Spectrophotometric determination of acidity and tautomeric constants and hydrogen bonding strength for a new Schiff base using hard modeling and multivariate curve resolution alternative least squares methods

The equilibria of a new Schiff base derived from 3,6-bis((aminoethyl)thio)pyridazine were studied spectrophotometrically with the aid of factor-analytical methods. Hard modeling program was used for determination of the acidity constants of the Schiff base in dimethylformamide (DMF)/water mixture (30:70 v/v). In this method acidity constant equations act as hard models and the score vectors obtained by decomposing of absorbance data matrix will be linear combinations of equilibrium concentrations of species that exist in the absorption matrix. Two rank annihilation factor analysis (TRAFA) was used as a standard method to investigate the accuracy of the method. The tautomerization constant, K_t, of the Schiff base solution in various DMF/water mixtures has also been determined using spectral variations of the Schiff base solutions in various volume ratios of water with the aid of evolving factor analysis (EFA) and multivariate curve resolution alternative least squares (MCR-ALS) methods. In addition the intramolecular hydrogen bonding strength and its related thermodynamic parameters have been determined using MCR-ALS and spectral variation of the Schiff base solutions in different temperatures.     

On the edge-density of 4-critical graphs

Gallai conjectured that every 4-critical graph on n vertices has at least 5/3n-2/3 edges. We prove this conjecture for 4-critical graphs in which the subgraph induced by vertices of degree 3 is connected.     

An initial-rate potentiometric method for the determination of uric acid using a fluoride ion-selective electrode

The principle of a general potentiometric method based on Emerson-Trinder reaction for the assay of various hydrogen peroxide generating systems is reported. Emerson-Trinder reaction, habitually employed as a spectrophotometric indicator reaction, is exploited in this method as a potentiometric indicator reaction. This method is based on the detection of F⁻ ions, liberated from the oxidation of a fluorophenol compound used as hydrogen-donor in Emerson-Trinder reaction, by a fluoride ion-selective electrode. The ability and usefulness of this method are illustrated by an initial-rate potentiometric determination of uric acid in aqueous and human serum samples, for which, initial-rate reaction progress curves, linear calibration curve, within-day precisions, upper and lower detection limits, and also its analytical recovery were reported.     

Photocatalytic Ring Opening of α-Epoxyketones: 1,3-Dioxolane Formation

Summary.  Photocatalytic ring opening of α-epoxyketones by 2,4,6-triphenylpyrylium tetrafluoroborate in acetone resulted in the formation of 1,3-dioxolanes as major products through C–O bond cleavage and the formation of alcoholic by-products through C–C bond cleavage. The type and nature of the substituent affects the rate of ring opening. Received June 19, 2001. Accepted (revised) November 28, 2001     

Optical parametric amplification beyond the slowly varying amplitude approximation

The coupled-wave equations describing optical parametric amplification (OPA) are usually solved in the slowly varying amplitude (SVA) approximation regime, in which the second-order derivatives of the signal and idler amplitudes are ignored and in fact the electromagnetic effects due to exit face of the medium is not involved. Here, an analytical plane-wave solution of these coupled-wave equations in a non-absorbing medium is presented. The solutions are derived beyond the SVA approximation up to order of κ/k (coupling constant over the wave number). The intensity distributions of the signal and the idler waves show a periodic behavior about their corresponding distributions of SVA-adapted solution. This behavior can be explained by the interference of the forward propagating signal (idler) wave and the corresponding backward one resulted from the reflection by the end face of the medium. Furthermore, this interference pattern in the medium can in turn serve as a periodic source for the next generations of the signal and idler waves. Therefore, the superposition of the waves, generated from different points of this periodic source, at the exit face of the medium shows an oscillatory behavior of the transmitted signal (idler) wave in terms of normalized coupling constant, κL. This study also shows that this effect is more considerable for high intensity pump beam, high relative refractive index and short length of the nonlinear medium.      

Hydrogen production by methanol steam reforming on a cordierite monolith reactor coated with Cu–Ni/LaZnAlO<Subscript>4</Subscript> and Cu–Ni/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

The performance of Cu–Ni/LaZnAlO₄ and Cu–Ni/γ-Al₂O₃ catalysts in the methanol reforming process in a monolith reactor in the temperature range of 200–350 °C, feed flow rate of WHSV = 20.8 h^?1 and atmospheric pressure has been investigated. In order to perform a more thorough investigation, surface area, morphology and crystalline structure of the synthetic catalysts have been studied using BET, FE-SEM, TPR, FT-IR, TEM, TGA and XRD analyses. The results have shown that Cu–Ni/LaZnAlO₄ catalyst synthesized by combustion reaction method under ultrasound irradiation has a very high efficiency and catalytic activity, low reduction temperature, high mechanical resistance and large pore sizes. The latter causes a higher percentage of active metal impregnation and better distribution on the support, greater resistance against sintering and maintenance of catalyst inertness at temperatures over 1000 °C, in comparison with conventional catalysts such as Cu–Ni/γ-Al₂O₃. This make its substitution for currently used catalysts affordable.     

Rapid Silylation of a Glass Surface: Choice of Reagent and Effect of Experimental Parameters on Hydrophobicity

The silylation of the surface of sodium borosilicate glass was investigated under conditions that might be applied to the fabrication of ion-selective microelectrodes of the liquid membrane type such as are used in physiology, Silylation was carried out for 15 min at different temperatures and with six different silanes. The hydrophobicity was measured and found to be greatest with reaction temperatures of 200–350°, the effectiveness of the different silylating agents being in the (increasing) order: trimethylchlorosilane, tributylchlorosilane, (dimethylamino)trimethylsilane, hexamethyldisilazane, dimethyldichlorosilane, bis(dimethylamino)dimethylsilane. The results give evidence of a catalytic effect of basic amine groups and show that the reactivity of the surface of freshly drawn glass is increased by acid leaching.     

Synthesis and antibacterial activity of Schiff bases of 5-substituted isatins

Based on the existing reports on the bioactive isatin derivatives, a number of Schiff bases were synthesized by reacting 5-substituted isatins with bioactive amines/hydrazides and their structures were confirmed using spectroscopic methods such as NMR, IR and mass spectrometry. Furthermore, Nbenzylation of isatin followed by the Schiff base formation furnished a new series of compounds(11a–13c) which allowed the analysis of the effect of isatin N-substitution on the bioactivity of the resulting compounds. The antibacterial activity of the synthesized derivatives was evaluated using a microtiter plate method on a series of gram positive and gram negative bacterial strains. Compounds 2d, 3b, 5c and 6a were among the most potent derivatives against Pseudomonas aeruginosa(MIC = 6.25 μg/m L).Analysis of the structure–activity relationship showed that the incorporation of(thio)urea-based Schiff bases lead to more potent derivatives with a broader spectrum of antibacterial activity. In addition,highly lipophilic compounds such as 11a–12c did not show any measurable antibacterial activity, which implies that an optimal lipophilicity might be an important requirement for the antibacterial activity of the studied isatins. Finally, the finding that hydantoin derivatives of N-benzylisatins(13a–13c) still exhibit some antibacterial activity prompted us to consider exploring the bioactivity of more diverse derivatives of isatin-aminohydantoin Schiff bases(compounds 1a–1d) in our future studies.     

Synthesis of water soluble quaternary chitosan derivative via protection-deprotection strategy and investigation of its antibacterial effect

New water-soluble trimethyl quaternary derivative of chitosan was synthesized by a multi step processes through protection-deprotection strategy. This derivative was characterized using FTIR and ¹H NMR spectroscopy. Finally, the antibacterial effect of this derivative against Gram-negative (Escherichia coli and Salmonella typhimurium) and Gram-positive (Staphylococcus aureus and Staphylococcus epidermidis) bacteria in acidic and nutral medium was invetigated. The results of this study demonstrated that this chitosan derivative did not have antibacterial activity against Gram-negative bacteria; however, it showed a relatively good antibacterical effect against Gram-positive bacteria in both acidic and neutral mediums.     

On the optimization of a vertical twisted tape arrangement in a channel subjected to MWCNT–water nanofluid by coupling numerical simulation and genetic algorithm

Journal of Thermal Analysis and Calorimetry - Heat transfer and fluid flow are optimized in a three-dimensional channel under the constant heat flux boundary condition employing a genetic algorithm...