Results 1 - 10 of 654132
Results 1 - 10 of 654132. Search took: 0.266 seconds
|Sort by: date | relevance|
[en] This paper describes a simple and efficient method involving domino Knovenegal/6π electrocyclization for the preparation of a variety of 2H-pyrans using microwave irradiation under solvent- and catalyst-free conditions. This method offers the advantages of a green approach, high yields, and short reaction times. Sixteen compounds (9a-p) were obtained in good to excellent yields using the procedure
[en] A kinetic study on nucleophilic substitution reactions of phenyl Y-substituted-phenyl carbonates (5a-5j) with ethylamine in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 .deg. C is reported. The plots of kobsd vs. [amine] are linear for the reactions of substrates possessing a strong electron-withdrawing group (EWG) but curve upward for those of substrates bearing a weak EWG, indicating that the electronic nature of the substituent Y in the leaving group governs the reaction mechanism. The reactions have been concluded to proceed through a stepwise mechanism with one or two intermediates (a zwitterionic tetrahedral intermediate T± and its deprotonated form T.) depending on the nature of the substituent Y. Analysis of Brφnsted-type plots and dissection of kobsd into microscopic rate constants have revealed that the reactions of substrates possessing a strong EWG (e. g., 5a-5f) proceed through T± with its formation being the rate-determining step, while those of substrates bearing a weak EWG (e. g., 5g-5j) proceed through T± and T-
[en] Highlights: ► We examined PBDE concentrations in various matrices from different industrial areas. ► Elevated PBDE levels were found in areas with low-voltage electrical manufactures. ► Areas with e-waste recycling activities also had higher PBDE concentrations. ► PBDE content and composition in water samples varied from one area to another. ► PBDE composition in sediment/soil and biological samples was predominated by BDE-209. - Abstract: Polybrominated diphenyl ethers (PBDEs) have been used extensively in electrical and electronic products, but little is known about their distribution in the environment surrounding the manufacturing factories. This study reports PBDE contamination in various matrices from the location (Liushi, Zhejiang province) that produces more than 70% of the low-voltage electrical appliances in China. Additionally, PBDE contamination was compared with other industries such as the e-waste recycling business (Fengjiang) in the same region. Specifically, we measured seven PBDE congeners (BDEs – 47, 99, 100, 153, 154, 183, and 209) in water, sediment, soil, plant, and animal tissues from four different areas in this region. The present study revealed elevated PBDE concentrations in all matrices collected from Liushi and Fengjiang in comparison with highly industrialized areas without significant PBDE contamination sources. In water samples, there were large variations of PBDE content and composition across different areas. In sediment/soil and biological samples, BDE-209 was the predominant congener and this could be due to the abundant usage of deca-BDE mixtures in China. Our findings provide the very first data on PBDE contamination in the local environments surrounding the electronics industry, and also reveal widespread PBDE contamination in highly industrialized coastal regions of China.
[en] In the last years atmospheric plasma has gained an increasing interest in the field of Cultural Heritage. Isolated experimentations that yet generated interesting results have drawn the attention of a wider community working on coating removal, oxide reduction and sterilization applications. Thanks to this growing multidisciplinary interest, the EU funded in 2011 the PANNA project, which includes the atmospheric plasma as a novel cleaning tool for cultural heritage. Plasma cleaning has some advantages compared to standard techniques because it avoids the use of solvents, it is contactless and differently from laser it works through a surface chemical action and not with mechanical shock or local heating. Moreover, atmospheric plasma has already shown good results in reducing mode for the conversion of corroded metal surface layers and in oxidation mode for biological cleaning and organic layer removal. In order to fulfil the Cultural Heritage requirements a novel plasma torch has been developed with a new design which allows to overcome the actual limits of commercial atmospheric plasma devices. The new device permits fast and effective treatments at temperatures lower than 50 °C, even at room temperature, without any electrode material deposition. These performances will be presented in relation to the removal of epoxy coatings and daguerreotype cleaning as examples. (paper)
[en] Electrochemical process of epicatechin, one of the flavonoids antioxidants, was studied here by cyclic voltammetry and semiempirical molecular orbital computation (MOPAC). Electrochemical oxidation of epicatechin showed a multistep mechanism with two anodic peaks being recognized at about +0.14 V and +0.52 V (vs. Ag/AgCl). The first peak is strong concentration dependent, showing an adsorptive feature between 1 × 10−8 M and 2 × 10−7 M, a diffusion controlled feature between 2 × 10−7 M and 1 × 10−5 M, and a surface polymerization feature between 1 × 10−5 M and 1 × 10−3 M. Computation showed that the first electron was released at 4′-hydroxyl group in B-ring. No charge delocalization occurs between A- and B-rings. Higher pH medium favors oxidation. The oxidation rate is faster in strong acidic or basic medium and slower in a weak acidic medium. This research may help to explain the complexity of antioxidant activity of flavonoids and as a complement method to characterize the role of flavonoids antioxidants in treating oxidative stress diseases
[en] Highlights: • Mechanism of Li-Se batteries is investigated by in-situ electrochemical tools. • In-situ cycling voltammetry (CV) and impedance (EIS) analyses are employed. • Self-discharge phenomenon is investigated by open circuit voltage monitoring. • Formation of dissolved polyselenides are evident in Ether-based electrolytes. • Carbonate-based can be used in Li-Se batteries. -- Abstract: Li-Se batteries have recently been concidered as one of the most promising battery systems to satisfy the future energy storage needs. On the other hand, in order to further improve the electrochemical cell performances, understanding of the Li-Se cell working mechanism, especially focusing on the formation of dissolved Li polyselenides, is essential. In this work, an in-situ study of intermediate polyselenide formation based on the 4-electrode cycling voltammetry (CV) and 3-electrode electrochemical impedance spectroscopy (EIS) are used to detect the existence of polyselenides in carbonate and ether-based electrolytes. CV measurements reveal the presence of dissolved polyselenide intermediate formations in ether-based solvent while no signatures are observed in carbonate-based electrolytes. Similar findings are also observed by EIS measurements as well as investigating the self-discharge behavior. Therefore, these two electrochemical characterizations can be easily implemented as prompt and cost-effective techniques to investigate the impact of the electrolyte solvents. Contrary to the Li-S counterparts, the outcome of the work suggests that carbonate-based electrolytes can be simply employed in the field of Li-Se batteries.
[en] Using EPR spectroscopy, it has been established that monomers affect the decomposition rate of their polymerization initiators by forming complexes with them. Several structures of the complexes have been optimized, and their decomposition mechanisms, the activation energies of the chain initiation and propagation reactions, and the heat of polymerization have been determined by means of quantum chemistry methods. In the absence of radical inhibitors (evacuation), the polymerization proceeds with high efficiency (98%) at room temperature without stimulation of the system with external energy.
[en] Trihalogenated anisoles (THAs) that have been identified at low concentration levels (ngL-1) in drinking water are suspected of causing odor episodes, which are a frequent source of complaint by consumers. Henry's law constant (KH) is an important parameter in controlling the diffusion of organic compounds from the water to the vapor-phase, so its evaluation is of significance in the study of odor events. In this paper, the KH of a wide range of trihalogenated anisoles - in its dimensionless form K'H - were calculated at two temperatures, 45 and 22deg. C using equilibration partitioning in a closed system and headspace microextraction (EPICS-SPME). Two methodological approaches, Ramachandran and Dewulf, were used for the assessment of the Henry's law constant. Nevertheless, to apply these methods to THAs, a relatively narrow headspace/water volume ratio range (80/1-8/1) is required. At these conditions, a linearity (r2) using Ramachandran's theoretical relationship from 0.9276 to 0.9989 was obtained and the variability (R.S.D.%) when Dewulf's theoretical relationship was employed was lower than 20% (n=5)
[en] Triethylammonium salt of pyran semisquaraine reacted with N-methyl-substituted heterocyclic salts to give unsymmetrical di-tert-butyl-substituted squarylium dyes. The effect of the terminal heterocycles on the spectral properties of the obtained dyes was studied, and the electronic structure of squaraines and electronic transitions therein were analyzed by quantum chemical methods.