgZnVO₄ (monoclinic space group P21/n, a = 5.68710, b = 12.54080, c = 5.65947 Å, β = 116.209°) and AgCd₄(VO₄)₃ (orthorhombic space group Pnma, a = 9.82438 b = 7.01250, c = 5.37393 Å). One double metavanadate Ag₂Cd(VO₃)₄ has been synthesized. A continuous solid solution formulated as Ag_{3 ? 2x} Cd_{3 + x} (VO₄)₃ has been found to exist between AgCd₄(VO₄)₃ and already described AgCdVO₄. The Ag₂O-ZnO (CdO)-V₂O₅ systems have been triangulated in the subsolidus region.     

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%.     

Characterization of the Photosynthetic Activity of Platinized Chloroplasts: A Study Using Fluorescence and Photoacoustic Techniques

In this report, the effect of platinization on the photosynthetic activity of the chloroplast membranes is studied. Oxygen evolution, fluorescence emission and thermal de-activation processes are modified after platinization. It is shown that photosystem II activity is affected by the hydrogen purging involved in the platinization procedure as seen by the reduced rates of oxygen evolution and a decrease in variable fluorescence. Depletion of bicarbonate from photosystem II during purging is suggested to be responsible partly for the decreased electron transfer rates and for a lower half-saturation light intensity required for energy storage as measured by photoacoustic spectroscopy. On the other hand, the electron sink created by the reduction of hydrogen at the acceptor side of photosystem I is shown to reoxidize efficiently the plas-toquinone pool of photosystem II.     

Antimalarial and cytotoxic activities of pregnene-type steroidal alkaloids from Holarrhena pubescens roots

The phytochemical investigation of an alkaloidal extract of Holarrhena pubescens roots led to the isolation and identification of a new pregnene-type alkaloid, mokluangin D (1), together with nine known steroidal alkaloids (2–10). The structure of the new metabolite was determined on the basis of spectroscopic analyses including 1D- and 2D-NMR spectroscopy and mass spectrometry. Compounds 3 and 4 showed potent antimalarial activity against Plasmodium falciparum K1 stain with IC₅₀ values of 1.2 and 2.0 μM, respectively, and showed weak cytotoxic activity against the NCI-H187 cell line with IC₅₀ values of 27.7 and 30.6 μM, respectively. The substituent groups at C-3 and the carbonyl group at C-18 are important for the activity against the P. falciparum K1 stain.     

Crystal structure and luminescence properties of the barium europium tetragermanate Ba2Eu2Ge4O13

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Hexametavanadates M<Stack><Subscript>4</Subscript><Superscript>+</Superscript></Stack>M<Superscript>2+</Superscript>(VO<Subscript>3</Subscript>)<Subscript>6</Subscript>: Thermal stability and luminescent characteristics

Rhombohedral hexametavanadates K₄Sr(VO₃)₆, K₄Ba(VO₃)₆, Rb₄ Ba(VO₃)₆, and Cs₄Ba(VO₃)₆ melt incongruently in the temperature range of 491 to 600°C. Cooling of peritectic melts yields mixtures of compounds typical of M₂⁺O-M²⁺O-V₂O₅ systems, far from equilibrium and depending on the cooling kinetics. The vanadate Cs₄Ba(VO₃)₆ undergoes reversible polymorphic transformation at 360°C. All compounds show broad-band luminescence with a maximum of the luminescence spectrum at 490–590 nm with three types of excitation. The vanadates K₄Sr(VO₃)₆ and Rb₄Ba(VO₃)₆ show the highest luminescence intensity at room temperature. The latter is also most efficient at liquid nitrogen temperatures. The luminescence spectra depend on the excitation of vanadates. Three hypotheses were put forward to interpret this finding. The nature of luminescence is attributed to the relaxation of electronic excitation in [VO₄]³⁻ structural tetrahedra present in the vanadates. The performance characteristics of luminophores were determined. These luminophores may be promising as X-ray luminescent screens, radioluminescence indicators, and light-emitting diode devices.