Preparation and stereoselectivity of 1,3-dipolar cycloaddition ofD-glucose-derived nitrones to N-arylmaleimides

Summary Nitrones2 derived fromD-glucose oxime and benzaldehydes without employing any protection of hydroxyl group were isolated in pure state. The 1,3-dipolar cycloaddition of2 to N-arylmaleimides gave predominantly theanti isoxazolidines3 and was rationalized byZ/E isomerization of N-glycosylnitrones2. The structure and steric configuration of the products have been assigned on the basis of¹H- and¹³C-NMR spectroscopy. AM1 calculations of the nitrones and MM2 calculations of the adducts were performed.

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Darstellung und Stereoselektivität der 1,3-dipolaren Cycloaddition vonD-Glucose-abgeleiteten Nitronen an N-Arylmaleimiden

Zusammenfassung Die Nitrone2 wurden ausD-Glucoseoxim und Benzaldehyden ohne Schutz von Hydroxylgruppen in reinem Zustand erhalten. Die 1,3-dipolare Cycloaddition von2 an N-Arylmaleimiden ergab bevorzugt dieanti-Isoxazolidine3; dies wurde über eineZ/E-Isomerisierung der N-Glycosylnitrone2 rationalisiert. Struktur und Stereochemie wurden auf Basis von¹H- und¹³C-NMR-Spektroskopie ermittelt. Außerdem wurden AM1-Berechnungen an den Nitronen und MM2-Rechnungen an den Addukten ausgeführt.

     

Antiferromagnetism of poly(3-dodecylthiophene) irradiated by γ-beams

The magnetic properties of poly(3-dodecyl-thiophene) (PDDT) irradiated by -beams have been studied by measuring the magnetic susceptibility and EPR spectra. The irradiated PDDT is characterized, similarly as the non-irradiated one, by an antiferromagnetic course of magnetic susceptibility with a Néel temperature of 237 K. It is suggested that the antiferromagnetic properties of PDDT are caused by transformation of polarons to bipolarons, which takes place at lower temperatures.     

Tautomerism of xanthine: The second-order MØller-Plesset study

Four 9H and four 7H tautomers of DNA base xanthine were studied by the ab initio LCAO-MO method at the MP2/6-311G^**//HF/6-31G^** and MP2/6-31G^**//HF/6-31G^** approximations. All calculated structures are minima at the HF/6-31G^** potential energy surface with the dioxo 7H tautomer (A1) being the global minimum. The second most stable tautomer, dioxo-9H (B1) is by 9 kcal/mol less stable. For the A1 B1 transition the calculated MP2 energy gap corresponds to the equilibrium constant of 2 × 10^–7. Therefore, only the major tautomeric form A1 is predicted to be detectable in the gas phase. The 7H and 9H groups of tautomers are discussed separately. Within both groups, the dioxo form (A1-7H, B1-9H) is the most stable one and is succeeded by the 2-dihydroxy (A2, B2) form. However, while the energy difference between A1 and A2 is 10 kcal/mol, the energy difference between B1 a B2 is only 2 kcal/mol. The effect of polar environment was estimated by the SCRF method, using a spherical cavity, at the HF/6-31G^** level. These calculations did not change the gas phase stability order of the tautomers. However, the energy difference between A1 and B1 decreased from 9 kcal/mol at the HF/6-31G^** level to 4 kcal/mol at the SCRF HF/6-31G^** level.     

Behavior of radiocesium in atmospheric arerosol and fallout in Prague

Radiocesium in ground layer atmospheric aerosol and fallout in Prague has been examined. After a decline in 1986–1988, controlled by a sum of two negative exponentials,¹³⁷Cs aerosol concentration reached a constant level based on an equilibrium between its deposition and resuspension. Deposition velocity was compared to that of⁷Be and²²⁶Ra. It confirmed the supposed source of¹³⁷Cs as resuspension from the ground. Resuspension factor of the order of magnitude of 10^–9 was determined, reminding similar values found earlier in Prague as well as those found in Munich. It also agrees well with the USAEC resuspension model. Solubility of¹³⁷Cs was measured in combined wet and dry fallout. The average undissolved fraction of¹³⁷Cs was found to be about 70% which can be explained by the conditions of the¹³⁷Cs aerosol formation in Chernobyl.     

Measurement of the electron distribution function in flowing afterglow plasma by means of Langmuir probe

Conclusion In the present report an attempt has been made to use the second derivative method for measurements of the electron distribution function in flowing afterglow plasma. It has been shown that using the cross-correlation technique, this method seems to be a useful tool for flowing afterglow plasma investigation.     

On general relations in relativistic kinematics and some of their applications

Two vector fields are considered, a timelike one,u , and an arbitrary one, . The relative expansion and rotation are defined with respect to these fields, their mutual relations are studied, and some general formulas obtained. Applications are made, first to vector fields which are mutually nonrotating in a plane-symmetric metric, then to the electromagnetic field of an arbitrary magnetohydrodynamic fluid.     

Exciton diffusion and interfacial charge separation in meso-tetraphenylporphyrin/TiO2 bilayers: effect of ethyl substituents

The photoinduced charge separation efficiency in porphyrin/TiO2 bilayers has been determined using the time-resolved microwave conductivity (TRMC) technique. Porphyrins investigated are unsubstituted meso-tetraphenylporphyrin (TPP) and meso-tetra(4-ethylphenyl)porphyrin (TEPP). TEPP/TiO2 bilayers exhibit a charge separation efficiency per incident photon at the Soret band maximum of 6.2%, which is considerably higher than the efficiency of 1.2% found for TPP/TiO2 bilayers. Exciton diffusion lengths of 7 A for TPP and 75 A for TEPP are obtained from fitting a model for the charge separation efficiency to the experimental data. Optical measurements on the porphyrin derivatives on quartz yield a 20 times higher fluorescence quantum yield and a 7 times higher fluorescence rate constant for TEPP layers as compared to TPP layers. The exciton lifetime of 800 ps found for TEPP layers is considerably longer than the lifetime of 260 ps in TPP layers. The exciton diffusion coefficients, determined from the exciton diffusion length and the exciton lifetime, are found to be 2.10(-9) m(2)/s for TPP and 7.10(-8) m(2)/s for TEPP. The difference is discussed in terms of the presence of face-to-face dimers or larger aggregates in TPP layers.     

A Novel Uranyl Complex with Dimeric Lacunary Polyoxoanion: [(A-α-SiW<Subscript>9</Subscript>O<Subscript>33</Subscript>)<Subscript>2</Subscript>K{UO<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)}<Subscript>2</Subscript>]<Superscript>11−</Superscript>

A novel uranyl complex with dimeric lacunary polyoxoanion like open-mouthed clam, Na₅[(A-α-SiW₉O₃₃H₃)₂K{UO₂(H₂O)}₂], was prepared and characterized by elemental analysis, infrared and ultraviolet–visible spectroscopy and single crystal X-ray diffraction. In the anion, two A-α-SiW₉O₃₄¹⁰⁻ groups share two terminal oxygen atoms O_d′ derived from removal of three corner-shared W atoms from saturated α-Keggin anion, forming a dimeric anion with an open mouth in which potassium ion and uranyl ions are coordinated. Uranium atom adopts a pentagonal bipyramidal geometry. The coordinating anions are linked by sodium ions via coordination of terminal or bridging oxygen atoms, forming two-dimensional layer arrangement. Between the layers are the hydrogen bonds from which a supramolecular architecture is created. UV–VIS spectrum gives W–O and U–O charge transfer transitions at 230–265 and 432 nm, showing the change of geometry of the polyanion and weakening of the U–O bonds of the uranyl cation. Electronic supplementary material The online version of this article () contains supplementary material, which is available to authorized users.     

Study of a fluorescence quenching mechanism of enoxacin and its determination in human serum and urine samples

The maximum emission wavelength of dopamine is 317 nm with excitation at 290 nm. The relative fluorescence intensity of dopamine decreased in the presence of enoxacin, which showed that fluorescence quenching occurred. The Stern-Volmer (S-V) plot showed a nonlinear relationship between the relative fluorescence intensity of dopamine and the concentration of enoxain. The quenching mechanism was studied and the results suggested that both dynamic and static quenching processes were responsible for the observed positive deviation in the S-V plot. When the S-V plot was modified by logarithm, the linear relationship was obtained between logF ₀/F and C in the range of 0.10 to 13.0 μg/mL (where F ₀ is the relative fluorescence intensity of dopamine, F is the relative fluorescence intensity of dopamine in the presence of enoxacin, and C is the concentration of enoxacin). The fluorescence quenching method for the determination of enoxacin was developed. The linear regression equation of the calibration graph of enoxacin was C = 13.70 (logF ₀/F) − 0.5836, with the correlation coefficient 0.9984. The detection limit was 2.0 ng/mL and the relative standard deviation was 2.52%. The effects of pH, the stability of dopamine in the presence of enoxacin, and foreign ions on the determination of enoxacin have been examined. The recovery of enoxacin was from 94.9 to 103.0% in a human serum sample and from 94.9 to 108.0% in a urine sample. The method is simple, rapid, and can be used for the determination of enoxacin in human serum and urine samples with satisfactory results. The text was submitted by the authors in English.