Kinetics of silver dissolution in sulfide containing thiocarbamide electrolytes

Effective values of reaction order with respect to ligand P, transfer coefficient α, and exchange current i ₀ at constant silver surface coverages θ by sulfide ions are measured. The employed solutions contained from 0.4 to 0.05 M thiocarbamide, 0.5 M HClO₄, 10^?4 M AgNO₃, and from 10^?5 to 10^?4 M Na₂S. It is shown that the exchange current grows approximately linearly from 10^?5 to 1.5 × 10^?4 A/cm² at θ increase in the range from zero to 0.8, while α and P values grow negligibly in the ranges of 0.4–0.45 and 0.9–1.1, accordingly. The obtained results are compared with the data of similar studies of the gold behavior in acidic thiocarbamide solutions. The possible reasons for the different effects of sulfide ion chemisorption on the anodic dissolution of gold and silver in the studied solutions are discussed.     

Manifestations of the pseudogap and superconducting gap of cuprates in photoemission

The degree to which the interpretation of the existence of a pseudogap and a superconducting gap in cuprates on the basis t-t′-U the Hubbard-model corresponds to the data obtained from the photoemission spectra is discussed. The pseudogap in the model is interpreted as the work function of electrons from the insulating parts of the Brillouin zone boundary. On this basis one can explain the angle dependence of the gap measured in the photoemission spectra and its evolution on changes in doping and temperature. In particular, an explanation is found for the decline in the ratio of the angle derivative of the gap near the site, v _Δ = (1/2)dΔ(?)/d?, to the maximum value of the gap, Δ_max, with decreasing doping. That behavior and the different temperature dependence of the gap Δ(?) for different angles are due to the presence of two contributions to Δ with different anisotropies—from the pseudogap and from the superconducting gap. The calculation of the spectral functions confirms the sharp Fermi boundary observed in the direction and the smeared edge of the distribution along the path Γ(0, 0)-M(π, 0)-Y(π, π).     

The impedance-characteristic method in the study of specific temperature features of dielectric anomaly suppression in triglycine sulfate

The reflection and absorption coefficients in a waveguide loaded on ferroelectric triglycine sulfate (TGS), which has a layered domain structure near the phase transition, are calculated by the impedance-characteristic method. The calculated and experimental data do not reveal any correlations with the specific temperature features in the TGS dielectric response to microwave irradiation. The mechanism of the microwave effect is discussed.     

Stereoselective synthesis of dihydropyrimidinethione podand in the presence of l-proline or 4-hydroxy-l-proline and metal nitrates

The asymmetric Biginelli reaction involving a 3-oxobutanoyl-containing podand, benzaldehyde, and thiourea was studied using secondary amines as a chiral inductor, Brönsted acid as a catalyst, and metal salts (especially metal nitrates) as an additive of asymmetric catalysis (AAC) was studied. The tuberculostatically active dihydropyrimidine-thione-containing podand was synthesized with an enantiomeric excess of 57% in the presence of 4-hydroxy-l-proline. In the presence of metal nitrates, the influence of the ionic radius of the cation on the enantioselective excess of the reaction under study was observed, which made it possible to propose a possible mechanism of chiral induction controlled by the complexing ability of the initial β-ketoester-containing podand with metal ions and coordination of the reagents in the transition states.

     

Methods for calculating the resonant part of the atomic factor in crystals with partial filling of the crystallographic position

The resonant part of the tensor atomic factor, which is important for incident radiation energies close to the atom absorption edges, is sensitive to the local environment of a scattering atom (near-range order). In this paper, a method for mathematical modeling of the resonant part of the atomic factor is developed by taking into account crystal-lattice relaxation caused by the partial filling of a crystallographic position. It is shown that an additional contribution leading to the appearance of purely resonance “forbidden” reflections can exist in the resonant atomic factor. As examples, crystals of potassium dihydrophosphate (KDP) and rubidium dihydrophosphate (RDP) are used to calculate such a contribution to the atomic factor of potassium and rubidium, and the last is compared to the thermally induced and dipole-quadrupole contributions to the atomic factor.     

Ab initio simulation of resonant forbidden reflections in Ge crystals

Forbidden reflections are observed in the case of diffraction of synchrotron radiation with wave-lengths close to the absorption edges in crystals. A new method for calculating the intensity of thermal-motion-induced (TMI) forbidden reflections is proposed in this paper. It includes two stages: simulation of instantaneous thermal atomic displacements using ab initio molecular dynamics and subsequent quantum-mechanical calculations of the resonance scattering amplitude for various configurations. This procedure is used to calculate the temperature dependence of the 600 reflection intensity for Ge. The proposed method for simulating forbidden TMI reflections is suitable for any crystal structures and can explain many results so far obtained using synchrotron.     

Sorption of Silver and Chemiluminescent Activity of Clinoptilolite

Sorption conditions of ions and colloidal particles of silver by a zeolite (clinoptilolite) and clays, and specific features of distribution of absorbed silver in clinoptilolite grains were studied. Temperature dependences of the intensity of heterogeneous chemiluminescence from samples of natural clinoptilolite and its modified forms were obtained. The activation energies of the luminescence-active chemisorption of oxygen on electron-donor centers of the zeolite surface and relative amount of centers were determined.     

Luminescence properties of humic substances

A detailed analysis of luminescence properties of humic substances and fulvic acids extracted from peat soil is carried out for the first time. Their fluorescence excitation and emission spectra, fluorescence lifetimes, phosphorescence excitation and emission spectra, and phosphorescence lifetimes at room temperature are investigated. The nature of chromophores of humic substances is discussed. Presented at the International Symposium on Peat Organic Substances, May 15–19, 1995, Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 489–493, July–August, 1997.     

Physical manifestations of valence-bond structures in correlated systems

Using the Hubbard model and the corresponding t-J model, we study the properties of correlated states with valence-bond structures. Mixed states with such structures and with antiferromagnetic spin ordering can be constructed by means of local unitary transformations of uncorrelated states. The latter turn out to have lower energy than mean-field antiferromagnetic solutions. Spin correlations for various degrees of doping δ=n−1 are in good agreement with the results of exact calculations for finite systems. In contrast to mean-field solutions, allowance for valence-bond correlations leads to a reasonable value of the critical δ, at which long-range antiferromagnetic order disappears. A calculation of the spectral functions that describe photoemission reveals typical behavior in two bands of effective hole (and electron) excitations, and energy transport in bands as the quasimomentum varies from (0,0) to (π,π), consistent with calculations in finite systems. We construct a homogeneous correlated state of fluctuating valence bonds (the band-model analog of states of fluctuating valence bonds), and demonstrate that its energy is lower than that of valence-bond alternant structures. Zh. éksp. Teor. Fiz. 112, 1409–1429 (October 1997)