Li(+) ion conductivity in rock salt-structured nickel-doped Li(3)NbO(4)

Two mechanisms of doping Li(3)NbO(4), which has an ordered, rock salt superstructure, have been established. In the "stoichiometric mechanism", the overall cation-to-anion ratio is maintained at 1:1 by means of the substitution 3Li(+) + Nb(5+) --> 4Ni(2+). In the "vacancy mechanism", Li(+) ion vacancies are created by means of the substitution 2Li(+) --> Ni(2+). Solid solution ranges have been determined for both mechanisms and a partial phase diagram constructed for the stoichiometric join. On the vacancy join, the substitution mechanism has been confirmed by powder neutron diffraction; associated with lithium vacancy creation, a dramatic increase in Li(+) ion conductivity occurs with increasing Ni content, reaching a value of 5 x 10(-4) Omega(-1) cm(-1) at 300 degrees C for composition x= 0.1 in the formula Li(3-2x)Ni(x)NbO(4). This is the first example of high Li(+) ion conductivity in complex oxides with rock salt-related structures.     

Highly enantioselective copper(I)-phosphoramidite-catalysed additions of organoaluminium reagents to enones

Simple phosphoramidite ligands afford good to excellent levels of enantioselectivity in 1,4-additions of AlR3 species to enones; sequential carboalumination-ACA cascades are possible.     

Thermochemistry of 1,3-dithiacyclohexane 1-oxide (1,3-dithiane sulfoxide): calorimetric and computational study

The enthalpies of combustion and sublimation of 1,3-dithiacyclohexane 1-oxide (1,3-dithiane sulfoxide, 2) were measured by a rotating-bomb combustion calorimeter and the Knudsen effusion technique, and the gas-phase enthalpy of formation was determined, DeltafH degrees m(g) = -98.0 +/- 1.9 kJ mol(-1). This value is not as large (negative) as could have been expected from comparison with thermochemical data available for the thiane/thiane oxide reference system. High-level ab initio molecular orbital calculations at the MP2(FULL)/6-31G(3df,2p) level were performed, and the optimized molecular and electronic structures of 2 afforded valuable information on (1) the relative conformational energies of 2-axial and 2-equatorial--the latter being 7.1 kJ mol(-1) more stable than 2-axial, (2) the possible involvement of nS --> sigma*(C-S(O)) hyperconjugation in 2-equatorial, (3) the lack of computational evidence for sigma(S-C) --> sigma*(S-O) stereoelectronic interaction in 2-equatorial, and (4) the relevance of a repulsive electrostatic interaction between sulfur atoms in 1,3-dithiane sulfoxide, which apparently counterbalances any nS --> sigma*(C-S(O)) stabilizing hyperconjugative interaction and accounts for the lower than expected enthalpy of formation for sulfoxide 2.     

Photophysical and Photochemical Studies of Pyridoxamine

The absorption and fluorescence emission of pyridoxamine were studied as function of pH and solvent properties. In the ground state, pyridoxamine exhibits different protonated forms in the range of pH 1.5–12. Fluorescence studies showed that the same species exist at the lowest singlet excited state but at different pH ranges. The phenol group is by ca. 8 units more acidic in the excited state than in the ground state. On the other hand, the pyridine N‐atom is slightly more basic in the lowest excited state than in the ground state. Excitation spectra and emission decays in the pH range of 8–10 indicate the protonation of the pyridine N‐atom by proton transfer from the amine group, in the ground and singlet excited states. Spectroscopic studies in different solvents showed that pyridoxamine in the ground or excited states exhibits intramolecular proton transfer from the pyridine N‐atom to the phenol group, which is more favorable in solvents of low hydrogen‐bonding capacity. The cationic form with the protonated phenolic group, which emits at shorter wavelength, is the dominant species in nonprotic solvents, but, in strong proton‐donor solvents, both forms exist. The fluorescence spectra of these species exhibit blue shift in protic solvents. These shifts are well‐correlated with the polarity and the H‐donor ability of the solvent.     

Reaction of a bis(bimetallic)-supported butadiyndiyl ligand with terminal acetylenes

The bis(bimetallic) diyndiyl complex [{Fe₂(CO)₆(μ-PPh₂)}₂(μ-C≡CC≡C)] reacts readily with terminal alkynes to give a number of products. One of these has been crystallographically and spectroscopically characterised, revealing facile insertion of a terminal alkyne into one metal-C σ-bond of the precursor to give a novel tetrametallic compound which features an extended “carbon-rich” ligand. The product is derived from a Fe–C bond cleavage reaction, coupled with C–C and C–P bond formation, and further illustrates the remarkable, and as yet poorly mapped, reactivity of C₄ fragments on polymetallic frameworks. Dedicated to Professor Dieter Fenske, an inspirational pioneer of organometallic chemistry.     

Structural analysis of metal ion ligation to nucleotides and nucleic acids using pulsed EPR spectroscopy

Metal ions play key structural and functional roles in many nucleic acid systems, particularly as required cofactors for many catalytic RNA molecules (ribozymes). We apply the pulsed EPR technologies of electron spin-echo envelope modulation and electron spin-echo-electron nuclear double resonance to the structural analysis of the paramagnetic metal ion Mn(II) bound to nucleotides and nucleic acids. We demonstrate that pulsed EPR, supplemented with specific isotope labeling, can characterize ligation to nucleotide base nitrogens, outer-sphere interactions with phosphate groups, distances to sites of specific (2)H atom labels, and the hydration level of the metal ion. These techniques allow a comprehensive structural analysis of the mononucleotide model system MnGMP. Spectra of phenylalanine-specific transfer RNA from budding yeast and of the hammerhead ribozyme demonstrate the applicability of the methods to larger, structured RNA systems. This suite of experiments opens the way to detailed structural characterization of specifically bound metal ions in a variety of ribozymes and other nucleic acids of biological interest.     

Electrochemistry of triphenyltin acetate at a mercury-film glassy carbon electrode

The electrochemical behaviour of triphenyltin acetate was investigated by cyclic voltammetry, differential-pulse voltammetry and controlled potential electrolysis at a mercury-film glassy carbon electrode. Effects on the electrochemical response of the composition of supporting electrolytes, pH, electrode rotation speed and triphenyltin acetate concentration were determined. The electrochemical reduction of this compound was found to involve a preliminary adsorption process (E_peak ≈ ?0.7 V vs. SCE), the reduction of triphenyltin acetate to the triphenyltin radical (E_peak ≈ ?1.0 V) and reduction of the radical to the triphenyltin anion (E_peak ≈ ?1.4 V). A procedure for the determination of trace amounts of this compound by differential-pulse anodic stripping voltammetry in 50% (v/v) ethanol with 0.1 M acetic acid + 0.1 M ammonia solution was developed and applied to the analysis of a commercial powder formulation and water and fish samples. The limit of detection was 2.5 × 10^?9 M triphenyltin acetate.     

Direct determination of five fluoroquinolones in chicken whole blood and in veterinary drugs by HPLC

A direct, accurate, and sensitive chromatographic analytical method for the quantitative determination of five fluoroquinolones (enoxacin, ofloxacin, norfloxacin, ciprofloxacin, and enrofloxacin) in chicken whole blood is proposed in the present study. For quantitative determination lamotrigine was used as internal standard at a concentration of 20 ng/microL. The developed method was successfully applied to the determination of enrofloxacin, as the main component of commercially available veterinary drugs. Fluoroquinolone antibiotics were separated on an Inertsil (250 x 4 mm) C8, 5 microm, analytical column, at ambient temperature. The mobile phase consisted of a mixture of citric acid (0.4 mol L(-1))-CH3OH-CH3CN (87:9:4% v/v) leading to retention times less than 14 min, at a flow rate 1.4 mL min(-1). UV detection at 275 nm provided limits of detection of 2 ng/mL per 20 microL injected volume for enoxacin, norfloxacin, and ciprofloxacin, 0.4 ng/mL for ofloxacin, and 4 ng/mL for enrofloxacin. Preparation of chicken blood samples is based on the deproteinization with acetonitrile while the pharmaceutical drug was simply diluted with water. Peaks of examined analytes in real samples were identified by means of a photodiode array detector. The method was validated in terms of within-day (n=6) precision and accuracy after chicken whole blood sample deproteinization by CH3CN. Using 50 microL of chicken blood sample, recovery rates at fortification levels of 40, 60, and 80 ng ranged from 86.7% to 103.7%. The applicability of the method was evaluated using real samples from chicken under fluoroquinolone treatment.     

Fused s-triazino heterocycles. XVII. 8,13-Dihydro-1,3,7,8,13c-pentaazabenzo[de]naphthacene, 7H-1,3,7,8,11b,12,14-heptaazadibenzo[de,hi]naphthacene and 8H-1,3,7,8,13,14c-hexaazabenzo[4,5]cyclohepta[1,2-a]phenalene,three new ring systems

The reaction of 7,9-dibromo-5-tribromornethyl-2-t-butyl-4-cyano-1,3,6,9b-tetraazaphenalene ( 1a ) with p-toluidine is shown to give 4,6-dibromo-2-t-butyl-8,13-dihydro-13-imino-11-methyl-1,3,7,8,13c-pentaazabenzo[de]naphthacene ( 4 ) in two steps with 7,9 dibromo-2-t-butyl-4-cyano-5-p-toluidino-1,3,6,9b-tetraazaphenalene ( 2b ) as the intermediate product. A related annulation reaction of 1a with N-(5-amino-2,4-dimethylphenyl)trimethylacetamide ( 8 ) leads in two steps to 9,11-dibromo-2,13-di-t-butyl-4,6-dimethyl-7H-1,3,7,8,11b,12,14-heptaazadibenzo[de,hi]naphthacene ( 6 ) with 7,9-dibromo-2-t-butyl-4-cyano-5N-(2,4-dimethyl-5-trimethylacetamidophenyl)amino-1,3,6,9b-tetraazaphenalene ( 2d ) as the intermediate product. In a similar fashion the reaction of 1a with o-phenylenediamine forms 14-amino-4,6-dibromo-2-t-butyl-8H-1,3,7,8,13,14c-hexaazobenzo[4,5]cyclohepta[1,2-a]-phenalene ( 12 ) by way of the intermediate 5-N-(2-aminophenyl)amino-7,9-dibromo-2-t-butyl-4-cyano-1,3,6,9b-tetraazaphenalene ( 2e ). The preparation of N-(2,4-dimethyl-5-nitrophenyl)-trimethylacetamide ( 11 ) and its reduction to N-(5-amino-2,4-dimethylphenyl)trimethylacetamide ( 8 ) is also described.     

Some novel binuclear group 13 metal tin hydrides formed in Ar matrices following the codeposition of the metal vapor with SnH4

IR measurements show that co-condensation of Al or Ga atoms (M) with SnH4 in a solid Ar matrix at about 12 K results mainly in the spontaneous insertion of the metal into an Sn-H bond to form the M(II) hydride HMSnH3. Simultaneously the Ga2 dimer also reacts with SnH4, possibly to form a nido-type cluster Ga2(mu-H)4Sn, with a metal-deficient cubane-like structure. All of these products are photolabile. Irradiation with visible light causes HMSnH3 to tautomerize to the novel dihydrido-bridged species H3M(mu-H)2Sn, which decomposes in turn under broad-band UV-visible light (lambda=200-800 nm); some H3Al(mu-H)2Sn is formed even on deposition. The data collected from experiments with SnH4 and SnD4 and different reagent concentrations, together with the results of quantum chemical calculations, are used to interpret the results and elucidate the structures and bonding of the new species.