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961.
Polynomials in the one-third power of the density were recently proposed to represent approximately the correlation energy density functional [S. Liu and R.G. Parr, Phys. Rev. A 53 (1996) 2211]. Studied in the present work is a Padé-approximant in that same variable which overcomes weaknesses in the polynomial form. Numerical results for atoms and molecules show that Padé forms fairly reproduce the experimental values, and are comparable in accuracy with other commonly used local functionals for the correlation energy. 相似文献
962.
Rudolf W. Soukup Valentin N. Sapunov Hossein Ateshykashi Roland Schmid 《Monatshefte für Chemie / Chemical Monthly》1982,113(10):1093-1105
The transfer of oxinate ions from thallium (III)oxinate to trivalent Fe(DMF)
6
3+
in propylenecarbonate takes place via rearrangements within a rapidly formed binuclear thallium(III)—iron(III) complex. In a last rapid step this rearranged complex reacts with excess reactants to the final products whose composition accordingly depends on the ratio of the reactant concentrations. 相似文献
963.
Valentin S. Enev 《Tetrahedron》2007,63(26):5930-5939
Quinic acid was used as a chiral scaffold for elaborating the highly substituted cis-decalin core system of branimycin via an Eschenmoser-Ireland rearrangement-INOC annulation sequence. 相似文献
964.
Valtchev VP Tosheva L Bozhilov KN 《Langmuir : the ACS journal of surfaces and colloids》2005,21(23):10724-10729
Zeolite A nanoparticles were synthesized under room-temperature conditions from a very reactive organic-template-free gel system. The optimization of the syntheses parameters, namely, the composition of the initial system and the careful choice of the reactants, allowed the crystallization to be accomplished within 3 days. At this stage the individual zeolite crystals were in the range of 100-300 nm without well-developed crystal faces. The prolongation of the synthesis time up to 10 days led to formation of larger well-faceted cubic crystals averaging about 400-500 nm in size. The high-resolution transmission electron microscopy (HRTEM) study revealed that a thin layer of amorphous material covers the zeolite particles acting as a binder between individual zeolite crystals. The postsynthesis treatment of the product in NH(3) media under ultrasonic radiation disintegrated the loosely attached zeolite particles and decreased the fraction of zeolite A particles with low colloidal stability. The employed approach, however, did not result in complete disintegration of aggregated crystals. The zeolite crystals obtained under ambient conditions were characterized by XRD, SEM, dynamic light scattering, and N(2) adsorption measurements. 相似文献
965.
Liviu V. Costea Vasile N. Bercean Valentin Badea Klaus Gerdes Ulrich Jordis 《Monatshefte für Chemie / Chemical Monthly》2006,137(6):737-744
Summary. 1H-3-Methyl-4-ethoxycarbonyl-5-(benzylidenehydrazino)pyrazoles are key intermediates in obtaining various heterocyclic systems
including pyrazolotriazoles. We present the voltammetric behavior of these compounds in nonaqueous media, with the following
para substituents grafted on the benzene ring: –N(CH3)2, –OH, –OCH3, –F, –Cl, –CF3, –NO2, as well as of the novel compounds with –Br, –I, and –SCH3 in the para position, in order to elucidate the influence of the various substituents on the mechanism of anodic oxidation. 相似文献
966.
Valentin Monev Milena Spassova Benoît Champagne 《International journal of quantum chemistry》2005,104(3):354-366
We present a comparative study of the AIM, CHELPG, GAPT, MK, Mulliken, NPA, and RESP charge distributions associated with a positively charged soliton on increasingly large trans‐polyacetylene chains, at HF, MP2, and DFT levels of theory. The charge storage in the soliton‐bearing systems is explored in detail, including charge magnitude, charge separation, charge alternation, and chain length effects. The grouping of the charge distributions at a given level of theory, as well as the sensitivity of a given charge distribution to the inclusion of electron correlation in its computation, are investigated using similarity analysis. Several of the charge definitions have been applied for the first time for charged soliton‐bearing systems, and there are substantial differences between the charge distributions for the charged and neutral systems. Thus, AIM charges are no longer one of the largest charge values, the AIM charges can be in counterphase with other definitions, and the GAPT charges for neutral systems are quite different from the GAPT charges for charged systems, e.g., the magnitudes of the GAPT charges are anomalously large and increase with the size of the charged system. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
967.
Alla A. KichaNatalia V. Ivanchina Anatoly I. KalinovskyPavel S. Dmitrenok Valentin A. Stonik 《Tetrahedron letters》2003,44(9):1935-1937
New ionic compounds containing an alkaloidal cation and a steroidal anion have been isolated by reverse-phase liquid chromatography from the extracts of the starfish Lethasterias nanimensis chelifera. Their structures have been elucidated by NMR and mass spectroscopy as 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinolinium salts of 3-O-sulfoasterone 1, 3-O-sulfoisoasterone 2 and 3-O-sulfothornasterol A 3. In addition, the alkaloid 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (MTCA) 4 was found in this starfish. 相似文献
968.
The epoxidation of olefins by peroxo complexes of Cr(VI), Mo(VI) and W(VI) was investigated using the B3LYP hybrid density functional method. For the mono- and bisperoxo model complexes with the structures (NH(3))(L)M(O)(2)(-)(n)()(eta(2)-O(2))(1+)(n)() (n = 0, 1; L = none, NH(3); M = Cr, Mo, W) and ethylene as model olefin, two reaction mechanism were considered, direct oxygen transfer and a two-step insertion into the metal-peroxo bond. The calculations reveal that direct attack of the nucleophilic olefin on an electrophilic peroxo oxygen center via a transition state of spiro structure is preferred as significantly higher activation barriers were calculated for the insertion mechanism than for the direct mechanism. W complexes are the most active in the series investigated with the calculated activation barriers of direct oxygen transfer to ethylene decreasing in the order Cr > Mo > W. Barriers of bisperoxo species are lower than those of the corresponding monoperoxo species. Coordination of a second NH(3) base ligand to the mono-coordinated species, (NH(3))M(O)(2)(eta(2)-O(2)) and (NH(3))MO(eta(2)-O(2))(2), results in a significant increase of the activation barrier which deactivates the complex. Finally, based on a molecular orbital analysis, we discuss factors that govern the activity of the metal peroxo group M(eta(2)-O(2)), in particular the role of metal center. 相似文献
969.
A study of the thermal behavior of n-alkanes in the solid state or in a hydrocarbon matrix has been carried out using a stepwise programming of temperature. In the case of pure n-alkanes the separation between the transition peak and the melting peak can be easily increased and the enthalpy of each thermal effect can be determined. The precipitated amount of n-alkanes contained in middle distillates has been obtained vs. temperature. 相似文献
970.
Oleg S. Chizhov Valentin I. Kadentsev Galina G. Palmbach Konstantin Ia. Burstein Svyatoslav A. Shevelev Albert A. Feinsilberg 《Journal of mass spectrometry : JMS》1978,13(11):611-617
The chemical ionization mass spectra of various aliphatic nitro compounds have been studied. Almost all the nitro compounds form stable [M+H]+ ions in sharp contrast to the instability of their ions produced by other ionization techniques. The fragmentations of the [M+H]+ ions are discussed, the mechanisms and structures of fragment ions proposed being supported by quantum mechanical calculations. Correlations between chemical ionization mass spectra and structures of compounds studied are outlined. Reliable identification of mono- and polynitroalkanes and some related derivatives may be made by analysis of their chemical ionization mass spectra. 相似文献