Organocatalytic photooxygenation of aldehydes at the α‐position proceeds via enamine catalysis, though enamines should be easily oxidized by singlet oxygen respectively to amides and carbonyl compounds. Moreover, the formation of a zwitterionic enamine peroxide intermediate was postulated based on experimental and theoretical data. The reaction affords desired diols (after in situ reduction) in a decent yield and (S)‐ or (R)‐enantioselectivity depending on a catalyst used. The (S)‐enantiomer predominated in imidazolidinone‐catalyzed reactions, while prolineamides assured the formation of the (R)‐stereoisomer. DFT calculation suggests that the enamine–oxygen complex with the lowest energy has the E,s‐cis conformation for the prolineamide derivative and E,s‐trans for the imidazolidinone catalyst, explaining the opposite stereoselectivity in the photooxygenation reaction.
This study investigates the phenomena preceding and accompanying the SHS reaction between Fe and Al elemental powders during sintering. SEM and XRD analysis were used to observe the mechanisms of formation of Fe-Al intermetallic phases. The analysis of sintered material just before the SHS reaction demonstrates that in addition to the well-known Fe2Al5 phase and the low-aluminum solid solution of iron, the high-aluminum phases FeAl2 and FeAl3 are formed. The kinetics of phase transformations under isothermal conditions were investigated by DSC using the JMA (Johnson-Mehl-Avrami) model. This approach allowed us to calculate Avrami coefficients, which characterize the speed and the manner of particular phase transformations. 相似文献
The study is concerned with the excitonic and intracenter emission spectra of planar structures with periodically arranged
Cd0.9Mn0.1Te layers (with a nominal thickness corresponding to one monolayer) separated by Cd0.5Mg0.5Te barriers with different thicknesses. The complex structure of the spectra and their variations in the temperature range
from 5 to 80 K are indicative of the inhomogeneity of the Cd0.9Mn0.1Te layers, the important role of vertical diffusion, and the correlation between irregularities in the direction of growth
at a small spacing between the narrow-gap Cd0.9Mn0.1Te layers. A substantial factor controlling the spectrum and quantum efficiency of excitonic emission is the relation between
the exciton radius and the period of the structure. 相似文献
In this work manganese cobalt spinel (MnCo2O4) coatings were deposited on steel substrates by spray pyrolysis at 390?°C. This is at much lower temperatures than previously reported (typically in excess of 900?°C). It was possible to produce coatings with well controlled thickness (2-5-10?μm). The as-deposited coatings were evaluated for their microstructural changes and electrical conductivity up to 800?°C. Results confirm the formation of a single phase spinel with high density and electrical conductivity. Based on the obtained results, it might be concluded that spray pyrolysis is a very promising method to develop protective coatings for steel substrates at low temperatures overcoming limitations of many other methods. 相似文献
Magnetization measurements of MBE grown epilayers of (Zn,Cr)Se with relatively large Cr content of 0.014 and 0.021 are presented. We evidence the presence of a strong ferromagnetic coupling between Cr ions, but also suggest a significant clustering due to a pronounced superparamagnetic behavior found in the layers. We estimate the intraparticle Curie temperature to be about 100 K, which combined with other arguments appears to indicate that some magnetic properties of Cr-rich layers might be dominated by the presence of small grains of AII–Cr2–B4VI spinels. 相似文献
Based on the results of optically detected magnetic resonance (ODMR) and time-resolved investigations we relate the observed lifetime shortening of intra-shell Mn2+ emission to spin dependent magnetic interactions between localized spins of Mn2+ ions and spins/magnetic moments of free carriers. We show that this mechanism is active both in bulk and in low dimensional structures, such as quantum wells (QWs), quantum dots (QDs) and nanostructures. 相似文献
Solid Oxide Electrolyzer Cells (SOECs) are very promising electrochemical devices for the production of syngas (H2/CO) by H2O and CO2 co-electrolysis. The structure, microstructure and electrical properties of the fuel electrode material play a crucial role in the performance of the whole cell and efficiency of electrocatalytic reduction of steam into hydrogen. In the present work, a novel Co and Pr co-doped SrTiO3-δ material attracted attention as a potential fuel electrode for SOFC/SOEC. Materials with different praseodymium content were prepared by a solid-state reaction process. XRD confirmed cubic perovskite structure in all obtained samples. SEM results showed porosity in doped materials and EDX proved ABO3 stoichiometry. TEC values were about 1.17–1.26•10−5 K−1 very close to the YSZ electrolyte value. XPS studies turn out that a praseodymium can be multivalent and exist on mixed +3 and + 4 oxidation state. Electrical conductivity of samples was measured by DC 4-wire method in range of 100-800 °C. Highest value of total conductivity was achieved for Sr0.7Pr0.3Ti0.93Co0.07O3-δ and reached 23.7 S∙cm−1. The obtained results were discussed and analyzed in term of defect chemistry. 相似文献
The reflection and photoluminescence spectra of modulation-doped CdTe/CdMgTe quantum-well structures have been studied. It was found that the magnitude and sign of the Zeeman splitting of the trion reflection line depend on the electron concentration in the quantum well, whereas the magnitude and sign of the splitting of the exciton line are absolutely the same for all the electron concentrations under study. In the photoluminescence spectra, the magnitude and sign of the Zeeman splittings for the exciton and trion were the same. This ??renormalization?? of the trion g factor is explained in terms of the model of combined exciton-electron processes. 相似文献
Exitonic emission from three structures consisting of CdMnTe layers separated by thick wide-gap CdMgTe layers are studied in the temperature range from 13 to 300 K. The thickness of the CdMnTe layers corresponds to the 0.5, 1.5, and 3.0 monolayers. To calculate the excitonic density-of-state spectra in these structures, a simple model that takes into account variations in the elemental composition of the alloy inside the exciton sphere under variations in its position with respect to the narrow-gap layer is used. It is shown that the energy extent of the emission and density-of-state spectra and the positions of the characteristic features of the spectra are in agreement. On low-intensity excitation (1–10 W/cm2), heating of the samples from 13 K yields a relative decrease in the emission intensity of excitions localized in the CdMgTe layers, so that this emission at 80 K is much lower in intensity than the emission of excitions bound at the narrow-gap layers. In the conditions of high-intensity excitation (103–105 W/cm2) at 80 K, the excitonic emission from the CdMgTe layers prevails; however, at room temperature, the emission from the narrow-gap layers dominates over the emission from the wide-gap layers. The intracenter luminescence of the Mn2+ ions in the sample with three monolayers CdMnTe presents a doublet structure, with the components corresponding to the internal and external layers of the three-layer planar inclusions containing manganese. 相似文献