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排序方式: 共有499条查询结果,搜索用时 15 毫秒
1.
Andrzej Czopnik Nataly Shitsevalova Alexander Krivchikov Vasyl Pluzhnikov Yuriy Paderno 《Journal of solid state chemistry》2004,177(2):507-514
We have measured heat capacity and thermal expansion of rare earth dodecaborides REB12 (RE=Y, Tb-Tm, Lu). YB12 and LuB12 are diamagnetics whereas the other dodecaborides are ordered antiferromagnetically. The amplitude of the heat capacity discontinuity at the Néel temperature and the shape of the heat capacity variation in the critical region for all these antiferromagnetics are characteristics for amplitude-modulated magnetic structures. In the ordered state TbB12 reveals two first-order phase transitions, most likely due to magnetic structure changes. The heat capacity of ErB12 just below the Néel point shows an anomaly of unclear origin. From the Schottky contribution to the heat capacity we have determined crystal field parameters. They are completely different than that is estimated from Point Charge Model. 相似文献
2.
A new Kohn-Sham formalism is developed for studying the lowest molecular electronic states of given space and spin symmetry whose densities are represented by weighted sums of several reference configurations. Unlike standard spin-density functional theory, the new formalism uses total spin conserving spin-density operators and spin-invariant density matrices so that the method is fully spin-adapted and solves the so-called spin-symmetry dilemma. The formalism permits the use of an arbitrary set of reference (noninteracting) configurations with any number of open shells. It is shown that the requirement of degeneracy of the total noninteracting energies of the reference configurations (or configuration state functions) is equivalent to the stationary condition of the exact energy relative to the weights of the configurations (or configuration state functions). Consequently, at any molecular geometry, the weights can be determined by minimization of the energy, and, for given reference weights, the Kohn-Sham orbitals can be determined. From this viewpoint, the developed theory can be interpreted as an analog of the multiconfiguration self-consistent field approach within density functional theory. 相似文献
3.
Alfaya RV Fujiwara ST Gushikem Y Kholin YV 《Journal of colloid and interface science》2004,269(1):32-36
3-n-propylpyridiniumsilsesquioxane chloride polymer, abbreviated as SiPy+Cl-, was used to coat a porous silica gel, SiO2, surface to form the chemically modified solid SiO2/SiPy+Cl-. The resulting polymer film was well adhered to the surface and presented an ion exchange capacity of 0.74 mmol g(-1). Metal halides, MClz [M=Fe(III), Cu(II), and Zn(II)], were adsorbed by the modified solid from ethanol solutions as neutral species by forming the surface anionic complexes described by the equation: mSiO2/SiPy+Cl-+ MClz <=> (SiO2/SiPy+)m[MCl(z+m)]m-, where the [MCl(z+m)]m- species adsorbed on the surface are FeCl4-, ZnCl4(2-), and CuCl4(2-). Accurate estimates of the specific sorption capacities and the heterogeneous stability constants of the immobilized metal complexes were determined with the aid of computational procedures. 相似文献
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6.
An O-methylated analog of protonated phenazine-di-N-oxide radical anion abstracts hydrogen from primary and secondary alcohols
in a slow (k
1 < 500 M−1 s−1) bimolecular reaction. No kinetic evidence has been found for the unimolecular release of free methoxyl radicals through
the homolytic N-OMe bond cleavage in these species. DFT calculations at the UB3LYP 6-31G(d) level indicate that protonated
and O-alkylated radical anions of pyrazine, quinoxaline and phenazine di-N-oxides are close analogues of aromatic nitroxyl
radicals with the highest spin density localized on the oxygen and nitrogen of the nitrone moiety. 相似文献
7.
Capillary electrophoretic approach to screen for enzyme inhibitors in natural extracts 总被引:2,自引:0,他引:2
This paper demonstrates development of electrophoretically mediated micro analysis (EMMA), for screening protein tyrosine phosphatase (PTP) inhibitors in natural extracts. It is demonstrated that capillary electrophoresis (CE) separation of the substrate and the product allows for using the assay in an on-column format to monitor the reaction without typically used fluorogenic substrates. Michaelis-Menten kinetics parameters calculated based on the EMMA results (Km = 1.2-1.5 microM) were in a good agreement (Km = 1.0-1.5 microM) obtained using an off-line CE functional assay (CE FA). EMMA of PTP titrated with different concentrations of ligand demonstrated the peak-shift phenomenon normally seen in affinity capillary electrophoresis. This feature of EMMA gives an indication of the binding affinity of the ligand in addition to its functional activity, providing another dimension in characterization of the protein-inhibitor interaction. It was demonstrated that simultaneous screening of the primary PTP target and a secondary, counter target (PTP-C) using the EMMA format can be used to prioritize hits based on their specificity. 相似文献
8.
Timoshenko VM Bouillon JP Chernega AN Shermolovich YG Portella C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(18):4324-4329
The ethyl ester of beta-bromoperfluorodithiocrotonic acid reacts with dimethyl acetylenedicarboxylate to give 1,4-difluoro-2,3-bis(trifluoromethyl)-but-2-ene-1,4-diylidene-2,2'-bis(4',5'-dicarbomethoxy-1',3'-dithiole) (4), a new type of vinylogue of tetrathiafulvalene. The thermal transformations of this compound lead, depending on the temperature, to the formation of the cyclization products: 11,14-difluoro-2,3,8,9-tetra(carbomethoxy)-12,13-bis(trifluoromethyl)-1,4,7,10-tetrathiadispiro[4.0.4.4]tetradeca-2,8,11,13-tetraene (8) or 5,8-difluoro-6,7-bis(trifluoromethyl)-2,3-bis(carboxymethyl)-1,4-benzodithiine (11). 相似文献
9.
Alexander V. Levdansky Natalya Yu. Vasilyeva Yuriy N. Malyar Alexander A. Kondrasenko Olga Yu. Fetisova Aleksandr S. Kazachenko Vladimir A. Levdansky Boris N. Kuznetsov 《Molecules (Basel, Switzerland)》2022,27(19)
For the first time, the process of birch ethanol lignin sulfation with a sulfamic acid-urea mixture in a 1,4-dioxane medium was optimized experimentally and numerically. The high yield of the sulfated ethanol lignin (more than 96%) and containing 7.1 and 7.9 wt % of sulfur was produced at process temperatures of 80 and 90 °C for 3 h. The sample with the highest sulfur content (8.1 wt %) was obtained at a temperature of 100 °C for 2 h. The structure and molecular weight distribution of the sulfated birch ethanol lignin was established by FTIR, 2D 1H and 13C NMR spectroscopy, and gel permeation chromatography. The introduction of sulfate groups into the lignin structure was confirmed by FTIR by the appearance of absorption bands characteristic of the vibrations of sulfate group bonds. According to 2D NMR spectroscopy data, both the alcohol and phenolic hydroxyl groups of the ethanol lignin were subjected to sulfation. The sulfated birch ethanol lignin with a weight average molecular weight of 7.6 kDa and a polydispersity index of 1.81 was obtained under the optimum process conditions. Differences in the structure of the phenylpropane units of birch ethanol lignin (syringyl-type predominates) and abies ethanol lignin (guaiacyl-type predominates) was manifested in the fact that the sulfation of the former proceeds more completely at moderate temperatures than the latter. In contrast to sulfated abies ethanol lignin, the sulfated birch ethanol lignin had a bimodal and wider molecular weight distribution, as well as less thermal stability. The introduction of sulfate groups into ethanol lignin reduced its thermal stability. 相似文献
10.
In this paper, we presented new J-oriented basis sets, pecJ-n (n = 1, 2), for phosphorus and silicon, purposed for the high-quality correlated calculations of the NMR spin–spin coupling constants involving these nuclei. The pecJ-n basis sets were generated using the modified version of the property-energy consistent (PEC) method, which was introduced in our earlier paper. The modifications applied to the original PEC procedure increased the overall accuracy and robustness of the generated basis sets in relation to the diversity of electronic systems. Our new basis sets were successfully tested on a great number of spin–spin coupling constants, involving phosphorus or/and silicon, calculated within the SOPPA(CCSD) method. In general, it was found that our new pecJ-1 and pecJ-2 basis sets are very efficient, providing the overall accuracy that can be characterized by MAEs of about 3.80 and 1.98 Hz, respectively, against the benchmark data obtained with a large dyall.aae4z+ basis set of quadruple-ζ quality. 相似文献