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41.
de Oliveira Neto Marçal Pires José M. Giambiagi Mario de Giambiagi Myriam Segre Alvarez Fernando A. 《Structural chemistry》1998,9(5):339-348
Electronic properties of lamotrigine (LTG) and two analogues (A1 and A2) are compared through MOPAC-AM1 calculations. Two stable conformers of LTG are calculated to exist in agreement with X-ray crystallography. In the three compounds and the two conformers for each of them, the more favorable protonation sites are N2 and N4; these should then be the sites appropriate for interaction with a receptor, and group valence reinforces the supposition. The molecular electrostatic potentials show that a region between the two chlorine atoms in LTG could be the site for an electrostatic interaction with a corresponding site in the receptor. The fluorine atom in A1 would play an equivalent role. A simple model for LTG-receptor interaction is proposed. 相似文献
42.
Hong SB Lee SH Shin CH Woo AJ Alvarez LJ Zicovich-Wilson CM Camblor MA 《Journal of the American Chemical Society》2004,126(42):13742-13751
Here, we report that synthetic gallosilicate molecular sieves with the NAT topology and Si/Ga ratios close to but slightly higher than 1.50 undergo an in situ transformation under their crystallization conditions. The materials have been studied ex situ by using powder X-ray diffraction, elemental and thermal analyses, and multinuclear MAS NMR. The transformation is characterized by a change in the distribution of Si and Ga of the NAT framework, from a quite (but not completely) disordered phase to a very highly (but not completely) ordered one, accompanied by a change from tetragonal to orthorhombic symmetry. During most of the solution-mediated transformation, no noticeable signs of fresh precipitation, phase segregation, or changes in the chemical composition are detected. Intermediate materials show variations in the degree of Si-Ga ordering and orthorhombic distortion and are not physical mixtures of the disordered and ordered phases. Ab initio calculations strongly suggest a preferential siting of Si in the tetrahedral sites involved in a smaller number of 4-rings in the NAT topology (i.e., the low multiplicity site). The cost of violations of Loewenstein's rule has also been calculated. For this topology and chemical composition the preferential siting and Loewenstein's rule drive together the system to the ordered configuration. A Monte Carlo sampling procedure affords a reasonable model for the initial, mainly disordered state, which fits well within the experimental disorder-order series. 相似文献
43.
44.
A. Montero Alvarez J. R. Estévez Alvarez R. Padilla Alvarez 《Journal of Radioanalytical and Nuclear Chemistry》2007,273(2):427-433
The aim of this work consisted on the implementation of sufficiently accurate and sensitive analytical procedure for the analysis
of metal concentration in rainwater. Different sample preparation procedures were tested to achieve the required concentration
prior to direct total reflection X-ray fluorescence (TXRF) analysis. TXRF and anodic stripping voltammetry (ASV) were compared
in regard to achieved detection limits, precision and accuracy. 相似文献
45.
The dinuclear complexes of transition metal ions of type [M(2)(mu,eta(1)-XY)(2)L(4)], where XY is an unsaturated ligand that can act as a four-electron or a two-electron donor through the X atom, appear in two molecular conformations depending on whether the coordination planes around the two metal atoms are coplanar or bent. In both structures the geometry of the X atom is planar, corresponding to an sp(2) hybridization. An ab initio theoretical study on 43 representative complexes, complemented with a structural database analysis, provides a rationale for the experimentally observed structures. 相似文献
46.
Vezmar I M.M. Alvarez J.T. Khoury B.E. Salisbury M.N. Shafigullin R.L. Whetten 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1997,40(1):147-151
Stable, intense beams of large metal clusters (from 102 to 103 Au or Ag atoms) can be produced by laser desorption of molecular films of passivated nanocrystals that have undergone fractionation by size and a separate structural characterization. The mass onset of the desorbed species corresponds directly to the dimensions of the nanocrystal core, indicating that only the surfactant shell is lost during desorption. Photofragmentation, photoionization, and photodetachment from beams of large metal clusters, generated in this way, have all been observed. 相似文献
47.
E. J. Alvarez V. Hornof L. P. Blanchard 《Journal of polymer science. Part A, Polymer chemistry》1972,10(7):1895-1903
Propylene oxide and tetrahydrofuran were polymerized cationically by an in situ catalytic system composed of antimony pentachloride and 1,2-propanediol. The rates of polymerization were measured by vapor-phase chromatography in the temperature range from ?20°C to +20°C. The Arrhenius parameters pertaining to the reaction of each of the comonomers were evaluated and compared with data published earlier for other catalytic systems. The present catalyst system was incapable of initiating a homo-polymerization of tetrahydrofuran in the absence of propylene oxide, while the latter was readily homopolymerized. In a copolymerization system, the rates of consumption of both monomers were first-order in respect to the catalyst, but the reaction ceased when all of the propylene oxide had been consumed. The relative reactivity of the two monomers as characterized by the copolymerization parameters r1 (PO) = 1.15 and r2 (THF) = 0.70 suggests that in the copolymerization system, tetrahydrofuran is capable of a reaction with its own active center. This is discussed in terms of a possible mechanism involving the effects of penultimate units and extensive chain transfer. The latter is well evident from the molecular weights of resulting copolyethers, which do not exceed one thousand. 相似文献
48.
The modeling of liquid–vapor equilibrium in ternary mixtures that include substances found in alcoholic distillation processes of wine and musts is analyzed. In particular, vapor–liquid equilibrium in ternary mixtures containing water + ethanol + cogener has been modeled using parameters obtained from binary mixture data only. The congeners are substances that although present in very low concentrations, of the order of part per million, 10−6 to 10−4 mg/L, are important enological parameters [1] and [2]. In this work two predictive models, the PSRK equation of state and the UNIFAC liquid phase model and two semipredictive activity coefficient models: NRTL and UNIQUAC have been used. The results given by these different models have been compared with literature data and conclusions about the accuracy of the models studied are drawn, recommending the best models for correlating and predicting the phase equilibrium in this type of mixtures. 相似文献
49.
Nicolaou KC Kim DW Baati R O'Brate A Giannakakou P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(24):6177-6191
Apicularen A (1) and related benzolactone acylenamines belong to a growing class of novel natural products possessing highly cytotoxic properties. The challenging structure of 1 includes a 10-membered macrolactone ring, a tetrahydropyran system, an o,m-substituted phenol and a doubly unsaturated acyl group attached on the side chain enamine functionality. The total synthesis of apicularen A described herein involves a strategy equivalent to its proposed biosynthesis and entails a reiterative two-step procedure featuring allylation and ozonolytic cleavage to grow the molecule's chain by one acetate unit at a time. The developed synthetic technology was applied to the construction of a series of apicularen A analogues whose biological evaluation established a set of structure-activity relationships in this new area of potential importance in cancer chemotherapy. 相似文献
50.
A nomogram is presented to estimate the radiooctivity of99mTc in99Mo−99mTc generators, based on equations of radioactive equilibrium. Since the parent element decays through two isomeric states
of the daughter, the ratio of the total mass of technetium per millicurie of99mTc is a function of the time elapsed between two consecutive elutions. Values of the amount of99mTc expressed as a mole fraction or in moles or grams of total technetium per millicurie of99mTcm are also presented. 相似文献