29Si-NMR-spektroskopische untersuchungen an substituierten disilanen

The ²⁹Si chemical shifts δ(Si) and coupling constants J(SiSi) and J(SiC) for a series of substituted tetra- and penta-methyldisilanes are reported and discussed as well as the corresponding data of di- and poly-silanes from the literature. The substituent effects of /gd(Si) can be explained satisfactorily by a simple quantum-chemical model without consideration of d-orbital effects and are mainly affected by the distribution of the /gs-electron density at the Si atom under study. The influence of steric interactions on δ(Si) is investigated in the case of alkoxy-substituted disilanes. A short discussion of the substituent effects on the coupling constants J(SiSi) and J(SiC) is given.     

Natural and artificial ageing of an alkyd based wood finish

Wood protection in exterior use is generally achieved with a stain. This protective product is often obtained from an alkyd resin. A natural and artificial agening have been studied by measuring the glass transition temperature (T _g) of the finish variations in terms of time. In both ageings, theT _g variations are the same; there is an increase inT _g during the first steps of ageing and then a stabilization. A behaviour equation is proposed and is perfectly suitable to both ageings. Time constants are calculated. This equation shows that the artificial ageing machine chosen is a good means of artificial ageing. It allows the reproduction and the acceleration by 10 times of phenomena which are observed during the natural ageing.     

Reduction of nitrophenols in an electrocatalytic system

Results of research into hydrogenation of o- and p-nitrophenols in an electrocatalytic system on a cathode activated with d- and s-metal catalysts are presented. The optimal conditions for o-aminophenol formation on Raney nickel were identified by the probability-determined method for the experimental design.     

Large sodium clusters in an electrostatic field

We study light scattering by sodium clusters generated in a metal cell [3] and subjected to an external electrostatic field. Scattered laser light intensities at right angle to the incoming laser beam and with polarization parallel I _V and perpendicular I _H to that of the laser show two maxima as a function of the electrostatic field (potential of electrode): the central maximum CM for the zero field (V = 0) and the side maximum SM for ca. -60 V (field strength 2400 V/m). This behavior can be understood on the basis of the Mie scattering theory by taking into account electrostatic charging of clusters due to the laser light ionization of the medium (Na₂ molecules). The presented model leads to the conclusion that the electrostatic field changes cluster size, mainly due to the influence on the supersaturation of the medium. Clusters are charged with charge proportional to the cluster radius, only at SM clusters are neutral (zero charge). For electrode potential higher than SM clusters are positively charged, for smaller potential (more negative than SM) clusters are negatively charged.     

The electrical properties of heterogeneous mixtures containing an oriented spheroidal dispersed phase

Theoretical equations for the permittivity and conductivity of a heterogeneous mixture containing an oriented dispersed phase of spheroidal shape are limited to low volume fractions of dispersed phase. Two new sets of equations have been derived here for such systems using approaches which have been shown previously to be applicable to higher concentrations. One set has generalised forms of equations given by Bruggeman and Hanai and includes simplifications applicable to oil in water (O/W) and water in oil (W/O) systems. The other set has generalised forms of an equation given by Looyenga. Tabulated mixture permittivityε and dielectric incrementΔε values are presented and, with the exception ofO 〈W systems containing prolate spheroids, the two types of equation show significant differences in bothε andΔε. For both typesΔε is strongly dependent on the ratio of the conductivities of the two media of aW/O system. The relevance of the mixture equations to the electrical behaviour of micellar, microemulsion and liquid crystalline systems is considered.     

The crystal and molecular structure of an oxodipyrromethene related to bilirubin

The structure of 5′-oxo-3′,4,4′-triethyl-3,5-dimethyl-1′,5′-dihydro[2.2′]-dipyrromethene 1, a model for bilirubin, has been determined from three dimensional counter data. The oxodipyrromethene skeleton shows only small deviations from planarity, and the structure assumes the Z-configuration with the nitrogen atoms syn. Bond lengths in the two rings are quite different. No intramolecular H-bonding is observed, but intermolecular H-bonded dimers are formed through the lactam oxygen with the lactam and pyrrole N-H groups. The compound crystallizes in monoclinic space group C2/c with cell dimensions a=25.106(10)A, b=7.132(7)A, c=19.144(10)A, β=112.87(3)°. There are eight molecules in the unit cell. 1423 independent reflections were measured. Of these, 600 had I>3 σ₁ and were used in the analysis. The structure was solved by direct methods and refined by least squares techniques to R=0.056.     

Facile synthesis of benzamides to mimic an α-helix

A new α-helix mimetic was designed by using a benzamide as a rigid scaffold. It presents three functional groups corresponding to side chains of amino acids found at the i, i + 4, and i + 7 positions of an ideal α-helix, which are displayed on the same helical face. Its efficient synthesis was achieved by employing simple alkylation and amidation reactions which can be easily adapted for solid-phase synthesis. As a result, two tris-benzamides were produced to mimic two helical regions found in a peptide hormone, glucagon.     

Ab-initio study of localization of an excess electron in sodium halide clusters

The equilibrium geometries of Na _n F _n and Na _n F _n?1 are optimized forn=2, 3, 4 at the SCF level. The Na _n F _n molecules appear as formed by Na⁺ and F^? ions. The paper studies the localization of the excess electron in the Na _n F _n?1 molecules. Na _n F _n?1 is obtained by removing from Na _n F _n a fluorine atom, and the excess electron takes the place of this lacking center, and allows a weak bond between the neighbouring Na atoms. Secondary minimum geometries, for which the excess electron has no defined place, are also studied.     

Untersuchungen an Tetranatrium-cyclo-tetraphosphat(V) und seinen Hydraten

A reinvestigation of the system (NaPo₃)₄/H₂O has been carried out. Besides the already known tetra-hydrate and anhydrous form, a mono-hydrate in the temperature range 100–120°C exists. Single crystals were obtained by crystallization from aqueous solutions containing higher alcohols (e.g. ethyleneglycol) at ～115°C. The crystal structure was solved by X-ray methods (orthorhombic;P2₁2₁2₁;a=1365.4(2),b=1 347.5(3),c=629.1(3)pm;Z=4; 2774 diffractometer data;R=0.053). The cyclic anions exhibit point symmetryD _2d with all terminal oxygens in eclipsed conformation. With respect to the (NaPO₃)₄-part of structure, mono-hydrate and anhydrous forms are isostructural; lattice parameters of the latter area=1 380.8(2),b=1 363.3(2),c=602.7(2) pm. The transition mono-hydrate to anhydrous form occurs topochemically and is reversible.     

Identification and characterization of an anomalous racemate

Direct crystallization is a convenient and generally efficient method to increase the optical purity of partially resolved enantiomers. The design of chiral purification processes based on crystallization requires measurement of solubility of all stable phases, i.e. enantiomers and racemic mixtures, in the process solvent. Enantiomeric systems have been classified by the nature of the solid phases formed as racemic compounds, conglomerates or solid solutions. In rare cases, however, enantiomers form crystalline addition compounds with a stoichiometry other than 1(S):1(R). These structures are generally known as “anomalous racemates”. The solubility and melting point diagrams of each type of enantiomeric system exhibit characteristic features which have significant implications on the efficiency of the crystallization to enhance optical purity. In this study, we report the formation of a non-equimolar 3:1 addition compound in the R/S 3-hydroxy-4-(2,4,5-trifluorophenyl) butanoic acid.     

Investigation of an Allosteric Deoxyhypusine Synthase Inhibitor in P. falciparum

The treatment of a variety of protozoal infections, in particular those causing disabling human diseases, is still hampered by a lack of drugs or increasing resistance to registered drugs. However, in recent years, remarkable progress has been achieved to combat neglected tropical diseases by sequencing the parasites’ genomes or the validation of new targets in the parasites by novel genetic manipulation techniques, leading to loss of function. The novel amino acid hypusine is a posttranslational modification (PTM) that occurs in eukaryotic initiation factor 5A (EIF5A) at a specific lysine residue. This modification occurs by two steps catalyzed by deoxyhypusine synthase (dhs) and deoxyhypusine hydroxylase (DOHH) enzymes. dhs from Plasmodium has been validated as a druggable target by small molecules and reverse genetics. Recently, the synthesis of a series of human dhs inhibitors led to 6-bromo-N-(1H-indol-4yl)-1-benzothiophene-2-carboxamide, a potent allosteric inhibitor with an IC₅₀ value of 0.062 µM. We investigated this allosteric dhs inhibitor in Plasmodium. In vitro P. falciparum growth assays showed weak inhibition activity, with IC₅₀ values of 46.1 µM for the Dd2 strain and 51.5 µM for the 3D7 strain, respectively. The antimalarial activity could not be attributed to the targeting of the Pfdhs gene, as shown by chemogenomic profiling with transgenically modified P. falciparum lines. Moreover, in dose-dependent enzymatic assays with purified recombinant P. falciparum dhs protein, only 45% inhibition was observed at an inhibitor dose of 0.4 µM. These data are in agreement with a homology-modeled Pfdhs, suggesting significant structural differences in the allosteric site between the human and parasite enzymes. Virtual screening of the allosteric database identified candidate ligand binding to novel binding pockets identified in P. falciparum dhs, which might foster the development of parasite-specific inhibitors.     

On the possibility of orienting rotationally cooled polar molecules in an electric field

We have investigated the effects of initial rotational stateJ and the gas temperature (T) on the rotationally elastic, inelastic and summed processes in CH₄ and SiH₄ molecules by low-energy electron impact. While rotationally summed differential, integral, momentum transfer and energy-loss cross sections are independent of initial stateJ (and hence independent onT), it is found that the same rotationally inelastic differential and integral cross sections, when averaged over rotational distribution function, show a qualitative improvement over the unaveraged results when compared with experimental results. Various theorems regarding the dependence of scattering parameters on the rotational distribution, as described in a series of papers by Shimamura, are discussed by presenting actual calculations on the two spherical tops, namely the CH₄ and SiH₄ molecules.     

Hexa-peri-benzocoronene with two extra K-regions in an ortho-configuration

There are three possible isomers for hexa-peri-hexabenzocoronene (HBC) with two extra K-regions, but only two of them have been reported, namely with the meta- and para-configurations. Herein, we describe the synthesis of HBC 4 with two extra K-regions in the ortho-configuration, forming a longer zigzag edge compared with the other two isomers. The structure of 4 was validated by laser desorption/ionization time-of-flight mass analysis and nuclear magnetic resonance spectra, as well as Raman and infrared spectroscopies supported by density functional theory calculations. The optical properties of 4 were investigated by UV/vis absorption and ultrafast transient absorption spectroscopy. Together with the analysis of aromaticity, the influence of the zigzag edge on the π-conjugation pathway and HOMO–LUMO gaps of the three isomers were investigated.

We reported the synthesis of hexa-peri-benzocoronene (HBC) with two extra K-regions adopting an ortho-configuration. The systematical study provides deep insights about the effect of zigzag edge on the π-conjugated pathway and molecular design.     

Detection of the reversibility of an excited-state reaction by phase-modulation fluorometry

We used phase and modulation fluorometry to investigate the excited-state proton transfer from 2-naphthol. Phase-sensitive detection of fluorescence allows determination of the phase angles of the two-excited species, naphthol and naphtholate. If the steady-state spectra of the individual species are known and are not identical, then this general procedure yields the phase angles irrespective of the extent of the spectral overlap. The phase difference (Δφ) between the naphthol and naphtholate emission is given by tan Δφ = ω/(Γ_R + k₂ where ω is the circular modulation frequency, Γ_R the decay rate of naphtholate fluorescence and k₂ the rate of the reverse reaction. Hence, Δφ reflects both the decay rate of the reaction product and the rate of the reverse action. This back reaction was also detected by comparison of phase shift (φ) and demodulation (m) values for the initially excited state. Specifically, the reverse reaction results in a double exponential decay of naphthol fluorescence, which is revealed by m/cosφ < 1. The concepts described herein are generally applicable to determination of the reversibility of excited-state reactions.     

Photoinitiation of methacrylate polymerization with an o-benzoquinone-amine system

The kinetic features of the photopolymerization of mono- and dimethacrylates in the presence of the binary initiating system comprising a substituted o-benzoquinone and a tertiary amine under the action of visible light are studied. In the case of methyl methacrylate, the limiting conversion of the monomer does not exceed 10%. The photopolymerization of oligo(ester methacrylates) yields a polymer glass with a monomer conversion of 60–90%. As is shown for a series of seven o-benzoquinones, the rate of photopolymerization increases with an increase in the volume of substituents in positions 3 and 6 of the quinoid ring of o-benzoquinone. It found that trialkylamines (dimethylethanolamine and dimethylcyclohexylamine) are more efficient as coinitiators of photopolymerization than N,N-dimethylaniline. For compositions based on 3,6-di-tert-butylbenzoquinone-1,2, the spectral-sensitivity range in the visible region is 400 to 650 nm with a maximum at 600 nm. This value coincides with the maximum of the absorption band of quinone, which corresponds to the S(n → π*) electronic transition of carbonyl groups.     

Selection and performance of adsorbents in an adsorption refrigeration cycle regenerated with water directly

An adsorption refrigeration cycle regenerated with water directly (ARRD), which has the advantage of excellent heat transfer performance in the adsorbent bed with half-cycling time 3–5 min, and operates close to atmospheric pressure (0.1–0.5 MPa) with n-butane as a refrigerant, is developed. The purpose of this study is to find out advanced adsorbent/n-butane pairs for the ARRD system. Adsorption equilibrium performances of n-butane on the adsorbents were measured by positive pressure gravimetric method, and the adsorption equilibrium data were fitted by D-A equation. Desorption properties were determined through thermal analysis. The coefficient of performance and specific cooling power (SCP) were calculated. The results show that adsorption capacities of n-butane with activated carbon are all above 0.31 kg/kg and the highest can reach 0.43 kg/kg. Adsorption data are fitted well by D-A equation, with the correlative coefficients R above 0.93. Desorption temperatures of n-butane from carbons are between 110 and 130 °C, prospective for using low grade heat source. The quasi-equilibrium of n-butane on carbons can be achieved within 150–250 s, which benefits SCP of the ARRD system. The SCP of n-butane/AC-5 pair estimated in the ARRD systems is from 180 to 200 W/kg.     

Molecular hydrogen emission in the vacuum ultraviolet from an inductively coupled plasma

Vacuum ultraviolet radiation is observed through a sampling orifice in a metal cone inserted directly into an Ar inductively coupled plasma (ICP). The many-lined spectrum of the H₂ Lyman bands is observed from an argon ICP with either H₂ gas or nebulized water added. The most prominent bands are from the ν = 3, 4, 5 or 6 vibrational levels of the B ¹Σ⁺_u electronic state of H₂. The energies of the ν = 3, 4 and 5 levels are close to those of the Ar metastable levels and of photons from the Ar resonance lines (11.5 to 11.8 eV). These H₂ levels are probably populated at least in part by selective energy transfer reactions between Ar metastable atoms and H₂ and/or by absorption of Ar resonance photons by H₂. The H₂ emission emanates only from the upstream reaches of the axial channel, i.e. the part of the axial channel that is inside the load coil and induction region.     

A facile preparation of o-acelated β-d-ribofuranosyl cyanide,an important intermediate of c-nucleoside synthesis

Tri-O-benzoyl and tri-O-acetyl derivatives of the title compound are obtained from the corresponding 1-O-acetates in good yields by treatment with cyanotrimethylsilane.     

Benzoin reaction in water as an aqueous medium catalyzed by benzimidazolium salt

Benzoin reactions are catalyzed by N,N-dialkylbenzimidazole to yield α-hydroxy ketones; the reaction proceeds in water as an aqueous medium under mild conditions. The utility of these salts as pre-catalysts in these reactions has been demonstrated.     

Thermal hazard evaluations of 18650 lithium-ion batteries by an adiabatic calorimeter

In this study, the thermal hazard features of various lithium-ion batteries, such as LiCoO₂ and LiFePO₄, were assessed properly by calorimetric techniques. Vent sizing package 2 (VSP2), an adiabatic calorimeter, was used to measure the thermal hazards and runaway characteristics of the 18650 lithium-ion batteries under an adiabatic condition. The thermal behaviors of the lithium-ion batteries were obtained at normal and abnormal conditions in this study. The critical parameters for thermal hazardous behavior of lithium-ion batteries were obtained including the exothermic onset temperature (T ₀), heat of decomposition (ΔH), maximum temperature (T _max), maximum pressure (P _max), self-heating rate (dT/dt), and pressure rise rate (dP/dt). Therefore, the result indicates the thermal runaway situation of the lithium-ion battery with different materials and voltages in view the of TNT-equivalent method by VSP2. The hazard gets greater with higher voltage. Without the consideration of other anti-pressure measurements, different voltages involving 3.3, 3.6, 3.7, and 4.2 V are evaluated to 0.11, 0.23, 0.88, and 1.77 g of TNT. Further estimation of thermal runaway reaction and decomposition reaction of lithium-ion battery can also be confirmed by VSP2. It shows that the battery of a fully charged state is more dangerous than that of a storage state. The technique results showed that VSP2 can be used to strictly evaluate thermal runaway reaction and thermal decomposition behaviors of lithium-ion batteries. The loss prevention and thermal hazard assessment are very important for development of electric vehicles as well as other appliances in the future. Therefore, our results could be applied to define important safety indices of lithium-ion batteries for safety concerns.