Stereochemical dependence of substituent γ-effects in the 19F NMR shielding constants

The substituent α-, β-, and γ-effects of the elements of the second and third periods on ¹⁹F NMR chemical shifts are evaluated including the establishment of stereochemical dependence of γ-effect, the latter particularly important in stereochemical studies of fluorine-containing compounds. Benchmark calculations performed for a series of 32 simple inorganic fluorine-containing molecules demonstrated a markedly good correlation between calculated and experimental fluorine chemical shifts characterized by a mean absolute error of 22.5 ppm in the range of about 900 ppm, which corresponds to a 2.5% error in the percentage terms.     

Simultaneous separation of actinides and technetium from large volumes of natural water for their determination

Journal of Radioanalytical and Nuclear Chemistry - Monitoring of radionuclide concentration in natural samples is important for assessing environmental safety. The low concentration of long-lived...     

Synthesis,metal ion binding,and photochromic properties of benzo- and naphthopyrans annelated by crown ether moieties

Combining a photochromic chromene with a crown ether moiety results in systems in which photochromism and ionophoric properties could significantly influence each other. In this paper, we report the synthesis of several chromenes annelated by 15(18)-crown-5(6) ethers. The approach involves the building of the photochromic fragment upon the initial crown ether via phenols. The two main routes for chromene preparation are discussed. The complex formation of the synthesized photochromic crown-containing naphthopyran with magnesium(II) and barium(II) cations was studied. The kinetic behavior of the colored form of the compound is affected by complex formation.     

Fast and highly efficient acetylation of xylans in ionic liquid systems

In this study high molecular weight pure rye arabinoxylan and spruce arabinoglucuronoxylan were acetylated in ionic liquid (IL) systems. Two different ILs were used in our study. In both IL, using optimized procedures, it was possible to achieve acetylation within 5 min. The first system involved direct dissolution into 1-ethyl-3-methylimidazolium dimethylphosphate ([emim][Me₂PO₄]), followed by addition of acetyl chloride/pyridine (AcCl/Pyr) and additional chloroform (CHCl₃), as co-solvent. The other system involved direct dissolution into the novel protic IL 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]), followed by acetic anhydride/1,5-diazabicyclo[4.3.0]non-5-ene (Ac₂O/DBN) and no co-solvent added. The full acetyl substitution of the xylans was confirmed by FT IR and ¹H NMR. The acetylated xylans maintained a high molecular weight, which was confirmed by gel permeation chromatography. The products were soluble in CHCl₃ and dimethyl carbonate, which is considered as a ‘green’ reagent or solvent. This allowed for the casting of the materials into clear transparent films, opening opportunity for further processing and evaluation of these materials.     

Mechanism of Framework Oxygen Exchange in Fe‐Zeolites: A Combined DFT and Mass Spectrometry Study

The role of framework oxygen atoms in N₂O decomposition [N₂O(g)→N₂(g) and 1/2O₂(g)] over Fe‐ferrierite is investigated employing a combined experimental (N₂¹⁸O decomposition in batch experiments followed by mass spectroscopy measurements) and theoretical (density functional theory calculations) approach. The occurrence of the isotope exchange indicates that framework oxygen atoms are involved in the N₂O decomposition catalyzed by Fe‐ferrierite. Our study, using an Fe‐ferrierite sample with iron exclusively present as Fe^II cations accommodated in the cationic sites, shows that the mobility of framework oxygen atoms in the temperature range: 553 to 593 K is limited to the four framework oxygen atoms of the two AlO₄^? tetrahedra forming cationic sites that accomodate Fe^II. They exchange with the Fe extra‐framework ¹⁸O atom originating from the decomposed N₂¹⁸O. We found, using DFT calculations, that O₂ molecules facilitate the oxygen exchange. However, the corresponding calculated energy barrier of 87 kcal mol^?1 is still very high and it is higher than the assumed experimental value based on the occurrence of the sluggish oxygen exchange at 553 K.     

Carbon nanotube—chalcogenide glass composite

This article describes the preparation of multi-walled carbon nanotube—chalcogenide glass composite by direct synthesis and the melt-quenching method. The carbon nanotubes—chalcogenide glass composite was characterized by high-resolution transmission electron microscopy (HRTEM), TEM/energy-dispersive X-ray spectroscopy, low energy electron excited X-ray spectroscopy, Raman spectroscopy, spectroscopic ellipsometry, microhardness, and impedance spectroscopy. CNTs-AgAsS₂ glass composite possess highly increased ionic conductivity, from σ_25 °C=4.38±0.0438×10⁻⁶ to σ_25 °C=6.57±0.0657×10⁻⁶ S cm⁻¹ and decreased refractive index from n=2.652 to 2.631 at the wavelength λ=1.55 μm.     

Direct palladium-catalyzed arylation of cyclopropenes

The first examples of direct palladium-catalyzed arylation and heteroarylation of cyclopropenes have been demonstrated. This method allows for efficient synthesis of various tetrasubstituted cyclopropenes, incuding nonracemic cyclopropenes, which are not available via known asymmetric cyclopropenation methods. Mechanistic studies strongly suggest an electrophilic path for this Heck-type transformation.     

Location of Framework Al Atoms in the Channels of ZSM‐5: Effect of the (Hydrothermal) Synthesis

²⁷Al 3Q MAS NMR and UV/Vis spectroscopy with bare Co^II ions as probes of Al pairs in the zeolite framework were employed to analyze the location of framework Al atoms in the channel system of zeolite ZSM‐5. Furthermore, the effect of Na⁺ ions together with tetrapropylammonium cation (TPA⁺) in the ZSM‐5 synthesis gel on the location of Al in the channel system was investigated. Zeolites prepared using exclusively TPA⁺ as a structure‐directing agent (i.e., in the absence of Na⁺ ions) led to 55–90 % of Al atoms located at the channel intersection, regardless the presence or absence of Al pairs [Al?O?(Si?O)₂?Al sequences in one ring] in the zeolite framework. The presence of Na⁺ ions in the synthesis gel did not modify the Al location at the channel intersection (55–95 % of Al atoms) and led only to changes in i) the distribution of framework Al atoms between Al pairs (decrease) and single isolated Al atoms (increase), and ii) the siting of Al in distinguishable framework tetrahedral sites.     

Development of a lipase-based optical assay for detection of DNA

A lipase-based assay for detection of specific DNA sequences has been developed. Lipase from Candida antarctica was conjugated to DNA and captured on magnetic beads in a sandwich assay, in which the binding was dependent on the presence of a specific target DNA. For amplification and to generate a detectable readout the captured lipase was applied to an optical assay that takes advantage of the enzymatic activity of lipase. The assay applies p-nitrophenol octanoate (NPO) as the substrate and in the presence of lipase the ester is hydrolyzed to p-nitrophenolate which has a strong absorbance at 405 nm. The method provides detection a detection limit of 200 fmol target DNA and it was able to distinguish single base mismatches from the fully complementary target.