Manganese dioxide graphite composite electrodes: application to the electroanalysis of hydrogen peroxide, ascorbic acid and nitrite.

The modification of carbon powder with manganese dioxide using a wet impregnation procedure with electrochemical characterisation of the modified powder is described. The process involves saturation of the carbon powder with manganese(II) nitrate followed by thermal treatment at ca. 773 K leading to formation of manganese(IV) oxide on the surface of the carbon powder. The construction of composite electrodes based on manganese dioxide modified carbon powder and epoxy resin is also described, including optimisation of the percentage of the modified carbon powder. Composite electrodes showed attractive performances for electroanalytical applications, proving to be suitable for the electrochemical detection of hydrogen peroxide, ascorbic acid and nitrite ions with limits of detection comparable to the detection limits achieved by other analytical techniques. The results obtained for detection of these analytes, together with composite electrodes flexible design and low cost offers potential application of composite electrodes in biosensors.     

New 3,4-Annelated Coumarin Derivatives: Synthesis, Antimicrobial Activity, Antioxidant Capacity, and Molecular Modeling

The one pot reaction preparation, spectral analysis, and molecular modeling experiments on the new 3,4-annelated coumarin systems with bioactivity associated structural features are described. These provided the insight into the equilibrium of the respective tautomeric forms making possible the reconciliation of previously published spectral data with the structure assignments as well as the correction of erroneously established structures. The synthesized compounds were tested in a standard disk diffusion assay and displayed strong to moderate antimicrobial activity, with a promising new lead prototype compound (7-amino-9-hydroxy-5-oxa-7a,8,11-triazacyclopenta[b]phenanthren-6-one (5)) possessing greater activity that the antibiotics Ampicillin and Nystatin. Antioxidant capacities of the compounds, determined spectrophotometrically using a phosphomolybdenum method, were greater, and in the case of compound 5 four times the activity of α-tocopherol acetate.     

Voltammetric and titrimetric study of acid-base properties of some μ-oxo dimeric iron(III) complexes inDMF

Summary The acid-base properties of the -oxo bridged dimeric iron complexes [FeL]₂O with the ligands based on S-alkyl-1,4-bis(substituted salicylidene)isothiosemicarbazide and a tetramer with S-methyl-1,4-bis(salicylidene)isothiosemicarbazide, {[FeL]₂O}₂I₃·I₂, were investigated by cyclic voltammetry at glassy carbon electrode inDMF. Studies were carried out in the presence of either a weak (phenol) or a strong (HClO₄ aq.) acid. The stoichimoetry of the reaction, changes in the general voltammetric pattern and the electrode reaction mechanism were discussed. These studies served as the basis for three-electrode amperometric titrations to determine the content of several of these complexes.

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Voltammetrische und titrimetrische Untersuchung der Säure-Base-Eigenschaften einiger -oxo dimerer Eisen(III)-Komplexe inDMF-Lösungen

Zusammenfassung Die Säure-Base-Eigenschaften einiger dimerer -oxo-Eisen(III)-Komplexe [FeL]₂O mit Liganden auf Basis von 1,4-bis(substituierten Salicyliden)S-(alkyl)isothiosemicarbaziden sowie eines tetrameren Komplexes mit 1,4-Bis(salicyliden)S-methylisothiosemicarbazid, {[FeL]₂O}₂I₃·I₂, wurden mittels zyklischer Voltammetrie an der Glaskohlenstoffelektrode inDMF-Lösungen untersucht. Die Messungen wurden in Gegenwart entweder einer schwachen (Phenol) oder einer starken (HClO₄ aq.) Säure ausgeführt. Die Stöchiometrie der Reaktion, die Änderungen der voltammetrischen Kurven und der Elektroden-Reaktionsmechanismus wurden diskutiert. Diese Untersuchungen stellten eine Basis für Drei-Elektroden amperometrische Titrationen dar, die für die Bestimmung des Gehalts einiger dieser Komplexe angewendet wurden.

     

Investigation of photoelectrical properties and relaxation dynamics in photoexcited CdSe nanocrystals in thin film form

Electrical and photoelectrical properties of cubic CdSe nanocrystals in thin film form (including the relaxation dynamics of photocarriers) are investigated. Photoelectrical properties of the obtained films are controlled by chemical (varying the reagent concentration in the reaction system) and physical means (controlling the crystal dimensions). In the case of thin films with optimal photoelectrical properties, the calculated band gap energy and ionization energies of impurity levels (on the basis of experimentally obtained temperature dependence of dark electrical resistance) at 0 K are 1.85, 0.74 and 0.43 eV, correspondingly. The calculated optical band gap energy (on the basis of spectral dependence of photoconductivity) at room temperature of 1.75 eV is in excellent agreement with the value of 1.77 eV which is obtained on the basis of electronic absorption spectrum in the framework of parabolic approximation for dispersion relation. Upon thermal treatment of chemically deposited thin films of cubic CdSe quantum dots, as a result of processes of coalescence and crystal growth, the electronic contact between nanocrystals increases and the confinement effects irreversibly disappear. Relaxation of non-equilibrium charge carriers is practically carried out according to the linear mechanism. The calculated relaxation time of photoexcited charge carriers is 0.4 ms.     

An improved procedure for phenylselenoetherification of some Δ5-alkenols using pyridine,Ag2O,and some Lewis acids as catalysts

An improved procedure for intramolecular cyclization of some Δ⁵-alkenols, using PhSeX (X = Cl, Br) has been developed. We found that cyclization can be facilitated in the presence of pyridine, Ag₂O, and some Lewis acids as catalysts. Thus catalytic amount of additives (pyridine and Ag₂O) influences higher yields but equimolar amount achieves almost quantitative yield under extremely mild experimental conditions. In the presence of Lewis acids (ZnCl₂ and FeCl₃) high yields of cyclic ether products are obtained with catalytic amounts. © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:146–149, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10227     

Metal ion-molecule kinetics at combustion temperatures: The reaction of Ca with O2

The high-temperature photochemistry (HTP) technique, previously used for reactions of neutral species, has been adapted to the study of atomic metal ion-molecule reactions. Ca⁺ ions were generated by 193 nm multi-photon photolysis of calcium acetyl acetonate and its pyrolysis fragments. The relative ion concentrations were monitored by laser-induced fluorescence at 393.4 nm. Ar was used as the bath gas. The data for the Ca⁺ + O₂ + M → CaO₂⁺ + M association reaction (1) are fitted by k₁(907-1425 K) = 3.5 × 10⁻³² exp(+3161 K/T) cm⁶ molecule⁻² s⁻¹. Combining with an approximate k₁(296 K) value in the literature leads to k₁(296-1425 K) = 5.8 × 10⁻²² (T/K)^−2.9 exp(−601 K/T) cm⁶ molecule⁻² s⁻¹. Over much of the observed temperature range reaction (1) has much smaller rate coefficients than the corresponding neutral Ca association reaction. Reaction (1) is shown to behave very similarly to the O₂ association reaction with neutral K atoms, with which Ca⁺ is iso-electronic. This suggests that the initial step is ion-pair complex formation of the superoxide Ca²⁺(O₂)⁻, which is also consistent with results from density functional calculations. The k₁ values are rationalized via Troe’s unimolecular formalism, which leads to good accord with the experiments.     

Chemoenzymatic Synthesis of Stannylated Metomidate as a Precursor for Electrophilic Radiohalogenations – Regioselective Alkylation of Methyl 1<Emphasis Type="Italic">H</Emphasis>-Imidazole-5-carboxylate [1]

Summary. Metomidate (ee 99%) substituted with iodine in the phenyl ring was prepared from (S)-1-(4-iodophenyl)ethanol (ee >98%) obtained by lipase-catalysed resolution and methyl 1H-imidazole- 5-carboxylate. The two fragments were joined highly regioselectively (alkylation only at N-1 of substituted imidazole) with inversion of configuration using the Mitsunobu reaction. Finally, p-iodometomidate was transformed into the p-trimethylstannyl derivative.     

Thickness-dependent ordering of water layers at the NaCl(100) surface

We have determined the ordering properties of water adsorbed at room temperature on the rock salt (100) surface under four different conditions: ultrahigh vacuum, dry nitrogen atmosphere, 45% and 75% relative humidity. Details of the atomic structure are determined for both sides of the solid-liquid interface. The top most layer of NaCl shows a small relaxation that changes from an expansion to a contraction with increasing humidity. Under all measured conditions water monolayers with different ordering properties are present at the interface. Surprisingly, we find that the amount of ordering in the first layer is increasing with increasing thickness of the water film. At a solid-liquid interface, the ordering appears to be correlated with the solubility.     

Reaction of Some Alkenols with Tetrachloromethane

Summary.  The reaction of some alkenols with tetrachloromethane in the presence of a radical initiator was investigated. Regarding the effects of structural features of the starting alkenol (number and position of methyl substituents at the double bond and at the carbinol carbon atom, constitutional relationship between the double bond and the hydroxyl group) there are two possible competing reactions: addition and cyclization. In the case of the simplest alkenols (without substituents and with a more remote double bond) addition occurs; mono- and disubstituted secondary and tertiary Δ⁴- and Δ⁵-alkenols cyclize in high yields to give the corresponding cyclic ethers. Received March 17, 2000. Accepted (revised) May 31, 2000     

Stepwise optimization approach for improving LC‐MS/MS analysis of zwitterionic antiepileptic drugs with implementation of experimental design

In this article, a step‐by‐step optimization procedure for improving analyte response with implementation of experimental design is described. Zwitterionic antiepileptics, namely vigabatrin, pregabalin and gabapentin, were chosen as model compounds to undergo chloroformate‐mediated derivatization followed by liquid chromatography coupled with tandem mass spectrometry (LC‐MS/MS) analysis. Application of a planned stepwise optimization procedure allowed responses of analytes, expressed as areas and signal‐to‐noise ratios, to be improved, enabling achievement of lower limit of detection values. Results from the current study demonstrate that optimization of parameters such as scan time, geometry of ion source, sheath and auxiliary gas pressure, capillary temperature, collision pressure and mobile phase composition can have a positive impact on sensitivity of LC‐MS/MS methods. Optimization of LC and MS parameters led to a total increment of 53.9%, 83.3% and 95.7% in areas of derivatized vigabatrin, pregabalin and gabapentin, respectively, while for signal‐to‐noise values, an improvement of 140.0%, 93.6% and 124.0% was achieved, compared to autotune settings. After defining the final optimal conditions, a time‐segmented method was validated for the determination of mentioned drugs in plasma. The method proved to be accurate and precise with excellent linearity for the tested concentration range (40.0 ng ml^?1–10.0 × 10³ ng ml^?1). Copyright © 2013 John Wiley & Sons, Ltd.