Lyapunov–Razumikhin method for impulsive functional differential equations and applications to the population dynamics

The present paper deals with the investigation of the stability of the zero solution of impulsive functional differential equations. By means of piecewise continuous functions coupled with the Razumikhin technique sufficient conditions for stability, uniform stability and asymptotic stability of the zero solution of such equations are found.     

Mechanism of No Conversion Over a Coprecipitated CuO-MnOx Spinel-Like Catalyst

A copper-manganese oxide catalyst prepared by coprecipitation of copper and manganese hydroxides with a Cu:Mn ratio of 1:1 was investigated in order to elucidate the mechanism of NO conversion with CO. A scheme for the NO-CO interaction based on the redox couple Cu²⁺ +Mn³⁺ Cu⁺+Mn⁴⁺ is proposed.     

Conformational Studies of Dibenzo-30-crown-10 Complexes. Syntheses and Crystal Structures of Potassium and Ammonium Hexafluorophosphate Complexes

Abstract

Crown ether complexes formed by the dibenzo–30-crown–10 (DB30C10) with potassium and ammonium hexafluorophosphate have been prepared and their crystal structures have been determined by single crystal X-ray analyses. The potassium complex (compound 1) consists of [K(DB30C10)]⁺ cation and PF₆ ^? anion. Crystals are monoclinic, space group P2/n, with a = 11.9106(3), b = 9.8382(5), c = 14.3062(3) Å, β = 97.581(3)°, V = 1661.7(1) ų, D_c = 1.440 g cm^?3, Z = 4, R = 0.0675 for 2528 unique observed reflections. The potassium atom is coordinated to the ten oxygen atoms of the crown ligand at the distance from 2.859(3) to 2.930(3) Å. The ammonium complex (compound 2) has also 1:1 crown—cation ratio. Crystals are monoclinic, space group P2₁/n, with a = 12.5061(6), b = 19.3724(5), c = 14.2203(9) Å, β = 102.476(5)°, V = 3363.8(3) ų, D_c = 1.501 g cm^?3, Z = 4, R = 0.0677 for 4172 unique observed reflections. The ammonium cation is completely enclosed with crown oxygen atoms forming seven hydrogen bonds. The conformation of previously reported dibenzo-30-crown-10 complexes with potassium salts were investigated using polar coordinate maps.     

Speciation of trace metals in natural waters: the influence of an adsorbed layer of natural organic matter (NOM) on voltammetric behaviour of copper

The influence of an adsorbed layer of the natural organic matter (NOM) on voltammetric behaviour of copper on a mercury drop electrode in natural water samples was studied. The adsorption of NOM strongly affects the differential pulse anodic stripping voltammogram (DPASV) of copper, leading to its distortion. Phase sensitive ac voltammetry confirmed that desorption of adsorbed NOM occurs in general at accumulation potentials more negative than −1.4 V. Accordingly, an application of negative potential (−1.6 V) for a very short time at the end of the accumulation time (1% of total accumulation time) to remove the adsorbed NOM was introduced in the measuring procedure. Using this protocol, a well-resolved peak without interferences was obtained. It was shown that stripping chronopotentiogram of copper (SCP) in the depletive mode is influenced by the adsorbed layer in the same manner as DPASV. The influence of the adsorbed NOM on pseudopolarographic measurements of copper and on determination of copper complexing capacity (CuCC) was demonstrated. A shift of the peak potential and the change of the half-peak width on the accumulation potential (for pseudopolarography) and on copper concentration in solution (for CuCC) were observed. By applying a desorption step these effects vanished, yielding different final results.     

Global Mittag-Leffler stability and synchronization of impulsive fractional-order neural networks with time-varying delays

In this paper we consider a class of impulsive Caputo fractional-order cellular neural networks with time-varying delays. Applying the fractional Lyapunov method and Mittag-Leffler functions, we give sufficient conditions for global Mittag-Leffler stability which implies global asymptotic stability of the network equilibrium. Our results provide a design method of impulsive control law which globally asymptotically stabilizes the impulse free fractional-order neural network time-delay model. The synchronization of fractional chaotic networks via non-impulsive linear controller is also considered. Illustrative examples are given to demonstrate the effectiveness of the obtained results.     

Mercury determination and speciation analysis in surface waters

A sorbent L-cysteine grafted silica gel has been evaluated for separation and enrichment of dissolved inorganic i-Hg(II) and methylmercury CH₃Hg(I) from surface waters at sub-μg L⁻¹ concentrations. Chemical parameters for mercury species enrichment and separation have been optimized. Analytical schemes for the determination of Hg species, using selective column solid phase extraction (SPE) with continuous flow chemical vapor generation atomic absorption spectrometry (CF-CVG-AAS) or inductively coupled plasma-mass spectrometry (ICP-MS) were developed. Possibilities for on-site SPE enrichment were demonstrated as well. The limits of quantification were 1.5 and 5 ng L⁻¹ for dissolved i-Hg(II) and CH₃Hg(I) by CF-CVG-AAS and 1 and 2.5 ng L⁻¹ by ICP-MS with relative standard deviations between 7–12% and 7–14%, respectively. The chemically modified SPE sorbent has demonstrated high regeneration ability, chemical and mechanical stability, acceptable capacity and good enrichment factors. Results for total dissolved mercury were in reasonable agreement with those from independent analyses by direct ICP-MS determinations for river waters and for estuarine water certified reference material.      

Lignans and Alkaloids from Haplophyllum suaveolens

Summary.  A new lignan of the dibenzylbutyrolactone type, haplotonin, was isolated from the aerial parts of Haplophyllum suaveolens together with five known compounds-the alkaloids flindersine, 6-methoxyflindersine, N-acetoxymethylflindersine, and evoxine and the lignan 4-acetyldiphyllin. This is the first report on the occurrence of N-acetoxymethylflindersine in the genus Haplophyllum and on 6-methoxyflindersine and 4-acetyldiphyllin as constituents of the investigated species. Received August 10, 1999. Accepted (revised) October 12, 1999     

Determination of metal traces in wine by argon stabilized d.c. arc

A spetroscopic method for the determination of metal traces (Mn, Cu, Fe) in wine has been developed based on argon stabilized d.c. arc plasma at atmospheric pressure. The experimental conditions were optimized using lateral distributions of spectral line intensities of the trace elements in aqueous and ethanol-aqueous solutions. The method was applied to the analysis of 6 wines from 3 Serbian wine-growing regions. Direct and standard addition methods were tested. The precision of the method is characterized by a relative standard deviation of 0.50– 3.00%. The accuracy of the method was assessed by flame AAS. Received: 8 October 1998 / Revised: 17 March 1999 / Accepted: 20 March 1999     

Methyl 3,6‐di‐O‐pivaloyl‐α‐d‐manno­pyran­oside

The X‐ray crystal structure analysis of the title compound, C₁₇H₃₀O₈, revealed a ⁴C₁ conformation of the pyran­osyl ring [Cremer–Pople puckering parameters of Q = 0.568 (2) Å, θ = 5.1 (2) and ϕ = 218 (3)°]. The structure shows no deviations from the geometric parameters of pyran­oside carbohydrates. The hydroxyl groups participate in O—H⃛O hydrogen bonds, forming a two‐dimensional pattern [O⃛O = 2.811 (3) and 2.995 (3) Å].