Use of volatile organic solvents in headspace liquid‐phase microextraction by direct cooling of the organic drop using a simple cooling capsule

A low‐cost and simple cooling‐assisted headspace liquid‐phase microextraction device for the extraction and determination of 2,6,6‐trimethyl‐1,3 cyclohexadiene‐1‐carboxaldehyde (safranal) in Saffron samples, using volatile organic solvents, was fabricated and evaluated. The main part of the cooling‐assisted headspace liquid‐phase microextraction system was a cooling capsule, with a Teflon microcup to hold the extracting organic solvent, which is able to directly cool down the extraction phase while the sample matrix is simultaneously heated. Different experimental factors such as type of organic extraction solvent, sample temperature, extraction solvent temperature, and extraction time were optimized. The optimal conditions were obtained as: extraction solvent, methanol (10 μL); extraction temperature, 60°C; extraction solvent temperature, 0°C; and extraction time, 20 min. Good linearity of the calibration curve (R² = 0.995) was obtained in the concentration range of 0.01–50.0 μg/mL. The limit of detection was 0.001 μg/mL. The relative standard deviation for 1.0 μg/mL of safranal was 10.7% (n = 6). The proposed cooling‐assisted headspace liquid‐phase microextraction device was coupled (off‐line) to high‐performance liquid chromatography and used for the determination of safranal in Saffron samples. Reasonable agreement was observed between the results of the cooling‐assisted headspace liquid‐phase microextraction high‐performance liquid chromatography method and those obtained by a validated ultrasound‐assisted solvent extraction procedure.     

Characterizations of B and N heteroatoms as substitutional doping on structure,stability, and aromaticity of novel heterofullerenes evolved from the smallest fullerene cage C20: a density functional theory perspective

The structural stabilities and electronic properties of C₂₀ fullerene and some its incorporated boron and nitrogen derivatives are probed at B3LYP/AUG‐cc‐pVTZ level of theory. According to density functional theory results, the topology of inserted B or N heteroatoms in [20]‐fullerene perturbs strongly the stability, energy, geometry, charge, polarity, nucleus‐independent chemical shifts, aromaticity, and highest‐occupied molecular orbital and lowest‐unoccupied molecular orbital (HOMO–LUMO) gap of the resulting heterofullerenes. Vibrational frequency (υ_min) calculations show that except N₁₀C₁₀, all other B_bN_nC_{20‐(b + n)} heterofullerenes with b, and n = 0, 4, 5, 8, and 10 are true minima. The calculated band gaps (?E_HOMO–LUMO) of B₈C₁₂, and N₈C₁₂ (2.86 eV), show them the most stable heterofullerenes against electronic excitations. While 10 B substituting in equatorial position increase the conductivity of B₁₀C₁₀ through decreasing its band gaps, 10 N doping in equatorial position enhance stability of N₁₀C₁₀ against electronic excitations via increasing its band gaps. High natural bond orbital and Mulliken charge transfer on the surfaces of B atoms, especially B₅N₅C₁₀with five B–N bonds in the equatorial position, provokes further investigation on its possible application for hydrogen storage. Nucleus‐independent chemical shift values show that B₅N₅C₁₀ is the most aromatic species. The calculated heat of atomization per carbon (ΔH_at/C) of B₈C₁₂ shows it the most thermodynamic stable heterofullerenes of each. Copyright © 2016 John Wiley & Sons, Ltd.     

An efficient tandem aldol condensation-thia-Michael addition process

An efficient synthesis of β-aryl-β-mercapto ketones is achieved via a tandem aldol condensation-thia-Michael addition process using an aqueous medium and diethylamine. Addition of different thiols to α,β-unsaturated ketones, formed in situ from the condensation of acetophenone derivatives with aldehydes, led to a rapid and high yielding synthesis of the products under very mild conditions using no expensive additive or catalyst. Products which precipitated spontaneously in the reaction mixtures were separated by simple filtration and purified by recrystallization.     

Highly efficient synthesis of 5,6-disubstituted-5H-pyrrolo[2,3-b]pyrazine-2,3-dicarbonitriles through a one-pot palladium-catalyzed coupling reaction/cyclization in water

A highly efficient one-pot synthesis of 5,6-disubstituted-5H-pyrrolo[2,3-b]pyrazine-2,3-dicarbonitriles is presented. The reaction of 5-(alkyl-arylamino)-6-chloropyrazine-2,3-dicarbonitriles with phenylacetylene, catalyzed by Pd–Cu, in the presence of SDS as the surfactant in water, leads to the desired products in good-to-high yields.     

Resource allocation in OFDMA networks based on interior point methods

This paper studies a joint optimization problem of sub‐carrier assignment and power allocation in orthogonal frequency division multiple access (OFDMA) wireless networks. A major challenge in solving the optimization problem is non‐convexity caused by the combinatorial nature of sub‐carrier assignment problem and/or non‐convex objective functions. To address the combinatorial complexity, we formulate the resource allocation problem as an optimization problem with continuous variables. We propose a novel approach based on a penalty function method and an interior point method (PM/IPM) to solve the problem. In specific, using a two‐step implementation, the penalty method is applied first to convert the non‐convex feasible region to a convex one. Then, the interior point method is deployed to solve the problem which is non‐convex only in the objective function. To evaluate the performance of PM/IPM, we apply a genetic algorithm (GA) that achieves near optimal solutions of the problem by iterative searching. Copyright © 2009 John Wiley & Sons, Ltd.     

An investigation into the role of polarity of diamines in controlling the electron migration mechanism of photoluminescent poly(amide–imide)s

Several diamines with remarkable different polarities were used to produce photoactive poly(amide–imide)s (PAI)s in a quantitative yield. The absorption, fluorescence and photophysical properties of series of poly(amide–imide)s containing fused aromatic systems as energy donor and energy acceptor with different diamines cores are described. Poly(amide–imide)s exhibit broad fluorescent characteristic, and its fluorescent intensity is related to the intermolecular chain–chain or chain–solvent interaction. The fluorescence spectra confirmed an efficient singlet–singlet energy transfer between fused aromatic systems. The self-quenching mechanism was studied according to the specific behavior of these polymers in different solvents. The self-quenching rate constant for the association reaction in the excited state (Kq) could be measured from the Stern–Volmer equation. The kind of fused system and diamines show different electron migration mechanisms and photoluminescent properties in the singlet-excited states. By using the exothermic energy transfer as a function of diamine polarity, the electron transfer mechanism was evaluated for aromatic poly(amide–imide)s. In principle, the fluorescence energy is absorbed by different (PAI)s and raises the molecules to one of its excited states. Afterwards this excitation energy transfers through the different relaxation channels, i.e. columbic or exchange energy transfer.     

A Simple Method for Melatonin Determination in the Presence of High Levels of Tryptophan using an Unmodified Carbon Paste Electrode and Square Wave Anodic Stripping Voltammetry

Electrochemical determination of melatonin in the presence of tryptophan is a challenge because of the coincidence of voltammetric signals of these compounds when executing a voltammetric technique. The new method for selective determination of melatonin based on the square wave anodic stripping voltammetry determination of an electroactive product of melatonin was suggested here. This product is produced by previously applied positive pre-potential to a carbon paste electrode, immersed in the test solution. By this means, the electrochemical signal of melatonin was separated effectively from that of tryptophan, making it possible to determine melatonin in the presence of a high concentration of tryptophan. The effect of important parameters on electrode performance was studied and optimized. The optimum response was obtained at pH=2 and utilizing the pre-potential magnitude of +0.8 V, applied for 10 s. A linear relationship was found between peak current intensity and melatonin concentrations over the range of 5.00×10⁻⁷ to 8.00×10⁻⁵ mol L⁻¹. A detection limit of 8.30×10⁻⁸ mol L⁻¹ was calculated for the method (S/N=3). The selectivity of the method was considerably high, because of the independence of melatonin signal to the presence of higher amounts of some potentially interfering agents such as ascorbic acid, tryptophane glucose, etc. As an analytical application, the proposed sensor was used for the determination of melatonin in pharmaceutical and food samples.