Orbital phase control of the conformations of α- and β-substituted enamines and vinyl ethers

The conformational stabilities of the α- and β-substituted enamines and vinyl ethers were predicted by orbital phase theory and confirmed by ab initio molecular orbital calculations. Cyclic interaction significantly occurs among the nonbonding orbital n _Y for the lone pair on the hetero atom Y (N in the enamines or O in the ethers), the π and π* orbitals of the CC bond, and the σ_C-H or σ*_C-X orbitals on the substituent CH₂X. The cyclic -n _Y-π-σ_C-H-π*- interaction is favored by the orbital phase continuity in the α-substituted molecules, while the cyclic -n _Y-π-σ*_C-X-π*- interaction is favored in the β-substituted molecules. The most stable conformation was then predicted to be synperiplanar or (pseudo)equatorial in the α-substituted molecules and anticlinical or (pseudo)axial in the β-substituted molecules. Received: 8 May 1998 / Accepted: 30 July 1998 / Published online: 16 November 1998     

Direct evidence for the plant origin of Brazilian propolis by the observation of honeybee behavior and phytochemical analysis

To identify the plant origin of Brazilian propolis directly, we observed the behavior of honeybees in Minas Gerais State of Brazil. Honeybee workers bit and chewed leaves of the plant, Baccharis dracunculifolia, packed the material into their pollen basket, brought it back to their nest, and used it as propolis. We collected the leaves of B. dracunculifolia and propolis, and compared their constituents by liquid chromatography-mass spectrometry (LC/MS) analysis. There was no difference between the chemical constituents of the ethanol extracts of B. dracunculifolia and those of propolis. This indicates directly that the plant origin of Brazilian propolis is B. dracunculifolia.     

Pressure and Temperature Effects on the Molecular Rotation in Acetonitrile-Water Mixtures

NMR spin-lattice relaxation time (T ₁) measurements were performed for ¹⁴N of acetonitrile in acetonitrile (CH₃CN)—H₂O mixtures and for ²H of heavy water in CH₃CN—D₂O mixtures at 30°C up to 294.2 MPa together with those for ²H in CH₃CN—D₂O mixtures at 10 and 20°C under atmospheric pressure over the whole composition range of the mixtures. IR absorption spectra for CH₃CN—H₂O and CH₃CN—10 mol% HDO/D₂O mixtures were obtained at 30°C under atmospheric pressure. Densities and viscosities of CH₃CN—H₂O mixtures were also measured under high pressure. The rotational correlation times for D₂O [τ _c (D)] and acetonitrile [τ _c (N)] were determined from T ₁ measurements. Under atmospheric pressure, τ _c (D) exhibits a small maximum around 10 mol% of acetonitrile at each temperature, and the maximum position is almost independent of temperature. These results suggest that the dipole–dipole interaction between acetonitrile and water molecules plays an important role in determining the rotational motion of water molecules in the mixtures. This is supported by the variation of the peak for the bending vibration of water molecules with composition. The decreases in τ _c (D) and τ _c (N) at higher acetonitrile contents are ascribed to the formation of acetonitrile dimer, trimer, and oligomer aggregates. Except for τ _c (D) in the water-rich region, the pressure coefficients of τ _c (D) and τ _c (N) are positive which is understood as a simple compression effect. Furthermore, the composition of mixture at which τ _c (D) and τ _c (N) show a maximum shifted to higher acetonitrile content with increasing pressure. These results are discussed in terms of the pressure effect on the equilibria of acetonitrile monomers with the aggregates of acetonitrile in the mixtures.     

Flow-injection catalytic spectrophotometic determination of molybdenum(VI) in plants using bromate oxidative coupling of p-hydrazinobensenesulfonic acid with N-(1-naphthyl)ethylenediamine

A novel flow-injection spectrophotometry has been developed for the determination of molybdenum(VI) at nanograms per milliliter levels. The method is based on the catalytic effect of molybdenum(VI) on the bromate oxidative coupling of p-hydrazinobenzenesulfonic acid with N-(1-naphthyl)ethylenediamine to form an azo dye (λ_max = 530 nm). Chromotropic acid (4,5-dihydroxy-2,7-naphthalenedisulfonic acid) acted as an effective activator for the molybdenum(VI)-catalyzed reaction and increased the sensitivity of the method. The reaction was monitored by measuring the change in absorbance of the dye produced. The proposed method allowed the determination of molybdenum(VI) in the range 1.0-20 ng mL⁻¹ with sample throughput of 15 h⁻¹. The limit of detection was 0.5 ng mL⁻¹ and a relative standard deviation for 10 ng mL⁻¹ molybdenum(VI) (n = 10) was 2.5%. The interfering ions were eliminated by using the combination of a masking agent and on-line minicolumn packed with cation exchanger. The present method was successfully applied to the determination of molybdenum(VI) in plant foodstuffs.     

Micro‐scale (1.5 µm) sulphur isotope analysis of contemporary and early Archean pyrite

We present a method for in situ sulphur (S) isotopic analysis of significantly small areas (1.5 µm in diameter) in pyrite using secondary ion mass spectrometry (NanoSIMS) to interpret microbial sulphur metabolism in the early earth. We evaluated the precision and accuracy of S isotopic ratios obtained by this method using hydrothermal pyrite samples with homogeneous S isotopic ratios. The internal precision of the δ³⁴S value was 1.5‰ at the level of 1 sigma of standard error (named 1SE) for a single spot, while the external reproducibility was estimated to be 1.6‰ at the level of 1 sigma of standard deviation (named 1SD, n = 25). For each separate sample, the average δ³⁴S value was comparable with that measured by a conventional method, and the accuracy was better than 2.3‰. Consequently, the in situ method is sufficiently accurate and precise to detect the S isotopic variations of small sample of the pyrite (less than 20 µm) that occurs ubiquitously in ancient sedimentary rocks. This method was applied to measure the S isotopic distribution of pyrite within black chert fragments in early Archean sandstone. The pyrite had isotopic zoning with a ³⁴S‐depleted core and ³⁴S‐enriched rim, suggesting isotopic evolution of the source H₂S from ?15 to ?5‰. Production of H₂S by microbial sulphate reduction (MSR) in a closed system provides a possible explanation for both the ³⁴S‐depleted initial H₂S and the progressive increase in the δ³⁴S_H2S value. Although more extensive data are necessary to strengthen the explanation for the origin of the MSR, the results show that the S isotopic distribution within pyrite crystals may be a key tracer for MSR activity in the early earth. Copyright © 2010 John Wiley & Sons, Ltd.     

Hard X-ray photoelectron spectroscopy study of core level shifts at buried GaP/Si(001) interfaces

We present a study of buried GaP/Si(001) heterointerfaces by hard X-ray photoelectron spectroscopy. Well-defined thin (4–50 nm) GaP films were grown on Si(001) substrates with 2° miscut orientations by metalorganic vapor phase epitaxy. Core level photoelectron intensities and valence band spectra were measured on heterostructures as well as on the corresponding reference (bulk) substrates. Detailed analysis of core level peaks revealed line broadening and energetic shifts. Valence band offsets were derived for the films with different thickness. Based on the observed variation of the valence band offsets with the GaP film thickness and on the experimental evidence of line broadening, the existence of charge displacement at the GaP/Si(001) interface is suggested.     

Chemical stability of La1−xSrxCrO3 in oxidizing atmospheres

The single-phase region of La_1−xSr_xCrO₃ (x=0.1, 0.2, 0.3) was precisely determined as a function of temperature, P_O2 and Sr content. The powders with the nominal composition of La_1−xSr_xCrO₃ were equilibrated under various conditions, and then identified by XRD analyses. To confirm the equilibration, two independent experiments were performed for each composition observing (i) the precipitation of the second phase from a single-phase solid solution, and (ii) the formation of the single phase from the constituent oxides. Two kinds of second phases, SrCrO₄ and an unknown phase, were observed depending on the conditions. The second phases tended to appear at low temperature, in high P_O2 and with a large Sr content. The single-phase regions obtained via the two equilibration routes were in good agreement with each other. The thermodynamic calculations on the supposition of ideality of the solid solution essentially reproduced the experimental results. When this material is used as the interconnects of solid oxide fuel cells, much attention should be paid to its relatively narrow solubility range of Sr; for example, the solubility limit is approximately 0.1 under a typical cathode-side condition (1273 K, air).     

A Rapid Sample Preparation Procedure Using MonoSpin CBA and Amide Columns for Tetrodotoxin Detection in Serum and Urine Using LC–MS/MS Analysis

Clinical diagnosis of tetrodotoxin (TTX) poisoning can be difficult because of the lack of characteristic morphological findings and a screening test, such as an immunoassay. Here, we present a fully validated method for the analysis of TTX in serum and urine. In this method, serum and urine samples were extracted using MonoSpin CBA or amide columns, followed by LC–MS/MS analysis. The TTX was eluted from the column by 0.1 mL of 10 % acetic acid solution, and was directly injected into LC–MS/MS. An Agilent 1200 HPLC system equipped with a HILIC separation column (Zorbax HILIC Plus 2.1 × 100 mm, 3.5 μm) was used for isocratic elution, with a mobile phase of 10 mM ammonium formate with formic acid (95:5, v/v), along with 5 mM trifluoroacetic acid and 2 % acetonitrile. TTX was detected with an Agilent 6410 mass spectrometer utilizing positive electrospray ionization and multiple reaction monitoring. Limits of quantification for serum and urine were established to be 1 and 0.5 ng mL^?1, respectively. Limits of detection for serum and urine were 0.5 and 0.25 ng mL^?1, respectively. Intra-day and inter-day precision varied from 1.5 to 8.5 %. The recovery was >86.5 % for both matrices. In this method, the sample preparation process prior to injection into the LC–MS/MS takes approximately 10–15 min, which reduces the extraction time to one-tenth of that of previous methods. The application of this method was further verified by analysis of biological materials from a patient suffering from puffer fish poisoning.     

Origin of hydrophilicity of cellulose hydrogel from aqueous LiOH/urea solvent coagulated with alkyl alcohols

Surface and structural properties of cellulose hydrogel prepared from LiOH/urea solvent with alcoholic coagulation were examined. As coagulants, alcohols from methanol to butanol were employed. Alcohol with high water miscibility (MeOH, EtOH, 1-PrOH, 2-PrOH, and t-BuOH) gave a nano-porous structure consisting of a fibrous network of cellulose, while alcohol with low water miscibility (1-BuOH, 2-BuOH, i-BuOH) showed aggregation of a fibrous structure because of large shrinkage during the coagulation process. Congo red adsorption measurement showed that an increase in the carbon number of alcohol brought about a less hydrophobic surface. This is likely to occur because the alkali/urea/cellulose complex was formed during the coagulation process in the case of ethanol, propanol, and butanol, leading to a higher crystalline content of cellulose II, the surface of which is thought to be highly hydrophilic.     

Phase stability of La1−xCaxCrO3−δ in oxidizing atmosphere

The chemical stability of perovskite-type La_1−xCa_xCrO_3−δ (x=0.1, 0.2, 0.3) in high oxygen partial pressure, P_O2, was investigated with three methods: thermogravimetry, XRD analysis, and thermodynamic calculation. The second phase, CaCrO₄ was observed by XRD analysis on the powder equilibrated in high P_O2. Thermogravimetry under fixed temperatures sensitively detected the segregation of the second phase in the form of oxygen incorporation, because oxidation of chromium ion accompanies the segregation. The second phase tended to appear in high P_O2 and at low temperature. The single-phase regions of La_1−xCa_xCrO_3−δ obtained from the two experimental methods well agreed with each other. The results of thermodynamic calculation on the assumption of ideality of the solid solution also agreed with the experimental results. These results suggested the sufficient chemical stability of La_1−xCa_xCrO_3−δ in high P_O2 concerning the application to an interconnector of high-temperature solid oxide fuel cells; for example, La_0.7Ca_0.3CrO_3−δ is stable at 1273 K in air.