KINETIC INVESTIGATION OF THE PHOTOLYSIS OF 2-OXO-2,5-DIHYDRO-1,3,4-OXADIAZOLES

Abstract. –A reaction scheme for the photolysis of 2-oxo-2,5-dihydro-1,3,4-oxadiazoles is given by kinetic analysis, quantum yield measurement and product determination. The different reaction pathways in acetonitrile and ether are discussed.     

Cooperative effects in the photophysical properties of self-associated triguanosine diphosphates

The present study deals with the photophysical properties of triguanosine diphosphate in aqueous solutions, which are compared with those of the 2'-deoxyguanosine monophosphate. They are studied by steady-state absorption and fluorescence spectroscopy as well as by time-resolved fluorescence spectroscopy with femtosecond resolution. The temperature, salt and concentration dependence of the absorption and fluorescence spectra reveal that association of the trimers takes place. The resulting aggregates could correspond to a tetraplex structure. The aggregate fluorescence quantum yield is higher and the fluorescence lifetime much longer than those of the monomer. These results show the interaction between guanosine residues that may manifest itself via self-solvation, hydrogen bonding and/or delocalization of the excitation.     

Stable isotope dilution analysis of wine fermentation products by HS-SPME-GC-MS

The aim of this study was to quantify, in a single analysis, 31 volatile fermentation-derived products that contribute to the aroma of red and white wine. We developed a multi-component method based on headspace solid-phase microextraction coupled with gas chromatography mass spectrometry (HS-SPME-GC-MS). The 31 volatile compounds analysed include ethyl esters, acetates, acids and alcohols. Although these compounds have a range of functional groups, chemical properties, volatilities, affinities for the SPME fibre, and are found in wine at various concentrations, the accuracy of the analysis was achieved with the use of polydeuterated internal standards for stable isotope dilution analyses (SIDA). Nine of the labelled standards were commercially available, while 22 were synthesised. The method was validated by a series of duplicate spiked standard additions to model, white and red wine matrices over the concentration range relevant for each compound in wine. This demonstrated that the appropriate use of SIDA helped to account for matrix effects, for instance potential sources of variation such as the relative response to the MS detector, ionic strength, ethanol content and pH of different wine matrices. The resultant calibration functions had correlation coefficients (R²) ranging from 0.995 to 1.000. Each compound could be quantified at levels below its aroma threshold in wine. Relative standard deviations were all <5%. The method was optimised for the best compromise (over the 31 compounds) of wine dilution factor, level of sodium chloride addition, SPME fibre, SPME temperature, SPME time, GC column and MS conditions. Confirmation of identity was achieved by retention time and peak shape, and measurement of at least three ions for each analyte and internal standard with the MS operating in selected ion monitoring mode to facilitate more precise quantitation with a high sampling rate. The method is a valuable research tool with many relevant applications. A novel method for the combined chiral separation and SIDA quantification of 2- and 3-methylbutanoic acid is also demonstrated.     

Factors affecting multiresidue determination of priority herbicides when using solid-phase microextraction

A solid-phase microextraction (SPME) procedure was developed for the determination of 10 selected organonitrogen herbicides (s-ethyl dibropylthiocarbamate [EPTC], molinate, propachlor, trifluralin, simazine, atrazine, propazine, terbuthylazine, alachlor, and prometryn) and was tested with various natural waters. Gas chromatography coupled with flame thermionic and mass spectrometric detection was used for quantitation. For this purpose, polydimethylsiloxane and polyacrylate fibers were used and the factors affecting the SPME process such as pH, ionic strength, methanol content, memory effect, stirring rate, and adsorption-time profile were investigated and optimized. By using spiked liquid chromatography water, optimal factors were determined to be 25% salt, <0.5% methanol, stirring rate of 960 rpm, pH 4, and an equilibrium time of 30 min. These conditions were used in further studies of the fibers and in analysis of natural water samples. The method was applied to spiked natural waters such as ground water, sea water, lake water, and river water at a concentration range of 0.5-10 microg/L. Limits of detection ranged from 5 to 90 ng/L, and precision ranged from 5 to 15% (as relative standard deviation), depending on the pesticide, fiber, and detector used. The recoveries of herbicides were 70.2-118.4%, and the average r2 values of the calibration curves were >0.99 for all analytes. The results demonstrate the suitability of the SPME method to determine these organonitrogen herbicides in various natural waters. River water samples originating from the Epirus region (Northwestern Greece) were analyzed to verify the performance of the optimized method by comparing the results obtained by SPME with those obtained by using conventional solid-phase extraction of the selected herbicides.     

The effect of β-cyclodextrin on tenoxicam photostability, studied by a new liquid chromatography method; the dependence on drug dimerisation

Tenoxicam (TXM) is an effective anti-inflammatory and analgesic drug, which presents fast photochemical decomposition. In this work in an attempt to investigate the potential β-CD photostabilizing effect on TXM, the photodegradation rate of β-CD complexed drug was monitored under simulated solar irradiation from Xenon arc lamp. The photodegradation was studied at pH 7.5. A new stability indicating Liquid Chromatography method, for TXM in the presence of β-CD was used. According to the obtained results, in the case of free molecules increasing the concentration the photostability is enhanced. The effect of complexation with CDs on the photodegradation rate seems to vary depending on TXM initial concentration. At low TXM concentrations photodecomposition is retarded upon CD complexation, while at high concentrations the process is accelerated. Molecular dimerisation was studied by ¹H(1D) NMR and 2D NOESY experiments. 2D ROESY spectra of complexed molecule were evaluated in order to confirm the complexation. TXM dimers could be considered as a critical parameter affecting oxicams photostability, in combination with the already described ESIPT phenomenon.     

Application of hollow fiber liquid phase microextraction for the determination of insecticides in water

In the present work, a novel sample pre-treatment technique for the determination of trace concentrations of some insecticide compounds in aqueous samples has been developed and applied to the determination of the selected analytes in environmental water samples. The extraction procedure is based on coupling polypropylene hollow fiber liquid phase microextraction (HF-LPME) with gas chromatography by flame thermionic detection (GC-FTD). For the development of the method, seven organophosphorous insecticides (dichlorvos, mevinphos-cis, ethoprophos, chlorpyrifos methyl, phenthoate, methidathion and carbofenothion) and one carbamate (carbofuran) were considered as target analytes. Several factors that influence the efficiency of HF-LPME were investigated and optimized including agitation, organic solvent, sample volume, exposure time, salt additives and pH. The optimized methodology exhibited good linearity with correlation coefficient = 0.990. The analytical precision for the target analytes ranged from 4.3 to 11.1 for within-day variation and 4.6 to 12.0% for between-day variation. The detection limits for all analytes were found in the range from 0.001 to 0.072 microg/L, well below the limits established by the EC Drinking Water Directive (EEC 80/778). Relative recoveries obtained by the proposed method from drinking and river water samples ranged from 80 to 104% with coefficient of variations ranging from 4.5 to 10.7%. The present methodology is easy, rapid, sensitive and requires small sample volumes to screen environmental water samples for insecticide residues.     

On the mechanism initiating bursting oscillatory patterns during the pitting corrosion of a passive rotating iron-disc electrode in halide-containing sulphuric acid solutions

Bursting current oscillations (trains of current spikes interrupted by a steady-state passive current) were found to occur during the iron anodic polarization in halide-containing sulphuric acid solutions. These oscillations occur at potentials located within a partially passive state of the iron. The partially passive state appears at potentials more positive than the unstable passive-active transition region of the current-potential curve of the Fe|2 M H₂SO₄ system for halide concentrations approximately ranged between 20 and 50 mM. The bursting oscillations appear after a certain induction period which ranged from minutes to several hours. Experiments were conducted in which an external resistance was connected in series between the ground and the working electrode for the Fe|2 M H₂SO₄ + 30 mM Br⁻ system. The results demonstrate that the appearance of oscillations is closely related to an IR-drop mechanism operating during the pitting corrosion of Fe induced by halides. When an appropriate value of the external resistance was imposed in the system the induction period was entirely eliminated. Bursting oscillations appear at once after setting the potential at the desired value. These results provide strong evidence that during the anodic electrodissolution of iron in halide-containing sulphuric acid solutions bursting current oscillations will be induced even though the electrode seems to be in the partially passive state. This is expected to occur after the passage of an induction period during which the system acquires a critical value of the ohmic potential drop. Received: 20 October 1997 / Accepted: 2 January 1998     

Coordination compounds of palladium(II) and platinum(II) with benzotriazoles

Summary The nitrogen-donor ligands 1-methylbenzotriazole (1Mebta), 5-methylbenzotriazole (5MebtaH), 5-chlorobenzotriazole (5ClbtaH) and 5-nitrobenzotriazole (5NO₂btaH) react with palladium(II) and platinum(II) to give cis-[PdL₂Cl₂], cis-[PtL₂Cl₂] (L = 1Mebta, 5MebtaH, 5ClbtaH or 5NO₂btaH), [Pt(5ClbtaH)₄]Cl₂, [Pd-(5MebtaH)Cl₂]₂, [Pd(5ClbtaH)Cl₂]₂ and [Pd(5NO₂btaH)-Cl₂]₂. The complexes were characterized by physicochemical and spectroscopic methods. The benzotriazoles act as monodentate ligands binding through N(3). Monomeric square planar structures are assigned for the 12 complexes and [Pt(5ClbtaH)₄]Cl₂ in the solid state. Centrosymmetric, chloro-bridged, dinuclear square planar structures of C_2h symmetry are proposed for the 11 palladium(II) compounds.     

Microwave-promoted Suzuki-Miyaura cross-coupling of aryl halides with phenylboronic acid under aerobic conditions catalyzed by a new palladium complex with a thiosemicarbazone ligand

A new air- and moisture-stable palladium complex with salicylaldehyde N(4)-hexamethyleneiminylthiosemicarbazone has been synthesized. According to its crystal structure, the metal is bonded to 2 equiv monoanionic thiosemicarbazone moieties in a N,S-bidentate fashion, forming two five-membered chelate rings, while additional intramolecular bonds stabilize the structure. In contrast to other palladium complexes with thiosemicarbazones, this complex was inactive towards the Suzuki-Miyaura coupling under aerobic conditions, by conventional heating. On the other hand, microwave irradiation promoted the effective catalytic activity of the complex for the coupling of aryl bromides and chlorides with phenylboronic acid in DMF/H₂O, under aerobic conditions, with turnover numbers of up to 37,000.