NMR investigation into the dynamic behaviour of a novel C2-symmetric macrocyclic ligand

The variable-temperature (1)H NMR spectra of a tosylated triazacyclononane are reported. The spectra show a dynamic process which is assigned to hindered rotation about the amide-like N--S bonds, and a value for the free energy of activation for the rotations is derived, DeltaG* = 50.2 kJ mol(-1) at 273 K. Steric and electronic factors which influence the barrier are discussed.     

Rigidity of critical circle mappings II

We prove that any two real-analytic critical circle maps with cubic critical point and the same irrational rotation number of bounded type are conjugate for some .

     

Experimental and theoretical investigation of the room-temperature photoluminescence of amorphized Pb(Zr,Ti)O3.

Ultrafine PbZr0.20Ti0.80O3 was amorphized through high-energy mechanical milling. The structural evolution through the amorphization process was accompanied by various characterization techniques, such as X-ray diffraction, Fourier-transformed IR spectroscopy (FTIR), high-resolution transmission electron microscopy (HR-TEM), and Raman spectroscopy. A strong photoluminescence was measured at room temperature for amorphized PbZr0.20Ti0.80O3, and interpreted by means of high-level quantum mechanical calculations in the density functional theory framework. Three periodic models were used to represent the crystalline and amorphized PbZr0.20Ti0.80O3, and they allowed the calculation of electronic properties that are consistent with the experimental data and that explain the appearance of photoluminescence.     

Electronic effects in the N-nitrosation of N-benzylpivalamides

A series of N-4-R-benzylpivalamides (R = MeO, Me, H, CF(3), and NO(2)) was nitrosated using a standardized solution of N(2)O(4) in CDCl(3) at -40 degrees C. The reactions, which produced the corresponding N-4-R-benzyl-N-nitrosopivalamides, were followed by (1)H NMR spectroscopy. The rate of nitrosation was found to vary in a systematic way with the nature of the 4-R-group on the aromatic ring. Thus, electron-releasing groups increased the rate of the reaction, whereas electron-withdrawing ones decelerated N-nitrosation. In a similar fashion, the nitrosations were accelerated in polar solvents but were slower in solvents of low polarity. The sensitivities of N-nitrosation to these intra- and intermolecular electronic effects are compared to those from a previous study examining the dependence of the kinetics of nitrosoamide thermolyses on the same factors.     

Calorimetry studies for interaction in solid/liquid interface between the modified cellulose and divalent cation

Cellulose (Cel) was initially modified with thionyl chloride in order to increase its reactivity. In another step, chlorinated cellulose (CelCl) was reacted with ethylenediamine (CelEn) molecule. The last reaction step was reacting CelEn with ethylene sulfide molecule, yielding the solid named CelEnEs. The two last steps occur in free solvent conditions. Elemental analysis showed the incorporation of an enormous amount of chlorine in the CelCl structure, the nitrogen entrance from the ethylenediamine molecule, as well as the huge amount of sulfur that was added due to entry of the ethylene sulfide molecule. Infrared spectroscopy, thermogravimetry, and ¹³C NMR in the solid state demonstrated the effectiveness of the reaction, in which the last one has shown signals at 30 and 32 ppm for CelEnEs due to the change in the methylene group environment. Divalent metal uptake by chemically modified biopolymer gave the order Ni²⁺ > Co²⁺ > Cu²⁺ > Zn²⁺, reflecting the corresponding acidity of these cations in bonding to the sulfur and the basic nitrogen atoms available on the pendant chains. The equilibrium data were fitted to Langmuir model. The maximum monolayer adsorption capacity for the cations was found to be 5.561 ± 0.017, 4.694 ± 0.013, 1.944 ± 0.062, and 1.733 ± 0.020 mmol g^?1 for Ni²⁺, Co²⁺, Cu²⁺, and Zn²⁺, respectively. Through calorimetric titrations thermodynamic parameters could be obtained, and the results proved the favor of the interactions between cations and basic centers of modified cellulose. This new synthesized biopolymer can be used as a material for cation removal from aqueous solution.     

Analytical methods for the determination of halogens in bioanalytical sciences: a review

Fluorine, chlorine, bromine, and iodine have been studied in biological samples and other related matrices owing to the need to understand the biochemical effects in living organisms. In this review, the works published in last 20 years are covered, and the main topics related to sample preparation methods and analytical techniques commonly used for fluorine, chlorine, bromine, and iodine determination in biological samples, food, drugs, and plants used as food or with medical applications are discussed. The commonest sample preparation methods, as extraction and decomposition using combustion and pyrohydrolysis, are reviewed, as well as spectrometric and electroanalytical techniques, spectrophotometry, total reflection X-ray fluorescence, neutron activation analysis, and separation systems using chromatography and electrophoresis. On this aspect, the main analytical challenges and drawbacks are highlighted. A discussion related to the availability of certified reference materials for evaluation of accuracy is also included, as well as a discussion of the official methods used as references for the determination of halogens in the samples covered in this review.

Biosynthesis of Bromoroquefortines in a High Saline Medium by Penicillium chrysogenum,a Terrestrial Endophyte Collected from Coffea arabica

Roquefortine C ( 1 ) and 11‐bromoroquefortine C ( 4 ) were isolated from Penicillium chrysogenum, an endophyte obtained from green leaves of Coffea arabica, during induction experiments by adding halide salts to the culture media. In our studies, 11‐bromoroquefortine D and 11‐bromo‐17‐hydroxybromoroquefortine C were identified by LC/HR‐MS. It is the first time that bromoroquefortine C and D are described as halogenated natural products. These halo alkaloids were shown to be biosynthesized from brominated tryptophan as the biosynthetic precursor. Cytotoxic and antibiotic assays using roquefortine C and 11‐bromoroquefortine C showed that the presence of the 11‐Br substituent impairs some biological activities of this alkaloid.