Three limit cycles for a first‐order exothermic chemical reaction in a continuous stirred‐tank reactor

We study a first‐order exothermic chemical reaction in a continuous stirred‐tank reactor modelled by a 3‐parameter family of vector fields in . We prove that there exist regions in which contain points that depend on parameters such that the chemical reaction has 0, 1, 2, or 3 small amplitude limit cycles that surround the origin. We conclude that this model can reach two stable small amplitude limit cycles. Finally, we show that one of these regions contains the point in the parameter space considered by Gurel and Lapidus [6] who proved numerically the existence of one stable limit cycle. This revised version was published online in July 2006 with corrections to the Cover Date.     

The elimination kinetics and mechanisms of ethyl piperidine‐3‐carboxylate,ethyl 1‐methylpiperidine‐3‐carboxylate,and ethyl 3‐(piperidin‐1‐yl)propionate in the gas phase

The gas‐phase elimination kinetics of the above‐mentioned compounds were determined in a static reaction system over the temperature range of 369–450.3°C and pressure range of 29–103.5 Torr. The reactions are homogeneous, unimolecular, and obey a first‐order rate law. The rate coefficients are given by the following Arrhenius expressions: ethyl 3‐(piperidin‐1‐yl) propionate, log k₁(s^?1) = (12.79 ± 0.16) ? (199.7 ± 2.0) kJ mol^?1 (2.303 RT)^?1; ethyl 1‐methylpiperidine‐3‐carboxylate, log k₁(s^?1) = (13.07 ± 0.12)–(212.8 ± 1.6) kJ mol^?1 (2.303 RT)^?1; ethyl piperidine‐3‐carboxylate, log k₁(s^?1) = (13.12 ± 0.13) ? (210.4 ± 1.7) kJ mol^?1 (2.303 RT)^?1; and 3‐piperidine carboxylic acid, log k₁(s^?1) = (14.24 ± 0.17) ? (234.4 ± 2.2) kJ mol^?1 (2.303 RT)^?1. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene through a concerted six‐membered cyclic transition state type of mechanism. The intermediate β‐amino acids decarboxylate as the α‐amino acids but in terms of a semipolar six‐membered cyclic transition state mechanism. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 106–114, 2006     

Some characterizations of Riesz operators by means of invariant subspaces

In this paper we shall give several characterizations of Riesz operatorsT on Banach spaces by means of some closedT-invariant subspaces. Moreover we also give a characterization of these operators dual to that give by Dieudonné in [5].     

Synthesis, gastroprotective effect and cytotoxicity of new amino acid diterpene monoamides and diamides

Following our studies on the gastroprotective effect and cytotoxicity of terpene derivatives, new amides were prepared from the diterpene 8(17)-labden-15,19-dioic acid (junicedric acid) and its 8(9)-en isomer with C-protected amino acids (amino acid esters). The new compounds were evaluated for their gastroprotective effect in the ethanol/HCl-induced gastric lesions model in mice, as well as for cytotoxicity using the following human cell lines: normal lung fibroblasts (MRC-5), gastric adenocarcinoma cells (AGS) and liver hepatocellular carcinoma (Hep G2). A dose-response experiment showed that at 25 mg/kg the C-15 leucyl and C-15,19-dileucylester amides of junicedric acid reduced gastric lesions by about 65.6 and 49.6%, respectively, with an effect comparable to lansoprazole at 20 mg/kg (79.3% lesion reduction). The comparison of the gastroprotective effect of 18 new amino acid ester amides was carried out at a single oral dose of 25 mg/kg. Several compounds presented a strong gastroprotective effect, reducing gastric lesions in the 70.9-87.8% range. The diprolyl derivative of junicedric acid, the most active product of this study (87.8% lesion reduction at 25 mg/kg) presented a cytotoxicity value comparable with that of the reference compound lansoprazole. The structure-activity relationships are discussed.     

An efficient enzymatic preparation of 20-pregnane succinates: chemoenzymatic synthesis of 20β-hemisuccinyloxy-5αH-pregnan-3-one

Lipase-catalyzed transesterification of the 20 hydroxyl group in a series of pregnanes afforded novel 20-ethyl succinates that are not possible to prepare following the traditional synthetic methods. The reaction is stereoselective. The enzyme reacts selectively with the 20β epimers therefore only the 20β-succinyloxy derivatives are obtained. These compounds are obtained in variable yield, depending on the substitution in the ring A. The enzymatic approach allowed, for the first time, the synthesis of 20β-hemisuccinyloxy-5αH-pregnan-3-one, novel compound useful as a precursor of steroid-protein conjugates.     

FOAMING PROPERTIES OF SURFACTANT-OIL-VATER SYSTEMS IN THE NEIGHBOURHOOD OF OPTIMUM FORMULATION

The foaming properties of. surfactant-oil-water systems are found to be closely related to the phase behavior at equilibrium. In Winsor I systems both the surfactant-oil-water emulsion and its aqueous phase produce stable foams. The foaminess and the foam stability follow the same pattern of variation than the emulsion stability. In Winsor III (three phase microemulsion-oil-water) and Winsor II systems no foam can be formed. The changes are the same no matter the formulation variable used to scan the physico-chemical formulation or the method used to measure the foam stability.     

Synthesis and pharmacological activity of diterpenylnaphthoquinone derivatives

New diterpenylquinones, combining a diterpene diacid and a naphthoquinone, were prepared from junicedric acid and lapachol. The new derivatives were assessed as gastroprotective agents by the HCl-EtOH-induced gastric lesions model in mice as well as for basal cytotoxicity on the following human cell lines: Normal lung fibroblasts (MRC-5), gastric epithelial adenocarcinoma (AGS), and hepatocellular carcinoma (Hep G2). Several of the new compounds were significantly active as antiulcer agents and showed selective cytotoxicity against AGS cells.     

New pyrazolino and pyrrolidino[60]fullerenes: the introduction of the hydrazone moiety for the formation of metal complexes

The [3 + 2] cycloaddition reaction of C₆₀ with pyridine‐derived hydrazones (acting as dipolar reagents) was successfully conducted resulting in fullerene derivatives 5a , 5b . The compounds were characterized by means of NMR, UV–Vis spectroscopy, and X‐ray crystallography. The electrochemical behavior was also investigated. The fulleropyrazoline 5a exhibits anodically shifted reduction potentials of about 100 mV when compared with those for C₆₀, whereas 5b exhibits cathodic shifts relative to pristine C₆₀. The complexation reaction of 5b with metallic ions (Zn²⁺, Cd²⁺, and Fe²⁺) was achieved. Job and Benesi–Hildebrand analysis confirmed the formation of complexes with a molar ratio of 1:1 and binding constants between 2.26 × 10⁵ and 1.59 × 10⁵ M^?1. Electrochemistry of these complexes showed a marked influence of the metal ion on the reduction potentials. Copyright © 2016 John Wiley & Sons, Ltd.     

A new inversion-free method for a rational matrix equation

Motivated by the classical Newton-Schulz method for finding the inverse of a nonsingular matrix, we develop a new inversion-free method for obtaining the minimal Hermitian positive definite solution of the matrix rational equation X+A^∗X^-1A=I, where I is the identity matrix and A is a given nonsingular matrix. We present convergence results and discuss stability properties when the method starts from the available matrix AA^∗. We also present numerical results to compare our proposal with some previously developed inversion-free techniques for solving the same rational matrix equation.     

On understanding the effect of benzotriazole during barrier-film growth on Al-Cu alloys

Abstrasct The effect of benzotriazole on the growth of anodic films on aluminium and an Al-3.5 wt% Cu alloy has been examined by transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS). Films grown to relatively high voltages reveal flaws and oxygen-filled voids as a result of the surface morphology and copper enrichment and oxidation at the alloy/film interface. Benzotriazole, apart from increasing the efficiency of anodic film growth on the artificially aged alloy, also reduces the population density of the local features. The impedance response of films formed at relatively low voltages has been used to probe the early influence of benzotriazole on the macroscopic surface as well as flaws of the oxide-covered substrate.