Study of the process e+e-→π+π-π0π0 at energies √s< 1 GeV

In an experiment with the Spherical Neutral Detector at VEPP-2M collider the cross section of the process e+e-→π+π-π0π0 was measured. At energies √s < 920 MeV this cross section was measured for the first time. The energy dependence of the cross section is well discribed by the vector dominance model with contributions from ρ, ρ' ρ", mesons. The decay probability ρ→π+π-π0π0was found to be Bρ = (1.60±0.74±0.18)×10-5. The upper limit for the decay ω→π+π-π0π0 was improved by two orders of magnitude compared to the previous measurements and is Bω < 2 × 10-4 at 90% confidence level.     

Attitude stabilization of a rigid body under the action of a vanishing control torque

The problem of attitude stabilization of a rigid body with the use of restoring and dissipative torques is studied. The possibility of implementing a control system in which the restoring torque tends to zero as time increases, and the only remaining control torque is a linear time-invariant dissipative one, is investigated. Both cases of linear and essentially nonlinear restoring torques are considered. With the aid of the Lyapunov direct method and the comparison method, conditions are derived under which we can guarantee stability or asymptotic stability of an equilibrium position of the body despite the vanishing of the restoring torque. A numerical simulation is provided to demonstrate the effectiveness of analytical results.     

Noise Insights into Electronic Transport

Typical experimental measurement is set up as a study of the system’s response to a stationary external excitation. This approach considers any random fluctuation of the signal as spurious contribution, which is to be eliminated via time-averaging, or, equivalently, bandwidth reduction. Beyond that lies a conceptually different paradigm—the measurement of the system’s spontaneous fluctuations. The goal of this overview article is to demonstrate how current noise measurements bring insight into hidden features of electronic transport in various mesoscopic conductors, ranging from 2D topological insulators to individual carbon nanotubes.     

The Role of Bile Acids in the Human Body and in the Development of Diseases

Bile acids are specific and quantitatively important organic components of bile, which are synthesized by hepatocytes from cholesterol and are involved in the osmotic process that ensures the outflow of bile. Bile acids include many varieties of amphipathic acid steroids. These are molecules that play a major role in the digestion of fats and the intestinal absorption of hydrophobic compounds and are also involved in the regulation of many functions of the liver, cholangiocytes, and extrahepatic tissues, acting essentially as hormones. The biological effects are realized through variable membrane or nuclear receptors. Hepatic synthesis, intestinal modifications, intestinal peristalsis and permeability, and receptor activity can affect the quantitative and qualitative bile acids composition significantly leading to extrahepatic pathologies. The complexity of bile acids receptors and the effects of cross-activations makes interpretation of the results of the studies rather difficult. In spite, this is a very perspective direction for pharmacology.     

Elucidating the Role of Oxygen Species in Oxidative Coupling of Methane over Supported MnOx−Na2WO4-containing Catalysts

The present study of oxidative coupling of methane (OCM) over MnO_x−Na₂WO₄/support catalysts demonstrated that the selectivity to C₂H₆ and C₂H₄ (C₂-hydrocarbons) is affected by the kind of support, co-fed water, and the kind of oxidant (O₂ vs. N₂O). In addition to previous studies with MnO_x−Na₂WO₄/SiO₂, an enhancing water effect was obtained using catalysts based on TiO₂- or ZrO₂-containing supports. However, a negative effect on methane conversion was established for SiO₂−Al₂O₃-supported catalysts. Temporal analysis of products with isotopic tracers suggests that the ability of MnO_x−Na₂WO₄ to generate diatomic adsorbed oxygen species depends on the kind of support and is the key property for the water effect. The strength of the water effect on the activity decreases with an increase in the surface area of working catalysts. The kind of support also affects products selectivity due to its influence on the mobility/releasability of lattice oxygen in supported MnO_x−Na₂WO₄. Among the prepared catalysts, MnO_x−Na₂WO₄/TiO₂ was found to be promising for H₂O-assisted OCM. The use of N₂O instead of O₂ further increases the selectivity to C₂-hydrocarbons to 84 % at 6.8 % CH₄ conversion due to the formation of predominantly monoatomic oxygen species from N₂O that selectively convert CH₄ into C₂H₆.     

Low-Spin and High-Spin Perferryl Intermediates in Non-Heme Iron Catalyzed Oxidations of Aliphatic C−H Groups

The selectivity patterns of iron catalysts of the Fe(PDP) family in aliphatic C−H oxidation with H₂O₂ have been studied (PDP=N,N′-bis(pyridine-2-ylmethyl)-2,2′-bipyrrolidine). Cyclohexane, adamantane, 1-bromo-3,7-dimethyloctane, 3,7-dimethyloctyl acetate, (−)-acetoxy-p-menthane, and cis-1,2-dimethylcyclohexane were used as substrates. The studied catalyst systems generate low-spin (S=1/2) oxoiron(V) intermediates or high-spin (S=3/2) oxoiron(V) intermediates, depending on the electron-donating ability of remote substituents at the pyridine rings. The low-spin perferryl intermediates demonstrate lower stability and higher reactivity toward aliphatic C−H groups of cyclohexane than their high-spin congeners, according to the measured self-decay and second-order rate constants k₁ and k₂. Unexpectedly, there appears to be no uniform correlation between the spin state of the oxoiron(V) intermediates, and the chemo- and regioselectivity of the corresponding catalyst systems in the oxidation of the considered substrates. This contrasts with the asymmetric epoxidations by the same catalyst systems, in which case the epoxidation enantioselectivity increases when passing from the systems featuring the more reactive low-spin perferryl intermediates to those with their less reactive high-spin congeners.     

Catalytic inhibition of olefin oxidation with Mn and Cu compounds

Russian Chemical Bulletin - The rate constants and kinetic coefficients of inhibition (fkin) for olefins oxidized with the transition metal compounds (Mn) were measured by highly sensitive...     

Controlling Metal-Oxide Reducibility for Efficient C−H Bond Activation in Hydrocarbons

Knowing the structure of catalytically active species/phases and providing methods for their purposeful generation are two prerequisites for the design of catalysts with desired performance. Herein, we introduce a simple method for precise preparation of supported/bulk catalysts. It utilizes the ability of metal oxides to dissolve and to simultaneously precipitate during their treatment in an aqueous ammonia solution. Applying this method for a conventional VO_x−Al₂O₃ catalyst, the concentration of coordinatively unsaturated Al sites was tuned simply by changing the pH value of the solution. These sites affect the strength of V−O−Al bonds of isolated VO_x species and thus the reducibility of the latter. This method is also applicable for controlling the reducibility of bulk catalysts as demonstrated for a CeO₂−ZrO₂−Al₂O₃ system. The application potential of the developed catalysts was confirmed in the oxidative dehydrogenation of ethylbenzene to styrene with CO₂ and in the non-oxidative propane dehydrogenation to propene. Our approach is extendable to the preparation of any metal oxide catalysts dissolvable in an ammonia solution.     

Ni- and Pd-based homogeneous catalyst systems for direct oxygenation of C(sp3)-H groups

Developing catalytic approaches to selective activation and functionalization of C–H bonds in hydrocarbons and complex organic molecules has been considered as a challenging goal. Recently, significant efforts have been aimed at the search for efficient nickel- and palladium-based catalyst systems, capable of conducting direct aliphatic C–H oxygenation with high and predictable chemoselectivity and regioselectivity. The present review focuses on the advances in homogeneous oxidation of hydrocarbon C(sp³)–H groups, catalyzed by nickel and palladium complexes, and covers the publications of the past 15 years. Correlations between the structure of the metal-based catalyst, steric and electronic properties of the ligands, catalytic conditions, and the catalytic reactivity (efficiency, chemoselectivity, and regioselectivity) are discussed.