New Ln[B6O9(OH)3] borates (Ln = Sm-Lu): Structure, properties, and structural relation to cationic Li4[B7O12]Cl conductors (Li-Boracites)

Crystals of new rare earth borates of the composition Ln[B₆O₉ (OH)₃] (Ln = Sm-Lu), sp. gr. R3c are synthesized under hydrothermal conditions. Their crystal structures are determined on single crystals with Ln = Ho, Gd without preliminary determination of their chemical formulas. The polar anionic framework of the crystals consists of BO₃ triangles and BO₄ tetrahedra and has wide channels along the threefold axis of the structure, which are similar to the channels along the a, b, and c axes in cubic Li₄[B₇O₁₂]Cl boracite with Li conductivity. Rare earth atoms are arranged in the structure over the cubic F pseudolattice, whereas the analogous positions in Li boracites are filled with Cl anions. The squared optical nonlinearity of the new crystals is comparable with the nonlinearity of quartz, whereas the electrical conductivity in borates at 300°C exceeds 10⁻⁶ S/cm. __________ Translated from Kristallografiya, Vol. 49, No. 4, 2004, pp. 681–691. Original Russian Text Copyright ? 2004 by Belokoneva, Ivanova, Stefanovich, Dimitrova, Kurazhkovskaya.     

Base-line model for identifying the bioaccumulation potential of chemicals

The base-line modeling concept presented in this work is based on the assumption of a maximum bioconcentration factor (BCF) with mitigating factors that reduce the BCF. The maximum bioconcentration potential was described by the multi-compartment partitioning model for passive diffusion. The significance of different mitigating factors associated either with interactions with an organism or bioavailability were investigated. The most important mitigating factor was found to be metabolism. Accordingly, a simulator for fish liver was used in the model, which has been trained to reproduce fish metabolism based on related mammalian metabolic pathways. Other significant mitigating factors, depending on the chemical structure, e.g. molecular size and ionization were also taken into account in the model. The results (r(2)=0.84) obtained for a training set of 511 chemicals demonstrate the usefulness of the BCF base line concept. The predictability of the model was evaluated on the basis of 176 chemicals not used in the model building. The correctness of predictions (abs(logBSF(Obs)-logBCF(Calc))=0.75)) for 59 chemicals included within the model applicability domain was 80%.     

Characterization of nickel loaded mordenite catalysts by temperature programmed reduction

Temperature programmed reduction (TPR) has been used to study the redox behavior of Ni(II) ions in nickel sodium mordenite (NiNaM) and decationated nickel mordenite (NiHM). The TPR profiles suggest that Ni(II) ions occupy nonequivalent sites with different cooridination states in the mordenite. The reducibility of Ni(II) depends strongly on the zeolite acidity.

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() Ni^II NiNaM NiHM. . .

     

Schnelle spektrophotometrische Tantalbestimmung in Hartmetallpulvermischungen mit Nilblau

Ohne Zusammenfassung

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Rapid spectrophotometric determination of tantalum in hard-metal alloy mixtures using nil blue

     

Theoretical study of hydrogen-bonded formaldehyde complexes

The hydrogen-bonded complexes involving formaldehyde and a series of proton donors of varying strengths, have been investigated at different levels of ab initio MO theory. The structures of the studied complexes were SCF optimized at the 6-31G basis set level. The binding energy was estimated employing basis set superposition correction, zero-point vibrations and MP2 correlation contribution at the different basis set: STO-3G; 6-31G; MP2/6-31G; 6-31G**; MP2/6-31G**; 6-311G(2d, 2p) and MP2/6-311G(2d, 2p). Linear relationships were found of the calculated binding energy with: the calculated shift in the carbonyl stretching frequency, the changes in carbonyl bond length and the optimum value of hydrogen-bond distance; furthermore the calculations confirm a parallel trend between the proton-donor ability and the strength of the hydrogen bond.     

Cyclohexane conversion on nickel mordenite

Two series of nickel mordenite catalysts have been prepared on the basis of Na-mordenite and H-mordenite. Their catalytic activity in cyclohexane conversion has been studied. It has been found that the properties of the catalysts are strongly dependent on the acidity of mordenite.

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NaM HM. .

     

On the conformational stability and dimerization of phosphotransferase enzyme I from Escherichia coli

The activity of enzyme I (EI), the first protein in the bacterial PEP:sugar phosphotransferase system, is regulated by a monomer–dimer equilibrium where a Mg²⁺-dependent autophosphorylation by PEP requires the homodimer. Using inactive EI(H189A), in which alanine is substituted for the active-site His189, substrate binding effects can be separated from those of phosphorylation. Whereas 1 mM PEP (with 2 mM Mg²⁺) strongly promotes dimerization of EI(H189A) at pH 7.5 and 20 °C, 5 mM pyruvate (with 2 mM Mg²⁺) has the opposite effect. A correlation between the coupling of N- and C-terminal domain unfolding, measured by differential scanning calorimetry, and the dimerization constant for EI, determined by sedimentation equilibrium, is observed. That is, when the coupling between N- and C-terminal domain unfolding produced by 0.2 or 1.0 mM PEP and 2 mM Mg²⁺ is inhibited by 5 mM pyruvate, the dimerization constant for EI(H189A) decreases from >10⁸ to <5 × 10⁵ or 3 × 10⁷ M⁻¹, respectively. With 2 mM Mg²⁺ at 15–25 °C and pH 7.5, PEP has been found to bind to one site/monomer of EI(H189A) with K_A′10⁶ M⁻¹ (ΔG′=−33.7±0.2 kJ mol⁻¹ and ΔH=+16.3 kJ mol⁻¹ at 20 °C with ΔC_p=−1.4 kJ K⁻¹ mol⁻¹). The binding of PEP to EI(H189A) is synergistic with that of Mg²⁺. Thus, physiological concentrations of PEP and Mg²⁺ increase, whereas pyruvate and Mg²⁺ decrease the amount of dimeric, active, dephospho-enzyme I.     

Ab initio prediction of the vibrational spectrum of the hydrogen bonded complex O2N...HONO2

The changes in the structural parameters and vibrational characteristics (vibrational frequencies, infrared intensities and Raman activities) arising from the hydrogen bonding between NO(2) and HONO(2) have been studied employing ab initio 6-31G(d, p)/UHF and 6-31+G(d, p)/UHF, and B3LYP/6-31G(d, p) calculations. The charge rearrangement upon hydrogen bonding have been, estimated using the Mulliken population analyses. It was established that the complexation between NO(2) and HONO(2) leads to changes in the structural parameters and the vibrational characteristics of the monomers. The most sensitive to the hydrogen bond formation are the vibrational characteristics of the normal modes of the monomer bonds participating in the hydrogen bonding. The predicted shifts in the vibrational frequencies by ab initio and B3LYP/6-31G(d, p) calculations are in very good agreement with the experimentally observed, which is an evidence for the reliance of the studied structure.     

Et3SiH + KOtBu provide multiple reactive intermediates that compete in the reactions and rearrangements of benzylnitriles and indolenines

The combination of potassium tert-butoxide and triethylsilane is unusual because it generates multiple different types of reactive intermediates simultaneously that provide access to (i) silyl radical reactions, (ii) hydrogen atom transfer reactions to closed shell molecules and to radicals, (iii) electron transfer reductions and (iv) hydride ion chemistry, giving scope for unprecedented outcomes. Until now, reactions with this reagent pair have generally been explained by reference to one of the intermediates, but we now highlight the interplay and competition between them.

The combination of potassium tert-butoxide and triethylsilane provides simultaneous access to multiple reactive intermediates, radicals, H-atom donors, hydride donors and electron donors, giving scope for unprecedented reaction outcomes.