Correlating Structure and KA2 Catalytic Activity of ZnII Hydrazone Complexes

Two new Zn(II) complexes bearing tridentate hydrazone-based ligands with NNO or NNS donor atoms were synthesised and characterised by elemental analysis, infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, and single crystal X-ray diffraction methods. These complexes, together with four previously synthesised analogues, having hydrazone ligands with a NNO donor set of atoms, were successfully employed as catalysts in the ketone-amine-alkyne (KA²) coupling reaction, furnishing tetrasubstituted propargylamines, compounds with unique applications in organic chemistry. DFT calculations at the CAM-B3LYP/TZP level of theory were performed to elucidate the electronic structure of the investigated Zn(II) complexes, excellently correlating the structure of the complexes to their catalytic reactivity.     

Biosynthesis of the Spermine Alkaloid Aphelandrine

Aphelandrine ( 1 ) is shown to be biosynthesized in the root cells of Aphelandra tetragona (VAHL ) NEES from labelled putrescine ( 4 ), spermidine ( 5 ), and cinnamic acid ( 3 ). Whether spermine ( 6 ) and the (p-hydroxycinnamoyl)spermidine 8 are precursors of 1 is uncertain, since the latter is hydrolysed to a large extent before incorporation, and the former is metabolized to 4 and 5 . Methionine ( 7 ) is the source of the 3-aminopropyl unit of 5 and 6 .     

Synthesis of 2D Germanane (GeH): a New,Fast, and Facile Approach

Germanane (GeH), a germanium analogue of graphane, has recently attracted considerable interest because its remarkable combination of properties makes it an extremely suitable candidate to be used as 2D material for field effect devices, photovoltaics, and photocatalysis. Up to now, the synthesis of GeH has been conducted by substituting Ca by H in a β‐CaGe₂ layered Zintl phase through topochemical deintercalation in aqueous HCl. This reaction is generally slow and takes place over 6 to 14 days. The new and facile protocol presented here allows to synthesize GeH at room temperature in a significantly shorter time (a few minutes), which renders this method highly attractive for technological applications. The GeH produced with this method is highly pure and has a band gap (E_g) close to 1.4 eV, a lower value than that reported for germanane synthesized using HCl, which is promising for incorporation of GeH in solar cells.     

Investigation of the incorporation of electrolyte anions in porous anodic Al2O3 films by employing a suitable probe catalytic reaction

A new method has been developed capable of describing the incorporation of electrolyte anions along the pore wall surface and across both the barrier layer and the pore wall oxide after the establishment of the steady state of growth of porous anodic Al₂O₃ where other methods cannot be applied to obtain reliable results. The knowledge of the nature/composition of anodic oxides as regards the incorporation of species like electrolyte anions is of specific importance for both the understanding of the electrochemical mechanism of oxide production and growth and the scientific and technological applications of porous anodic Al₂O₃ films. The method consists of the selection and use of a suitable catalytic probe reaction on porous anodic oxides at thicknesses varying from a value near zero up to the maximum limiting thickness and the treatment of the experimental reaction rate results by a properly developed mathematical formalism. This method was employed in anodic Al₂O₃ films prepared in H₂SO₄ anodizing electrolyte at a constant bath temperature and different current densities using as a probe reaction the decomposition of HCOOH on these oxides, which is almost exclusively a dehydration reaction, at relatively high reaction temperatures, 350 °C and 390 °C, where the effect of other species except SO₄ ²⁻ incorporated in the oxide on the reaction rate is eliminated. It has been shown that the fraction of the intercrystallite surfaces occupied by SO₄ ²⁻ follows a parabola-like distribution. It has a significant value at the pore base surface, depending on the current density, then it passes through a maximum along the pore wall surface and across both the barrier layer and the pore walls near the pore bases at positions depending on the current density and then becomes almost zero at the mouths of the pores of the oxide with the maximum limiting thickness and at both the Al₂O₃/Al interface and cell boundaries. The maximum value of the surface coverage is almost independent of the current density and is always near 1, showing an almost complete saturation of intercrystalline surfaces at these positions. The above distribution of surface coverage predicts a qualitatively similar distribution of the SO₄ ²⁻ bulk concentration across both the barrier layer and pore wall oxide around the pore bases. The method may be improved and developed further either for a more detailed investigation of the above films or to investigate films prepared in other pore-forming electrolytes. Received: 30 July 1998 / Accepted: 30 September 1998     

HPLC study of the inhibiting effect of phosphate and bicarbonate buffers on the leaching pattern of dental resin composites

A high-performance liquid chromatographic method was applied to evaluate the effect of bicarbonate (NaHCO₃) and phosphate (NaH₂PO₄, Na₂HPO₄) buffers on monomer leachability of dental resin composites. Evetric was the restorative resin composite that was investigated in current study. Forty disk-shaped resin composite specimens (n?=?40) were immersed by means of a silk string in the following solutions: Distilled water (A), phosphate buffer (B), bicarbonate buffer (C), artificial saliva (D). After seven (7) and fourteen (14) days, the eluates were analyzed by means of High Performance Liquid Chromatography (HPLC). Subsequently, new specimens were fabricated and immersed in fresh solutions in which methanol 12% (v/v) was added. After seven and fourteen days of storage, the leaching of monomers was examined by means of HPLC, which was proved to be an effective tool to buffers demonstrate the inhibiting effect of phosphate and bicarbonate ions on the leaching pattern of dental resin composites.     

Induced secondary metabolites from the endophytic fungus Aspergillus versicolor through bacterial co-culture and OSMAC approaches

Two new cryptic 3,4-dihydronaphthalen-(2H)-1-one (1-tetralone) derivatives, aspvanicin A (1) and its epimer aspvanicin B (2), as well as several known cryptic metabolites (3–8), were obtained from the ethyl acetate extract of the co-culture of the endophytic fungus Aspergillus versicolor KU258497 with the bacterium Bacillus subtilis 168 trpC2 on solid rice medium. When A. versicolor was cultured axenically in liquid Wickerham medium supplemented with 3.5% DMSO, an additional three known secondary metabolites (9–11) were isolated that were lacking when the fungus was fermented on rice medium. The structures of the new compounds were elucidated using one- and two-dimensional NMR spectroscopy as well as HRESIMS. The relative and absolute configurations of 1 and 2 were determined by the combination of NMR and electronic circular dichroism (ECD) analysis aided by DFT conformational analysis and TDDFT-ECD calculations. The ECD calculations revealed that although the sign of the blue-shifted overlapping n-π¹ ECD transition follows the helicity rule of cyclic aryl ketones, the calculation of low-energy conformers and ECD spectra was necessary to determine the stereochemistry. All metabolites were assessed for their antibacterial and cytotoxic activities; one of the new diastereomers, compound 2, showed moderate cytotoxic activity against the mouse lymphoma cell line L5178Y.     

High‐Pressure Synthesis of a Nitrogen‐Rich Inclusion Compound ReN8⋅x N2 with Conjugated Polymeric Nitrogen Chains

A nitrogen‐rich compound, ReN₈?x N₂, was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser‐heated diamond anvil cell. Single‐crystal X‐ray diffraction revealed that the crystal structure, which is based on the ReN₈ framework, has rectangular‐shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100 GPa, ReN₈?x N₂ is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [?N=N?]_∞ that constitute the framework have not been previously observed in any compound. Ab initio calculations on ReN₈?x N₂ provide strong support for the experimental results and conclusions.     

Robust regression credibility: The influence function approach

In classical credibility theory we assume that the vector of claims conditionally on has independent components with identical means. However, this assumption is sometimes unrealistic. To relax this condition Hachemeister (Hachemeister, C.A., 1975. Credibility for regression models with application to trend. In: Kahn, P. (Ed.), Credibility, Theory and Applications. Academic Press, New York) introduced regressors. The presence of large claims can perturb the credibility premium estimation. The lack of robustness of regression credibility estimators, as well as the fairness of tariff evaluation, led to the development of this paper. Our proposal is to apply robust statistics to the regression credibility estimation by using the robust influence function approach of M-estimators.     

On the limiting distribution of sample central moments

We investigate the limiting behavior of sample central moments, examining the special cases where the limiting (as the sample size tends to infinity) distribution is degenerate. Parent (non-degenerate) distributions with this property are called singular, and we show in this article that the singular distributions contain at most three supporting points. Moreover, using the delta-method, we show that the (second-order) limiting distribution of sample central moments from a singular distribution is either a multiple, or a difference of two multiples of independent Chi-square random variables with one degree of freedom. Finally, we present a new characterization of normality through the asymptotic independence of the sample mean and all sample central moments.