Cyclic Conjugation in Benzo-Annelated Perylenes. How Empty is the “Empty” Ring?

Summary. Cyclic conjugation in benzo-annelated perylenes is studied by means of the energy-effects of their six-membered rings. Classical theoretical approaches (based on Kekulé structures, Clar formulae, or conjugated circuits) predict that the central ring in benzo-annelated perylenes is empty and thus negligibly contributes to cyclic conjugation. Our calculations show that this is true only to a limited degree. In particular, rings angularly annelated relative to the central ring significantly increase the extent of its cyclic conjugation.     

1H NMR and DFT study of proton exchange in heterogeneous structures of pyridine-N-oxide/HCl/DCl/H2O

Moderately narrow 1H NMR signals were observed in the solid-phase obtained from pyridine-N-oxide (PyO)...HCl solutions in acetonitrile/H2O after heterogeneous phase separation. High-resolution 1H NMR spectra are compared with those of crystalline PyO...HCl and PyO...DCl. It is concluded that partially resolved peaks in 1H NMR spectra of solids are related with heterogeneity of the spin system and the presence of different mobile H-bond clusters containing PyO, HCl, DCl and water molecules. Some part of non-bonded water or HCl molecules is captured in the cavities of crystalline samples. The attribution of the 1H NMR signals was based on the density functional theory calculation of proton magnetic screening tensor of the most expected H-bond structures in the 6-311G** basis taking into account the solvent effect by the polarized continuum model.     

Study of Thin Oxide Films by Electron, Ion and Synchrotron Radiation Beams

 Titanium oxide and zirconium oxide thin films deposited on silicon substrates were characterised using electron probe microanalysis (EPMA), Rutherford backscattering spectroscopy (RBS), time-of-flight elastic recoil detection analysis (TOF-ERDA) and scanning photoelectron microscopy (SPEM). The composition and mass thickness of the films were determined and the results of different methods compared. It was revealed that the synchrotron radiation used for SPEM studies caused considerable modification of zirconia films grown at low temperatures.     

Analysis of ozone monitoring data

The concentration data obtained by monitoring pollutants do not follow a symmetric or even regular, for example log-normal, distribution so that statistical methods which are insensitive to the type of distribution are required. Such a robust statistical method has been applied for the analysis of data obtained by monitoring ozone in different environments. Since ozone is formed at ground level through photochemical reactions, its volume fractions follow a typical diurnal behaviour with a maximum at about noon and a minimum during the night. A convenient way of representing ozone monitoring data is described. The distribution is represented by percentiles, and for each hour of a day averaged over the whole period. Five most characteric percentiles are represented by so-called boxes and whiskers.     

Isopiestic determination of the osmotic and activity coefficients of the {yKCl + (1 − y)K2HPO4}(aq) system at T = 298.15 K

The osmotic coefficients of aqueous mixtures of KCl and K₂HPO₄ have been measured at T = (298.15 ± 0.01) K by the isopiestic vapor pressure method over the range of ionic strengths from (2.3700 to 11.250) mol · kg⁻¹ using CaCl₂(aq) as the reference solution. Our new experimental results were modeled with an extended form of Pitzer’s ion-interaction model equations, both with the usual mixing terms and with Scatchard’s neutral–electrolyte mixing terms, and with the Clegg–Pitzer–Brimblecombe equations based on the mole-fraction-composition scale. There is a dearth of previously published isopiestic data for mixtures containing salts of and, consequently, no previous measurements are available for comparison with the present results. The present study yields mixing parameters for these three models that are needed for modeling the thermodynamic activities of solute components of natural waters and other complex aqueous electrolyte mixtures.     

Influence of electron-electron scattering on electron relaxation rates in three and four-level quantum cascade lasers in magnetic fields

The aim of this work is to calculate carrier relaxation rates from the upper laser level due to electron-electron interactions in three and four-level quantum cascade lasers (QCLs) in a strong magnetic field. The comparison between calculated results and previously obtained values for acoustical and optical-phonon scattering processes indicates that carrier-carrier scattering might have noticeable influence on laser output properties, depending on the structural design. Numerical results are presented for two λ ∼ 9 μm GaAs-based QCLs in magnetic fields between 20 T and 60 T and the band nonparabolicity is taken into account.     

ab plane ac conductivity in the cuprates: pseudogap effects below Tc

Building on our understanding of the superfluid density rho(s)(T), we show how the pseudogap enters the in-plane optical conductivity sigma(omega,T) for temperatures T相似文献   

Integral averages and oscillation of second order sublinear differential equations

New oscillation criteria are given for the second order sublinear differential equation

Polarization control and sensing with two-dimensional coupled photonic crystal microcavity arrays

We have experimentally studied polarization properties of the two-dimensional coupled photonic crystal microcavity arrays and observed a strong polarization dependence of the transmission and reflection of light from the structures-effects that can be employed in building miniaturized polarizing optical components. Moreover, by combining these properties with a strong sensitivity of the coupled bands on the surrounding refractive index, we have demonstrated a detection of small refractive-index changes in the environment, which is useful for construction of biochemical sensors.     

Synthesis of leucascandrolide A via a spontaneous macrolactolization

We have developed a concise, convergent, and stereocontrolled synthesis of (+/-)-leucascandrolide A (18 steps from commercially available precursors), featuring a complete relay of the initial stereochemical information via a series of diastereoselective transformations. Spontaneous macrolactolization discovered during this synthetic exercise has provided unprecedented access to this macrolide and demonstrated the possibility of accessing even large-ring systems in a highly controlled and efficient manner.