Diimidazo[4,5-b:4′,5′-e]pyridine: synthesis and nucleophilic aromatic substitution reaction

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Problems of synthesis of tetrahedral organosilsesquioxanes. Hydrolytic condensation of hexafunctional branched organotetrasiloxanes

The hydrolytic condensation of methyltris(methyldichlorosiloxy)silane (1b), methyl(1a), and vinyltris(methyldiethoxysiloxy)silanes (1c) in dilute homogeneous solutions was studied. It was found by X-ray diffraction and¹H NMR methods that, irrespective of reaction conditions, only octaorganooctasilsesquioxanes (T ₈), namely, octamethyloctasilsesquioxane and 1,4-divinylhexamethyloctasilsesquioxane, were obtained instead of the expected tetraorganotetrasilsesquioxanes (T ₄). These data suggest that the condensation processes involved in the hydrolysis of1a–c are predominantly intermolecular, and the molecules of the starting oligomers do not undergo rearrangements. Probably, the formation ofT ₄, whose molecules are built of four strained six-membered organosiloxane cycles, is less favorable thermodynamically than the formation of their homologs (T ₈) built of eightmembered organosiloxane cycles.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1332–1339, July, 1995.     

Spectral functions in the hubbard model with half-filling

Under the assumption of long-range antiferromagnetic order at low temperatures, the spectral functions and the density of states are calculated in the two-dimensional Hubbard model with half-filling in the Hubbard-I approximation. The results are compared with the data obtained using an exact numerical technique, namely, the quantum Monte Carlo method. The influence of hopping to the next-to-nearest neighbor on the formation of the electronic structure is considered.     

Underestimating the width of the bandgap in the electronic spectra of La2CuO4

It is shown that the width of the bandgap determined by the simultaneous analysis of the experimental photoelectron and inverse photoemission spectra of the surface of La₂CuO₄ using a common energy scale is underestimated by 1 eV. The electronic and satellite structures of the spectra of La₂CuO₄ are calculated on the basis of the multiband p-d model and the sudden perturbation approximation. It is shown that shakedown processes shift the one-electron contour of the final two-hole configuration of the photoelectron spectrum 1 eV down the energy scale and shift the contour of the final d ¹⁰ configuration of the inverse photoemission spectrum downward by 2 eV; these shifts cause the energy splitting between the filled and empty bands to be underestimated. Fiz. Tverd. Tela (St. Petersburg) 39, 449–451 (March 1997)     

BCS instability of a two-layer system of composite fermions

Pairing instability is considered for a two-layer electron system in a strong magnetic field with an even-fractional filling ν=1/(2m) (m is an integer) of the lowest Landau level in each of the layers. The limit of large distance d between the layers is analyzed. Microscopic analysis is carried out in the eikonal approximation in the composite-fermion formalism. It is found that the condition for pairing instability in this model is independent of d. Due to the marginal character of the composite-fermion system, pairing instability in the particle-particle (BCS) channel arises only for η<2 or η=2, but Mv ₀>43, where η and v ₀ are the parameters of the assumed electron-electron interaction, v∝v ₀/r ^η, and M is the band electron mass. In the particle-hole (isospin density wave) channel, instability is not observed.     

d-symmetry superconductivity due to valence bond correlations

It is shown that d-symmetry superconductivity due to valence bond correlations is possible. Valence bond correlations are compatible with antiferromagnetic spin order. In order to explictly construct a homogeneous state with the valence bond structure in the two-dimensional Hubbard model for an arbitrary doping, we have used the variational method based on unitary local transformation. Attraction between holes in the d-channel is due to modulation of hopping by the site population in course of the valence bond formation, and corresponding parameters have been calculated variationally. An important factor for the gap width is the increase in the density of states on the Fermi level due to antiferromagnetic splitting of the band. The gap width and its ratio to the T _c are 2Δ≃0.1t and 2Δ/kT _c≃4.5−4 for U/t≃8. The correspondence between the theoretical phase diagram and experimental data is discussed. The dependence of T _c on the doping δ=|n−1| and the Fermi surface shape are highly sensitive to the weak interaction t′ leading to diagonal hoppings. In the case of t′>0 and p-doping, the peak on the curve of T _c(δ) occurs at the doping δ _opt, when the energy of the flattest part of the lower Hubbard subband crosses the Fermi level at k∼(π,0). In underdoped samples with δ<δ _opt, the anisotropic pseudogap in the normal state corresponds to the energy difference |E(π,0)−μ| between this part of the spectrum and the Fermi level. Zh. éksp. Teor. Fiz. 114, 985–1005 (September 1998)     

Fault Detection in DC Electro Motors Using the Continuous Wavelet Transform

Two time–frequency methods were used to detect typical faults in DC electro motors: the windowed Fourier transform and the continuous wavelet transform. Four groups containing three electro motors each were manufactured with typical faults and examined. These faults included a bearing fault, an increased unbalance, a fragmented brush and a fragmented collector. The velocity of the vibrations at selected points on the electro motors was measured with a laser probe. The parameters of both transforms were selected in order to make both methods comparable. Because of the poor frequency variance, the windowed Fourier transform was, in this case, proven to be inferior to the continuous wavelet transform. Therefore, the continuous wavelet transform was chosen as the primary tool for fault detection.Three criteria were found that successfully discriminated between the typical faults. These were the highest magnitude level, the frequency of the first and second harmonics and the time period between the magnitude pulses in the third (highest) frequency region. If the maximum magnitude levels versus the period of the pulses in the third frequency range are plotted, four distinct regions corresponding to four different faults are obtained. Since the regions do not overlap, linear classifiers can be used with the presented criteria.     

Excitation of Complicated Shapes in Three Dimensions

We experimentally verified a recently proposed technique for the excitation of a complicated three-dimensional profile (CARVE,completelyarbitraryregionalvolumeexcitation). CARVE is based on a generalized DANTE RF pulse sequence and a synchronous string of gradient steps. Provided there is no limitation in the number of pulses, CARVE can generate an excitation profile of any shape with any resolution. However, hardware limitations and sample properties restrict the number of RF pulses and gradient steps and, thus, limit attainable resolution of the excitation profile. We theoretically and experimentally showed that spatial resolution can be increased by distributing a long sequence among several CARVE experiments and summing up their signals. This is particularly important for three-dimensional excitation profiles where ann-fold increase in resolution requires ann³-fold increase of the number of events in the sequence. The potential use of three-dimensional CARVE might be in spectroscopic imaging where the excitation profile can be tailored to match the shape of a selected organ or body part.     

New progress in the cobalt-catalysed synthesis of aromatic organozinc compounds by reduction of aromatic halides by zinc dust

The experimental conditions of the cobalt-catalysed synthesis of aromatic organozinc species from the corresponding bromides have been optimised. The preparation of numerous aromatic organozinc reagents under these optimised conditions is reported, with yields between 67 and 99%.     

Magnetic collapse and electronic phase transitions at high pressure in transition metal oxides

A review of electronic and magnetic phase transition in metal oxides with strong electron correlations (SEC) is given. The bandwidth control of the insulator gap is expected in the Hubbard model when the decreasing of the interatomic distance results in the bandwidth W(P) increase and at some critical value P_c, W(P_c)∼U and the Mott–Hubbard gap disappears. The other situation takes place in transition metal boroxides FeBO₃ and GdFe₃(BO₃)₄, where the increase of crystal field parameter Δ(P) results in the high spin–low spin crossover.