Energy contribution of heterocyclic sulfur and a disulfide bond in solid and gaseous phase

Structural Chemistry - In the present work, the energy contribution of the sulfur atom in a heterocycle and in a disulfide bond in solid and gas phases was calculated. To achieve this goal,...     

Synthesis of zinc oxide microrods and nano-fibers with dominant exciton emission at room temperature

Employing a simple chemical synthesis method, hexagonal-shaped zinc oxide microrods and zinc oxide nano-fibers were deposited on pyrex-glass and aluminum substrates, respectively. Both kinds of deposits showed zincite crystalline phase with lattice parameters: a=3.2498 Å and c=5.2066 Å. Microrods showed very uniform wide and large sizes of around 1 and 10 μm, respectively. Both deposits were homogeneous over all substrate surfaces. Microrods and nano-fibers resulted with good optical quality and with preferential crystalline growth in [1 0 1 0] and [0 0 0 1] directions. The principal optical characteristics for both microrods and nano-fibers were: a) room-temperature photo and cathodo-luminescent spectra with strong exciton emission centered around 390 nm and with FWHMs around 125 and 160 meV, respectively, b) poor photo and cathode-luminescent emissions in the visible region of the electromagnetic spectrum, c) energy band gap of 3.32 eV, d) good emission efficiency supported by the not-required high energy densities to obtain strong exciton emission and e) good ZnO stoichiometry endorsed by photoluminescent results. These characteristics make of these microrods and nano-fibers good for potential photonic applications.     

Optical properties of LiNbO3:Cr crystals co-doped with germanium oxide

Lithium niobate (LiNbO₃) crystals doped with chromium ions show a clear green colouring reflecting the absorption profile of the dominating [Cr]_Li defect centres. A significant change in its colouration takes place when it is co-doped with other valency impurities such as Mg²⁺, Sc³⁺ and W⁶⁺, above a certain threshold concentration. This concentration singularity has been attributed to the formation of [Cr]_Nb centres coexisting with the [Cr]_Li centres.In this work, we extended the investigation on the effect of co-dopant ions in Cr:LiNbO₃ to tetravalent cation such as GeO₂. A singularity in the relative intensity of the ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ absorption band was observed for a concentration of ～1.5 mol%, compared with 4.5 mol% for Mg²⁺. The photoluminescence emission spectra also reveal a new emission band, at a lower energy than the [Cr]_Li centre, corresponding to this threshold concentration. A charge compensation model is proposed to explain the role of cation impurities and results are compared with those of other valence impurities.     

Accurate standard‐based quantification of X‐ray fluorescence data using metal‐contaminated plant tissue

Quantitative X‐ray fluorescence (XRF) measurements have been conducted on naturally lead‐contaminated samples. The calibration procedure using the ratio of fluorescence to Compton scattered radiation was investigated using Monte Carlo simulation. Experimental results with low‐energy photons (14 keV) and simulations show a very good linearity of the fluorescence to Compton ratio as a function of metal concentration. Lead (Pb), iron (Fe) and zinc (Zn) are measured in samples of Phaseolus vulgaris (bean seeds) that have been grown using a nutritive solution with different Pb dopings. Naturally contaminated samples are thus obtained. The calibration must be done for fixed conditions of X‐ray energy and scattering angle, while X‐ray beam intensity and detector to sample distance can change from one sample to another. Simulation allows to evaluate the matrix effect on the calibration curve, and shows that linearity is preserved even in the presence of other heavy elements in the fluorescence spectrum. However, calibration must be done using samples with similar matrix as it affects the slope of the curve. Copyright © 2010 John Wiley & Sons, Ltd.     

Cold white light generation through the simultaneous emission from Ce3+, Dy3+ and Mn2+ in 90Al2O3·2CeCl3·3DyCl3·5MnCl2 thin film

The photoluminescence of a CeCl₃, DyCl₃ and MnCl₂ doped aluminum oxide film deposited by ultrasonic spray pyrolysis was characterized by excitation, emission and decay time spectroscopy. A nonradiative energy transfer from Ce³⁺ to Dy³⁺ and Mn²⁺ is observed upon UV excitation at 278 nm (peak emission wavelength of AlGaN-based LEDs). Such energy transfer leads to a simultaneous emission of these ions in the blue, green, yellow and red regions, resulting in white light emission with CIE1931 chromaticity coordinates, x=0.34 and y=0.23, which correspond to cold white light with a color temperature of 4900 K.     

Ethanol induces cell-cycle activity and reduces stem cell diversity to alter both regenerative capacity and differentiation potential of cerebral cortical neuroepithelial precursors

Background  

The fetal cortical neuroepithelium is a mosaic of distinct progenitor populations that elaborate diverse cellular fates. Ethanol induces apoptosis and interferes with the survival of differentiating neurons. However, we know little about ethanol's effects on neuronal progenitors. We therefore exposed neurosphere cultures from fetal rat cerebral cortex, to varying ethanol concentrations, to examine the impact of ethanol on stem cell fate.     

The standard molar enthalpies of combustion and formation of 2-methylbenzothiazole, 2-methylbenzoxazole, and 2-methyl-2-thiazoline

A static-bomb combustion calorimeter and a rotating-bomb combustion calorimeter were used to determine the energies of combustion of 2-methylbenzothiazole, 2-methylbenzoxazole, and 2-methyl-2-thiazoline. The static- and rotating-bomb calorimeters were recently calibrated by the standard benzoic acid combustion runs and they were tested with adequate secondary combustion standards. The rotating-bomb calorimeter was tested using thianthrene and, in the present work, 1,2,4-triazole was used to test the static-bomb calorimeter. From the energies of combustion of the compounds under study, the liquid-phase standard molar enthalpies of formation were derived, at T = 298.15 K, as: (72.5 ± 1.5), (?50.7 ± 2.1), and (?88.5 ± 2.8) kJ mol^?1, respectively.     

The standard molar enthalpies of formation of chromone-3-carboxylic acid, 6-methylchromone-2-carboxylic acid,and 6-methyl-4-chromanone determined by micro-combustion calorimetry

The energies of combustion of chromone-3-carboxylic acid (C3CA), 6-methylchromone-2-carboxylic acid (6MCC), and 6-methyl-4-chromanone (6M4C) were determined using an isoperibolic micro-combustion calorimeter. The calorimeter used in the present work has been assembled, calibrated, and tested in our laboratory with the desired results. Prior to the measurement of the energies of combustion, the purities, heat capacities (C _p), fusion temperatures (T _fus), and enthalpies of melting (Δ_fus H) for each compound were determined by differential scanning calorimetry. The values of the energies of combustion were used to derive standard molar enthalpies of combustion ( \( \Delta _{{\text{c}}} H_{{\text{m}}}^{\circ } \) ) and standard molar enthalpies of formation ( \( \Delta _{{\text{f}}} H_{{\text{m}}}^{\circ } \) ) in the crystalline phase at T = 298.15 K. The values found for the \( \Delta _{{\text{f}}} H_{{\text{m}}}^{\circ } \) of C3CA, 6MCC, and 6M4C were ?(619.5 ± 2.6), ?(656.2 ± 2.2), and ?(308.9 ± 3.0) kJ mol^?1, respectively.     

Dielectric anomalous response of water at 60 °C

Recently, the paraelectric response of water was investigated in the range 0–100 °C. It showed an almost perfect Curie–Weiss behaviour up to 60 °C, but a slight change in slope of 1/ε_d versus T at 60 °C was overlooked. In this work, we report optical extinction measurements on metallic (gold and silver) nanoparticles dispersed in water, annealed at various temperatures in the range from 20 to 90 °C. An anomalous response at 60 °C is clearly detectable, which we associate to a subtle structural transformation in the water molecules at that temperature. This water anomaly is also manifested by means of a blue shift in the longitudinal surface plasmon resonance of the metallic nanoparticles for the solutions annealed at temperatures higher than about 60 °C. A reanalysis of 1/ε_d (T) for water in the whole temperature range leads us to conclude that the water molecule undergoes a subtle transformation from a low temperature (0–60 °C) configuration with a dipole moment μ₁ = 2.18 D (close to the molecular dipole moment of ice) to a high temperature (60–100 °C) configuration with μ₂ = 1.87 D (identical to the molecular dipole moment in water vapour).