Synthèse de dérivés du tétrathiafulvalène à propriétés mésomorphogènes

We report the synthesis of di(4-alkoxyphenyl)- and di(4-alkanoyloxy-phenyl)tetrathiafulvalenes. Their polymorphism has been investigated.     

Rectilinear shortest paths in the presence of rectangular barriers

In this paper we address the following shortest-path problem. Given a point in the plane andn disjoint isothetic rectangles (barriers), we want to construct a shortestL ₁ path (not crossing any of the barriers) from the source point to any given query point. A restricted version of this problem (where the source and destination points are knowna priori) had been solved earlier inO(n ²) time. Our approach consists of preprocessing the source point and the barriers to obtain a planar subdivision where a query point can be located and a shortest path connecting it to the source point quickly transvered. By showing that any such path is monotone in at least one ofx ory directions, we are able to apply a plane sweep technique to divide the plane intoO(n) rectangular regions. This leads to an algorithm whose complexity isO(n logn) preprocessing time,O(n) space, andO(logn+k) query time, wherek is the number of turns on the reported path. If only the length of the path is sought,O(logn) query time suffices. Furthermore, we show an (n logn) time lower bound for the case where the source and destination points are known in advance, which implies the optimality of our algorithm in this case.A preliminary version of this paper appeared in theProceedings of the First Symposium on Computational Geometry (1985).Supported in part by CNPq-Conselho Nacional de Desenvolvimento Científico e Tecnológico (Brazil).Supported in part by the National Science Foundation under Grants MCS 8420814 and ECS 8340031.     

A new cytotoxic naphthoquinone from Paepalanthus latipes

Quinones constitute an important class of naturally occurring compounds. They are found in plants, fungi and bacteria. Large number of quinones has been associated with antitumor, antibacterial, antimalarial and antifungal activities. In this work we describe the isolation, structure determination and the cytotoxic index of a new 1,4-naphthoquinone isolated from the capitula of Paepalanthus latipes.     

The synthesis and spectroscopic properties of 3-(2′,4′-Xylyl)-1H,3H-quinazoline-2,4-diones

The synthesis of 3-(2′,4′-xylyl)-1H,3H-quinazoline-2,4-diones ( 1a, 9a-j ) has been accomplished. Their pmr and ir spectra data for all compounds are presented.     

A comparison of the conductimetric behavior of cesium chloride in water-tetrahydrofuran,water-dioxane,and water-1,2-dimethoxyethane mixtures

The association constants of CsCl in water-THF, water-dioxane, and water-1,2-DME mixtures have been evaluated from conductance data using the Pitts, the Fuoss-Hsia, and a new conductance equation which includes the Chen electrophoretic effect. It is shown that the three sets of results confirm the Bjerrum concept of association. The differences observed for the ion association in the three types of mixtures are interpreted in terms of differences in the dipolar nature of the three organic molecules and that of H₂O.     

Solute–Solvent Interactions of Flavonoids in Organic Solvents

The diffusion coefficients of one flavone and five isoflavones in methanol, ethanol, 2-propanol, and acetonitrile were measured at 25°C by the Taylor–Aris dispersion method. The observed variations of the D values were interpreted in terms of solute–solvent hydrogen-bond interactions and the interpretation was supported by molecular dynamics simulations of two solutes in methanol.     

Direct determination of lead in produced waters from petroleum exploration by electrothermal atomic absorption spectrometry X-ray fluorescence using Ir-W permanent modifier combined with hydrofluoric acid

The present work reports the development of a methodology for the direct determination of lead in high saline waters derived from petroleum exploration employing electrothermal atomic absorption spectrometry with permanent Ir-W and HF as modifiers. These waters, so-called produced waters, have complex composition containing several types of organic and inorganic substances. In order to attain best conditions (highest analytical signal besides lowest background) for the methodology studies about the effect of several variables and the convenient calibration strategy were performed. Also, the efficiency of other modification approaches was evaluated. At best conditions, pyrolysis and atomization temperature were 800 and 2200 °C, respectively, when the modifiers cited above were utilized. Obtained results indicate that, in this kind of sample, lead can be determined by standard addition method or employing external calibration with standard solutions prepared in 0.8 mol l⁻¹ NaCl medium. In order to evaluate the accuracy of the procedure, a recovery test was performed with six spiked samples of produced waters. The detection limit, quantification limit and the relative standard deviation in 0.8 mol l⁻¹ NaCl were also calculated and the values are 1.5 μg l⁻¹, 5.0 μg l⁻¹ and 5.0% (at 10 μg l⁻¹ level), respectively.     

Cycle rank of Lyapunov graphs and the genera of manifolds

We show that the cycle-rank of a Lyapunov graph on a manifold satisfies: , where is the genus of . This generalizes a theorem of Franks. We also show that given any integer with , for some Lyapunov graph on .