The Fermi doublet V2−V3 + V4 of CH3CN as a probe of molecular interactions

The Fermi doublet V₂?V₃ + V₄ of CH₃CN in basic, inert and acidic solvents has been studied by IR and Raman spectroscopy. The values of W, the Fermi coupling coefficient, obtained from IR spectra varies with the nature of the solvent while W evaluated from Raman data remains constant at 12.5 ± 0.5 cm^?1. The similar effects of Bronsted and Lewis acids on the band frequencies and intensities is evidence that the CN group complexes with acids via the N atom “n” electron pair and not the π bond.     

Ab Initio (GIAO) Calculations of Absolute Nuclear Shieldings for Representative Compounds Containing 1(2)H, 6(7)Li, 11B, 13C, 14(15)N, 17O, 19F, 29Si, 31P, 33S,and 35Cl Nuclei

Two kinds of ab initio calculations (B3LYP and HF) using the GIAO approach have been carried out for fifteen H, three Li, three B, eleven C, seven N, nine O, four F, three Si, four P, six S and five Cl containing derivatives. The calculated absolute nuclear shieldings have been compared with experimental absolute values (either measured or estimated from relative chemical shifts) when available or with relative chemical shifts (¹¹B, ³⁵Cl). The correlations range from good to excellent and only three compounds (PN, P₂H₂, SO₂) deviate significantly.     

Complexes of divalent nickel and copper with the Schiff base derived from 2-(2-aminophenyl)benzimidazole and benzaldehyde

Summary The reactions of divalent nickel and copper salts with the Schiff base derived from 2-(2-aminophenyl)benzimidazole and benzaldehyde, L, yield complexes of general formulae [CuL₂X₂] (X=Cl, Br, or ClO₄), [CuL(SO₄)], [CuL(SO₄)] · 4H₂O, and NiLX₂ (X=Cl, Br, or NCS).All the complexes have been characterized by elemental analyses, magnetic measurements, e.s.r. electronic and i.r. spectral studies. The results show that the Schiff base acts as bidentate ligand through the pyridine-like imine nitrogen of imidazole ring and the azomethine nitrogen. Tentative structures of the complexes are suggested.     

Determination of manganese in brain samples by slurry sampling graphite furnace atomic absorption spectrometry

A simple procedure for the determination of manganese in different sections of human brain samples by graphite furnace atomic absorption spectrometry has been developed. Brain sections included cerebellum, hypothalamus, frontal cortex, vermix and encephalic trunk. Two sample preparation procedures were evaluated, namely, slurry sampling and microwave-assisted acid digestion. Brain slurries (2% w/v) could be prepared in distilled, de-ionized water, with good stability for up to 30 min. Brain samples were also digested in a domestic microwave oven using 5 ml of concentrated HNO₃. A mixed palladium+magnesium nitrate chemical modifier was used for thermal stabilization of the analyte in the electrothermal atomizer up to pyrolysis temperatures of 1300 °C, irrespective of the matrix. Quantitation of manganese was conducted in both cases by means of aqueous standards calibration. The detection limits were 0.3 and 0.4 ng ml⁻¹ for the slurry and the digested samples, respectively. The accuracy of the procedure was checked by comparing the results obtained in the analysis of slurries and digested brain samples, and by analysis of the NIST Bovine Liver standard reference material (SRM 1577a). The ease of slurry preparation, together with the conventional set of analytical and instrumental conditions selected for the determination of manganese make such methodology suitable for routine clinical applications.     

Revealing structural effects: electrochemical reactions of butanols on platinum

Spectroelectrochemical studies on the reactivity of butanol isomers on Pt electrodes in perchloric acid medium led to the observation of structural effects that result from the different arrangements of atoms in the organic molecules. The use of differential electrochemical mass spectrometry (DEMS) to detect volatile products showed that all four isomers react on the electrode, though different product yields were observed for each compound. In spite of the differences in the electrochemical behaviour of the butanol isomers, a series of general processes accounts for the results obtained. The formation of strongly adsorbed residues by a dehydration process leading to the formation of a C=C bond was proposed for all isomers. Electroreduction of the adsorbates produces C(4) and C(3) alkanes, and the latter reveal the existence of a fragmentation process. The C(4) hydrocarbons can be formed by hydrogenation of these residues and by hydrogenolysis of alcohol molecules in the bulk solution which react at the electrode with adsorbed hydrogen. On the other hand, CO(2) is formed during electrooxidation of the adsorbed species. Partial-oxidation products containing a carbonyl group were detected from 0.2 M solutions of 1-butanol, isobutyl alcohol and sec-butyl alcohol. The tertiary alcohol tert-butyl alcohol only reacts in its adsorbed state.     

Carbon fiber cylindrical microelectrode-based detector for the determination of antithyroid drugs

An amperometric flow-injection method for the determination of antithyroid drugs such as 6-methyl-2-thiouracil (MTU) using a carbon fiber cylindrical microelectrode-based detector is reported. A home-made flow-cell specially adapted for working with cylindrical microelectrodes ranging between 4 and 10 mm in length was used for this purpose. Methanol containing 0.05 mol l(-1) tetrabutylammonium perchlorate as the supporting electrolyte was employed, and a potential of +1.6 V was selected for the amperometric detection of MTU. No cleaning or electrode surface regeneration of the fiber was necessary during the whole working day. A limit of detection of 2.6x10(-7) mol l(-1) (37 mug MTU l(-1)) was achieved. The method was applied to the determination of MTU in spiked feed samples (peas and corn flour), at the 142 mug g(-1) level. The procedure involved the extraction of the drug in methanol, a clean-up step using Florisil cartridges and the use of the standard additions method. Recoveries of 81+/-6 and 85+/-8% were obtained for peas and corn flour, respectively.     

Totally selective ring-opening of amino epoxides with ketones: a general entry to enantiopure (2R,3S)- and (2S,3S)-3-aminoalkano-1,2-diols

[Reaction: see text] Transformation of enantiopure diastereoisomers (2R,1'S)- and (2S,1'S)-2-(1-aminoalkyl)epoxides into the corresponding 4-(1-aminoalkyl)-1,3-dioxolanes is achieved by reaction with different ketones in the presence of BF3.Et2O. The conversion takes place in very high yields, total selectivity, and without epimerization. A mechanism to explain this transformation is proposed. The obtained 1,3-dioxolanes can be deprotected, and (2R,3S)- and (2S,3S)-3-aminoalkano-1,2-diols were isolated.     

Structural characterization of single-walled carbon nanotube bundles by experiment and molecular simulation

A procedure, combining molecular simulation, Raman spectroscopy, and standard nitrogen adsorption, is developed for structural characterization of single-walled carbon nanotube (SWNT) samples. Grand canonical Monte Carlo simulations of nitrogen adsorption are performed on the external and internal adsorption sites of homogeneous arrays of SWNTs of diameters previously determined by Raman spectroscopy of the sample. The results show the importance of the peripheral grooves of a nanotube bundle at low relative pressure and the insensitivity of nanotube diameter toward adsorption on the external surface of the bundle at higher pressures. Simulations also reveal that samples containing thin nanotubes have less internal adsorption capacity that saturates at lower pressure than those comprising large diameter nanotubes. The fraction of open-ended nanotubes in a sample can be estimated by scaling the simulated internal adsorption inside nanotubes to obtain a near perfect fit between simulated and experimental isotherms. This procedure allows extrapolation of adsorption properties to conditions in which all nanotubes in the sample are open-ended.     

Crystal structure of potassium tetramethylammonium hexacyanocobaltate(III) trihydrate: K3(Me4N)3[Co(CN)6]2·3H2O

The structure of K₃(Me₄N)₃[Co(CN)₆]₂·3H₂O has been determined from three-dimensional X-ray diffraction data. The unit cell is formed by parallel layers of cobalt octahedra [CoC₆] and potassium octahedra, [K(1)N₅O(1)], separated byc/2. In each layer both types of octahedra are located alternatively. The [MeN₄]⁺ tetrahedra are located in the cavities between the two layers of octahedra. The crystal structure of this compound is the first example of its type. TMC 2483     

Trehalose-based cyclodextrin analogs: cyclotrehalans (CTs)

A new concept for the de novo synthesis of artificial glyconanocavities is presented. The use of alternating α,α′-trehalose building blocks and (thio)urea segments allows the efficient synthesis of a new family of cyclooligosaccharides, namely cyclotrehalans (CTs), featuring a convex-shaped cavity with an apolar environment. CTs are designed to exhibit molecular inclusion abilities similar to that of cyclodextrins (CDs). Contrary to CDs, CTs expose the monosaccharide β-face to the inner cavity, while the (thio)urea tethers provides some conformational adaptability. High-yielding syntheses of a series of CTs and a preliminary evaluation of their inclusion properties are reported.