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11.
Jesus P. Lopez 《Journal of Molecular Structure》1983,94(3-4):203-207
MNDO calculations on several substituted 1,2,4-oxadiazole compounds are discussed. Changes in geometry, atomic charge distribution, rotational barriers and molecular orbital energies are examined. A correlation exists between the thermal stability of the heterocycle and the excess electronic charge accumulated inside the ring as well as its distribution. For the case of phenylated derivatives, the nonplanar conformation is energetically more stable than the corresponding planar one, which might explain the relative decrease in thermal stability of phenylated derivatives. It is observed that as the planar conformation starts being populated, a greater amount of electrons are donated to the 1,2, 4-oxadiazole ring. 相似文献
12.
Janis V. Musso Jordan De Jesus Silva Mathis J. Benedikter Jonas Groos Wolfgang Frey Christophe Copret Michael R. Buchmeiser 《Chemical science》2022,13(29):8649
Cationic d0 group 6 olefin metathesis catalysts have been recently shown to display in most instances superior activity in comparison to their neutral congeners. Furthermore, their catalytic performance is greatly improved upon immobilization on silica. In this context, we have developed the new family of molecular cationic molybdenum oxo alkylidene complexes stabilized by N-heterocyclic carbenes of the general formula [Mo(O)(CHCMe3)(IMes)(OR)[X−]] (IMes = 1,3-dimesitylimidazol-2-ylidene; R = 1,3-dimesityl-C6H3, C6F5; X− = B(3,5-(CF3)2C6H3)4−, B(ArF)4, tetrakis(perfluoro-t-butoxy)aluminate (PFTA)). Immobilization of [Mo(O)(CHCMe3)(IMes)(O-1,3-dimesityl-C6H3)+B(ArF)4−] on silica via surface organometallic chemistry yields an active alkene metathesis catalyst that shows the highest productivity towards terminal olefins amongst all existing molybdenum oxo alkylidene catalysts.The first cationic molybdenum oxo complexes were synthesized and immobilized on partially dehydroxylated silica. Vastly enhanced catalytic activity for terminal olefins was found compared to their neutral congeners. 相似文献
13.
Maria do Carmo Rangel Varela Milton Francisco de Jesus Filho Fernando Galembeck 《Hyperfine Interactions》1994,83(1):159-167
Mössbauer spectroscopy was used to follow the aging of ferrihydrite with different amounts of acetate, to achieve more crystalline products. Mössbauer spectra of fresh samples did not show any magnetic components. After two years aging, the solids presented crystalline fractions. Hematite formation was inhibited as acetate content increased in solids. For an acetate/iron molar ratio equal to 0.63 goethite was formed instead of hematite. X-ray diffraction confirmed these results. This work shows that synthetic samples can be used for elucidating the anti-hematitic effect of organic matter already detected in soils. 相似文献
14.
A. Basillais R. Benzerga H. Sanchez E. Le Menn C. Boulmer-Leborgne J. Perrière 《Applied Physics A: Materials Science & Processing》2005,80(4):851-859
The pulsed-laser-deposition (PLD) method is particularly well suited for the growth of oxide thin films, but in the case of other compounds, such as nitrides, PLD presents some limitations which are mainly due to the low reactivity of nitrogen in comparison with oxygen. A possible way to overcome this problem is to increase the reactivity of the constituent species, via plasma assisted-pulsed-laser deposition. A plasma is coupled to the ablation chamber, in order to increase the density of reactive atomic species, which can be further incorporated in the growing film. This approach is described in this paper as well as the nature, energy, and concentration of the atomic and molecular species in the plasma as determined by various plasma diagnostics. These results are correlated to the growth of thin films in the particular case of the aluminum nitride compound. The composition and structure of the films are studied as a function of the growth conditions, and the positive effects of the additional discharge are evidenced on the film purity and properties. The fundamental problem with the PLD technique, especially with metallic targets, is the production of unwanted droplets that significantly worsen the properties of the films. To eliminate these droplets, a thin film has been grown with an experimental setup using two targets and crossed laser beams which gave positive results. PACS 81.15.Fg; 52.80.Pi; 77.84.Bw 相似文献
15.
Steatite mineral rocks, soapstone, have been studied by X-ray diffraction, optical microscopic analysis (modal analysis), electron probe micro analysis and Mössbauer spectroscopy for characterization, mineral percentages and chemical composition. Mössbauer spectra show both, magnetic interactions corresponding to magnetite and doublets corresponding to talc. chlorite, dolomite and tremolite. The temperature dependence of the quadrupole splitting in dolomite has been explained in terms of crystal field interaction. 相似文献
16.
Svetlakova A. V. Mendez M. Sanchez Tuchin E. S. Khodan A. N. Traore M. Azouani R. Kanaev A. Tuchin V. V. 《Optics and Spectroscopy》2021,129(8):846-850
Optics and Spectroscopy - We study the photocatalytic activity of nanocomposites based on θ-phase alumina (Al2O3) with different TiO2 concentrations for heterogeneous photocatalysis and... 相似文献
17.
We study the X-ray intensities of 142 light curves of cataclysmic variables, galaxies, pulsars, supernova remnants and other X-ray sources present in the public data collected by the instrument All Sky Monitor on board the satellite Rossi X-ray Timing Explorer. We show that the X-ray light curves coming from astrophysical systems obey Tsallis’s q-Gaussian distribution as probability density. This fact strongly suggests that these astrophysical systems behave in a non-extensive manner. Furthermore, the q entropic indices for these systems were obtained and they provide an indication of the nonextensivity degree of each of these astrophysical systems. The q-value increases for systems if the Tsallis entropy decreases. 相似文献
18.
Abstract During these last twenty five years a great efforts have been made in the Laboratoire de Chimie du Solide du CNRS in order to develop high oxygen pressure for the stabilization of unusual oxidation state of transition elements. The aims of such research works was to correlate the increase of the Mn+-O bond covalency versus the increase of the oxidation state n+ to the physicochemical properties of resulting oxides [1]. Iridium was an interesting element due to its position (5d) in the Periodic Table. In 1980 Ir(V) (d4) was stabilized in La2LiIrO6 in the perovskite structure [2]. Such a study had underlined the strong value of the spin-orbit coupling associated to Ir(V) [3]. The stabilization of Ir(VI) is interesting from two points of view : (i) its isotropic electronic configuration (d3), (ii) such a high oxidation state could lead to the strongest M-O bond in an oxygen lattice. Selecting, through a specific methodology, the most appropriate local structural and chemical factors and with the help of high oxygen pressure, Ir(V1) was stabilized in the perovskite Ba2CaIrO6. Structural, magnetic and Mössbauer studies have been carried out in order to characterize the physicochemical properties induced by this unusual oxidation state. 相似文献
19.
Gheorghe Dinca Luis Sanchez 《NoDEA : Nonlinear Differential Equations and Applications》1994,1(2):163-178
In this paper we derive the existence of multiple solutions for boundary value problems of the type $$u\prime \prime + f\left( {t,u} \right) = 0,u\left( 0 \right) = 0,u\left( \pi \right) = 0$$ , in terms of the behaviour of the ratiof(t,u)/u nearu=0 and near infinity. The nonlinear termf is assumed to be locally Lipschitz inu, so that the shooting method can be used. (AMS Subject Classification: 34B15). 相似文献
20.
We have found the two-phonon mode of the disorder-induced line at 1370 cm-1 for the single crystal and polycrystalline graphites having disorder. The degenerate E2g mode of the D6h symmetry for graphite is split for the disordered graphites, having frequencies at 1590 cm-1 and 1627 cm-1. Brewster angle measurements using lasers at 6328A and 10.6 μm yield n and k with good consistency even for surfaces with moderate roughness. Our results suggested that distortion of the crystal structure is present. 相似文献