Micellar-stabilized room-temperature phosphorimetric determination of the fungicide thiabendazole in canned pineapple samples

A method for the determination of the fungicide thiabendazole (TBZ) by micellar-stabilized room-temperature phosphorescence is described. It does not require any separation step and allows the direct determination of the fungicide in canned pineapple samples. The effect of various experimental conditions is discussed in detail. The analytical curve of thiabendazole gives a linear dynamic range of 23.8–500.0 ng mL^–1 and a detection limit of 23.8 ng mL^–1. Recoveries of 103.9 and 89.2% for syrup and canned pineapple pulp, respectively, were obtained for 250 ng mL^–1 thiabendazole.     

Interpretation of vicinal proton–proton coupling constants of heteroaromatic molecules in terms of topological electron density descriptors

The indirect vicinal proton–proton coupling constants for pyrrole, furan, thiophene and 15 related heteroaromatic compounds were calculated using the Khon–Sham approximation. An analysis of the four Ramsey contributions to the coupling constants was carried out showing that the Fermi contact term is always positive and dominant, although the remaining contributions have a nonnegligible net negative contribution. The trends observed for the proton–proton coupling constants were rationalized in terms of the properties of the electron density. It was found that electron delocalization between the corresponding hydrogen atoms plays a major role on the observed behavior with the charges of the carbon atoms bonded to them and the accompanying geometric variations being also of importance in the coupling mechanism. Copyright © 2010 John Wiley & Sons, Ltd.     

Positron spectroscopy analysis in metallocene propylene/1‐octadecene copolymers: Parameters dependence on monoclinic and mesomorphic polymorphs

Copolymers of propylene and 1‐octadecene synthesized by a metallocene catalyst were characterized by PALS, WAXD, DSC, and density measurements. The change in the sizes and the number density of free‐volume holes as a function of the 1‐octadecene content were compared to the master laws published for copolymers of propylene with α‐olefins of shorter lengths. It was found that the samples were separated into two groups. The first set of samples obeyed the master laws while the second group deviated. This behavior was explained by the difference in the copolymer structure. The WAXD analysis confirmed the separation as well. The deviation was explained as due to the mesomorphic crystalline structure of the second group of samples, in contrast to the monoclinic one for the first group, which was a result of the relatively fast cooling from the molten state and from the high comonomer molar content. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1994–2002, 2010     

Chromium(VI) oxide oxidation of non‐ethoxylated and ethoxylated alcohols for determination by electrospray ionization mass spectrometry

A new derivatization procedure to increase the sensitivity of electrospray ionization mass spectrometry (ESI‐MS) to non‐ethoxylated and ethoxylated alcohols was investigated. The analytes were oxidized with chromium(VI) oxide and the resulting carboxylic and ethoxy‐carboxylic acids were isolated by extraction with ethyl acetate; the extracts were alkalinized and infused into the ESI‐MS system working in the negative‐ion mode. The yields of the combined oxidation‐extraction were ca. 100% for non‐ethoxylated fatty alcohols dissolved in acetone and they decreased moderately in samples containing increasing amounts of water (e.g., a 75% yield was obtained with 50% water). Ethoxylated alcohols with more than two ethylene oxide units resulted in yields of ca. 60%. Low limits of detection (LODs) were obtained when the procedure was applied to the analysis of body‐care products and cosmetics containing fatty alcohols, e.g., in a varicose‐vein cream, the LODs were 25 µg cetyl alcohol and 7.5 µg stearyl alcohol (detected as palmitic acid and stearic acid, respectively) per gram of sample. High molecular mass alcohols were also detected in seawater after pre‐concentration by solid‐phase extraction. Thus, the proposed method is particularly valuable for use in industrial samples having complex matrices and in environmental samples and it is competitive with other methods for the analysis of trace amounts of fatty alcohols. Copyright © 2010 John Wiley & Sons, Ltd.     

Solid phase extraction of chromium(VI) using Aliquat336 immobilized on a thin film of multiwall carbon nanotubes

We report on a novel and selective method for the preconcentration and determination of Cr(VI) in aqueous samples. Cr(VI) is adsorbed - in a “batch mode” - on multiwalled carbon nanotubes covered with Aliquat 336 and then determined directly, i.e., on the solid, by X-ray fluorescence spectrometry. This reduces the number of reagents and minimizes sample handling. The method combines the advantages of solid-phase extraction with the benefits of the XRF method in that the large areas required by the carbon nanotubes make them a promising solid sorbent for preconcentration. The enrichment factor was calculated after considering that the thin film obtained from the 10?mL solution of 1?mg?L^?1 of Cr(VI) has a real thickness of 0.04?mm and a final diameter of 16.7?mm, so that the volume deposited on the pellet is 0.0088 cm³ and the preconcentration factor is 1000.

Reversible Double C?H Bond Activation of Linear and Cyclic Ethers To Form Iridium Carbenes

The double C? H bond activation of a series of linear and cyclic ethers by the iridium complex [Tp^tol′Ir(C₆H₅)(N₂)] ( 2? N₂), which features a cyclometalated hydrotris(3‐p‐tolylpyrazol‐1‐yl)borate ligand (Tp^tol′) coordinated in a κ⁴‐N,N′,N′′,C manner, has been studied. Two methyl ethers, namely, Me₂O and MeOtBu, along with diethyl ether and the cyclic ethers tetrahydrofuran, tetrahydropyran (THP), and 1,4‐dioxane have been investigated with formation in every case of the corresponding hydride carbene complexes 3 – 8 , which are stabilized by κ⁴‐coordination of the ancillary Tp^tol′ ligand. Five of the compounds have been structurally authenticated by X‐ray crystallography. A remarkable feature of these rearrangements is the reversibility of the double C? H bond activation of Me₂O, MeOtBu, Et₂O, and THP. This has permitted catalytic deuterium incorporation into the methyl groups of the two methyl ethers, although in a rather inefficient manner (for synthetic purposes). Although possible in all cases, C? C coupling by migratory insertion of the carbene into the Ir? C σ bond of the metalated linkage has only been observed for complex 8 that contains a cyclic carbene that results from α,α‐C? H activation of 1,4‐dioxane. Computational studies on the formation of iridium carbenes are also reported, which show a role for metalated Tp ligands in the double C? H activation and account for the reversibility of the reaction in terms of the relative stability of the reagents and the products of the reaction.     

Molecular modeling (MM2 and PM3) and experimental (NMR and thermal analysis) studies on the inclusion complex of salbutamol and beta-cyclodextrin

The inclusion complex of salbutamol and beta-cyclodextrin (beta-CD) is studied by computational (MM2 and PM3) and experimental techniques. Molecular modeling calculations predict two different orientations of salbutamol in the beta-CD cavity in vacuo and in aqueous solution. In vacuo calculations show that the introduction of the aromatic ring of salbutamol is preferred to the introduction of the tert-butyl group into the beta-CD cavity. However, in aqueous solution both computational methods predict the introduction of the alkyl chain instead of the aromatic ring in the beta-CD cavity contrary to experimental results published previously. These quantitative predictions were experimentally confirmed here by studying the inclusion complex in solution by NMR. A 1:1 stoichiometry was found by (1)H NMR studies for this complex. A 2D ROESY (rotating-frame Overhauser enhancement spectroscopy) experiment shows that there are no cross-peaks between the aromatic protons of salbutamol and any of the protons of beta-CD. Cross-peaks for the protons of the tert-butyl group and protons inside the cavity of beta-CD demonstrate the full involvement of this group in the complexation process and confirm the orientation of the complex predicted by molecular modeling. The solid-state complex was prepared and its stoichiometry (beta-CD.C(13)H(21)NO(3).8H(2)O) and dissociation process studied by thermogravimetric analysis.     

Effect of the oxygen transfer coefficient on xylitol production from sugarcane bagasse hydrolysate by continuous stirred-tank reactor fermentation

The effect of the oxygen transfer coefficient on the production of xylitol by biocon version of xylose present in sugarcane bagasse hemicellulosic hydrolysate using the yeast Candiada guilliermondii was investigated. Continuous cultivation was carried out in a 1.25-L fermentor at 30°C, pH 5.5, 300 rpm, and a dilution rate of 0.03/h, using oxygen transfer coefficients of 10,20, and 30/h. The results showed that the microbial xylitol production (11 g/L) increased by 108% with the decrease in the oxygen volumetric transfer coefficient from 30 to 20/h. The maximum values of xylitol productivity (0.7g/[L…h]) and yield (0.58 g/g) were obtained at k _L a 20/h.