One Singularly Perturbed Problem of Turbulent Gas Dynamics

The problem of the interaction of a Prandtl–Mayer wave with a shear layer is solved using the small parameter method for the case where the flow vorticity in the shear layer is small. A direct expansion is constructed and its inadequacy at large distances from the vortex layer is proved. The strained coordinate method is used to obtain a uniformly adequate expansion. It is shown that for certain velocity distributions in the shear layer, the characteristics in the reflected simple wave resulting from the interaction intersect each other and a shock arises in the flow. There coordinates of the shock origin and the function describing the shock shape are obtained.     

Chemoselective reduction of a lactam carbonyl group in the presence of a gem-dicarboxylate by sodium borohydride and iodine: a facile entry to N-aryl trisubstituted pyrroles

Several N-aryl γ-lactam gem dicarboxylates were chemoselectively reduced to cyclic amine diesters by using sodium borohydride-iodine system. The reduction in all cases was completed within 2.5 h after refluxing in THF. The cyclic amine products were isolated after aqueous (acidic) workup in good yields. Hydrolytic decarboxylation followed by dehydrogenation produced N-aryl carboethoxy pyrroles.     

Covalence reduction factors of four-coordinated pseudo-tetrahedral paramagnetic compounds inD 2d symmetry

Theoretical expressions for the covalence reduction factors of orbital angular momentum and spin-orbit coupling in pseudo-tetrahedral four-coordinated paramagnetic complexes withD _2d symmetry, denoted ask's andR's respectively, have been derived. p ]The mixing coefficients in the antibonding MO's for the CuCl ₄ ^2- ion in three complexes are estimated using suitable approximations. It is shown thatk's must be less thanR's in Tinkham's approximation. Certain misconceptions existing in the literature regarding the value of the integral 〈p _u |???u|s〉 have been clarified.     

Computational Screening of 3 d Transition Metal Atoms Anchored on Defective Graphene for Efficient Electrocatalytic N2 Fixation

The synthesis of ammonia (NH₃) through the electrochemical reduction of molecular nitrogen (N₂) is a promising strategy for significantly reducing energy consumption compared to traditional industrial processes. Herein, we report the design of a series of monovacancy and divacancy defective graphenes decorated with single 3d transition metal atoms (TM@MVG and TM@DVG; TM=Sc−Zn) as electrocatalysts for the nitrogen-reduction reaction (NRR) aided by density functional theory (DFT) calculations. By comparing energies for N₂ adsorption as well as the free energies associated with *N₂ activation and *N₂H formation, we successfully identified V@MVG, with the lowest potential of −0.63 V, to be an effective catalytic substrate for the NRR in an enzymatic mechanism. Electronic properties, including Bader charges, charge density differences, partial densities of states, and crystal orbital Hamilton populations, are further analyzed in detail. We believe that these results help to explain recent observations in this field and provide guidance for the exploration of efficient electrocatalysts for the NRR.     

Structural and Electronic Influences on Rates of Tertpyridine−Amine CoIII−H Formation During Catalytic H2 Evolution in an Aqueous Environment

In this work, the differences in catalytic performance for a series of Co hydrogen evolution catalysts with different pentadentate polypyridyl ligands (L), have been rationalized by examining elementary steps of the catalytic cycle using a combination of electrochemical and transient pulse radiolysis (PR) studies in aqueous solution. Solvolysis of the [Co^II−Cl]⁺ species results in the formation of [Co^II(κ⁴-L)(OH₂)]²⁺. Further reduction produces [Co^I(κ⁴-L)(OH₂)]⁺, which undergoes a rate-limiting structural rearrangement to [Co^I(κ⁵-L)]⁺ before being protonated to form [Co^III−H]²⁺. The rate of [Co^III−H]²⁺ formation is similar for all complexes in the series. Using E_1/2 values of various Co species and pK_a values of [Co^III−H]²⁺ estimated from PR experiments, we found that while the protonation of [Co^III−H]²⁺ is unfavorable, [Co^II−H]⁺ reacts with protons to produce H₂. The catalytic activity for H₂ evolution tracks the hydricity of the [Co^II−H]⁺ intermediate.     

Phenolic resin as a carbon source for the synthesis of monometallic Mo and bimetallic CoMo carbides via carbothermal reduction route

It was the first time that phenolic resin (PR) was used as a carbon source for the synthesis of nanostructured monometallic Mo and bimetallic CoMo carbides via carbothermal reduction route. The results showed that phase-pure β-Mo₂C can be formed under an Ar atmosphere at 900°C or a H₂ atmosphere above 700°C. However, almost pure CoMo carbides (Co₃Mo₃C and Co₆Mo₆C) can be obtained only under a H₂ atmosphere at a low temperature of 630°C for 24 and 48 h, respectively. The role of PR in the preparation process has been investigated and a detailed formation mechanism was proposed based on the experimental results.     

Structure elucidation and stereoselective total synthesis of pavettamine, the causal agent of gousiekte

The structure elucidation of a novel natural product pavettamine (1), the causal agent of the plant toxicosis gousiekte, is reported. The structure was defined by analysis of NMR and MS data and the relative configuration followed from the ¹³C NMR data of the acetonide derivative. The absolute stereochemistry was established by total synthesis from (2S)-malic acid using chiral sulfoxide methodology as (2S,4R,8R,10S)-1,11-diamino-6-aza-undecane-2,4,8,10-tetraol.     

Underpotential surface reduction of mesoporous CeO<Subscript>2</Subscript> nanoparticle films

The formation of variable-thickness CeO₂ nanoparticle mesoporous films from a colloidal nanoparticle solution (approximately 1–3-nm-diameter CeO₂) is demonstrated using a layer-by-layer deposition process with small organic binder molecules such as cyclohexanehexacarboxylate and phytate. Film growth is characterised by scanning and transmission electron microscopies, X-ray scattering and quartz crystal microbalance techniques. The surface electrochemistry of CeO₂ films before and after calcination at 500 °C in air is investigated. A well-defined Ce(IV/III) redox process confined to the oxide surface is observed. Beyond a threshold potential, a new phosphate phase, presumably CePO₄, is formed during electrochemical reduction of CeO₂ in aqueous phosphate buffer solution. The voltammetric signal is sensitive to (1) thermal pre-treatment, (2) film thickness, (3) phosphate concentration and (4) pH. The reversible ‘underpotential reduction’ of CeO₂ is demonstrated at potentials positive of the threshold. A transition occurs from the reversible ‘underpotential region’ in which no phosphate phase is formed to the irreversible ‘overpotential region’ in which the formation of the cerium(III) phosphate phase is observed. The experimental results are rationalised based on surface reactivity and nucleation effects.     

Protonation of 1,3- and 1,4-dinitrobenzene dianions

The protonation of the electrochemically generated (0.1 M solution of tetrabutylammonium perchlorate in DMF) 1,3-dinitrobenzene (1) dianions with phenol, ethanol, tert-butanol, and 1-butyl-3-methylimidazolium cation was studied by cyclic voltammetry and chronoamperometry at the carbositall working electrode. The rate constants for the protonation (k) of the dianion of compound 1 by phenol, 1-butyl-3-methylimidazolium cation, ethanol, and tert-butanol were estimated by the comparison of the data of cyclic voltammetry and chronoamperometry with the digital simulation results, being 100, 50, 1.5, and 0.01 L mol⁻¹ s⁻¹, respectively. The similarly obtained k value for the protonation of the dianion of 1,4-dinitrobenzene (2) with phenol is 13 L mol⁻¹ s⁻¹. According to the quantum chemical calculation results, the protonation of the dianions of compounds 1 and 2 with phenol and 1-butyl-3-methylimidazolium cation can be classified as orbital-controlled reactions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1463–1466, July, 2008.     

Synthese von ungesättigten und gesättigten Aminosäurederivaten durch kathodische Reduktion von Azidozimtsäureester

Cathodic reduction of -azidocinnamic ester under aprotic conditions on Hg, Pt, or graphite electrodes can be directed to high yields of N,N-diacylated dehydroaminoacid derivates (f.i. addition of acetic anhydride) or to almost quantitative yields of -aminocinnamic ester in very pure form by careful addition of H⁺-donors. The dehydroamino compounds in turn can be further reduced to the corresponding saturated compounds by following H⁺-addition and changed electrolysis potential. Almost no dimerization occurs.