Quasi-Rayleigh waves in sandwich structures: Dispersion equation, eigenmodes, and resonance reflection

The dispersion equation for quasi-Rayleigh acoustic modes in transversely isotropic sandwich-type structures (an elastic layer between two substrates) has been obtained in the analytical form. The equation is analyzed for three types of structures—a layer between two soft substrates, a layer between a soft and a hard substrate, and a structure consisting of two contacting substrates with markedly different values of hardness. Eigenmodes are determined for all three structures. Depending on the combination of the material parameters in these structures, these eigenwaves are either Rayleigh-type waves localized at interfaces in the substrates and accompanying a Lamb wave in the interlayer (in the absence of an interlayer, forming a Stoneley wave) or the leaky modes characterized by a small imaginary addition to the phase velocity and the presence of bulk partial waves of leakage providing some energy removal from the interfaces. Without any calculations, simple criteria were established that allow one to predict the existence of a leaky mode in the given structure and the number of leakage fluxes it contains. The resonance reflection is analized for the case where the incidence angle of the wave in the substrate corresponding to the velocity v of the wave-front propagation along the interface is close to the value of the real part of the phase velocity of the leaky mode.     

The use of new chiral phosphite and amidophosphite ligands in the Rh-catalyzed hydrogenation of dehydro-??-amino acid derivatives

New chiral phosphite-type ligands were synthesized. The tests of these ligands in the asymmetric Rh-catalyzed hydrogenation of N-acetyl derivatives of dehydro-??-amino acid esters showed their high enantioselectivity (up to 75% ee). The reaction of nucleophilic addition of phthalimide to disubstituted alkynes offering access to esters of N-phthaloyldehydro-??-amino acids was discovered. Higher conversion and enantioselectivity in the hydrogenation of N-acetyl and N-phtaloyl derivatives of dehydro-??-amino acids were observed in fluorinated alcohols as compared to common organic solvents.     

Allyl substitution in a supercritical carbon dioxide medium catalyzed by complexes of palladium with phosphite-type ligands

Allyl amination and alkylation catalyzed by a palladium complex-phosphite-type-ligand catalyst in a supercritical carbon dioxide (SC-CO₂) medium is studied. It is demonstrated that that the Pd-containing catalysts dissolved in SC-CO₂ provide a high regioselectivity of the reaction and, thus, offer a convenient method for synthesizing allylamines.     

The Onset and Nonlinear Regimes of Convection in a Two-Layer System of Fluid and Porous Medium Saturated by the Fluid

The onset of convection and its nonlinear regimes in a heated from below two-layer system consisting of a horizontal pure fluid layer and porous medium saturated by the same fluid is studied under the conditions of static gravitational field. The problem is solved numerically by the finite-difference method. The competition between the long-wave and short-wave convective modes at various ratios of the porous layer to the fluid layer thicknesses is analyzed. The data on the nature of convective motion excitation and flow structure transformation are obtained for the range of the Rayleigh numbers up to quintuple supercriticality. It has been found that in the case of a thick porous layer the steady-state convective regime occurring after the establishment of the mechanical equilibrium becomes unstable and gives way to the oscillatory regime at some value of the Rayleigh number. As the Rayleigh number grows further the oscillatory regime of convection is again replaced by the steady-state convective regime.     

Intensification of acoustic beams in crystals at mode conversion near total internal reflection

Geometrical conditions under which mode conversion occurs in a crystal near total internal reflection of an acoustic beam are found. In this case, the entire energy of an incident pump wave is spent on the excitation of a narrow intense reflected beam close in structure to an eigenmode. A consistent choice of orientations of the sagittal plane and surface that excludes the reflection of a parasitic wave of leakage is found.     

Diamidophosphites with isomeric carborane fragments: a comparison of catalytic activity in asymmetric Pd-catalyzed allylic substitution reactions

New chiral diamidophosphite ligands containing electron-donating 9-meta-carborane and electron-withdrawing 1-meta-carborane substituents have been synthesized. The ligand 3a with an electron-donating group demonstrated high enantioselectivity in Pd-catalyzed allylic substitution reactions with C-, S- and N-nucleophiles (up to 98% ee). The isomeric diamidophosphite 3b bearing an electron-withdrawing substituent showed in all cases, moderate-to-poor conversion and lower enantioselectivity.     

Silica gel modified with tetraalkylammonium halides as an available and efficient catalyst for the synthesis of cyclic organic carbonates from epoxides and CO2

Russian Chemical Bulletin - Silica gel with the additives of tetraalkylammonium halides serves as an efficient catalyst for the solvent-free insertion of carbon dioxide into epoxides (10–56...     

Size of the proton emission region inp-C,d-C, α-C and C-C interactions at 4.2 GeV/c per incident nucleon from two-proton correlations

Correlations between secondary protons with small relative momenta have been studied in interactions of various light nuclei with the carbon target at 4.2 GeV/c per incident nucleon with the aim to determine the radius of the proton emission region. This radius is found to decrease with increasing momentum and transverse momentum of the proton pair, but it does not show any dependence on the mass of the projectile nucleus.     

Dispersion of photon density waves in a strongly scattering medium

In the approximation of the radiation transfer equation, the damping decrement and the velocity of photon density waves propagating in a strongly scattering medium are studied as functions of the increasing modulation frequency of the source of radiation up to the microwave frequency range. The results obtained for microwave frequencies are found to be in qualitative agreement with those derived from the Bethe-Salpeter equation.