Home – Home – Jornals – Advanced Search

This paper presents a solution of a sequence of one-dimensional boundary-value problems of thermal stresses defining the elastic-plastic deformation processes used in the shrink fitting of cylindrical bodies. The origin and development of plastic flow in the materials of the assembly elements are studied taking into account the temperature dependence of the yield strength of these materials. During temperature equalization, the flow can slow down, followed by unloading and formation of a residual stress field providing an interference fit. The conditions of formation and motion of the boundaries of the elastic and plastic states in both plastic flow and during unloading are determined.

By means of B3LYP/6-311++G(3df,3pd) the electron density distribution in the propargyl radical CH₂CCH is obtained. Within the Quantum Theory of Atoms in Molecules the phenomenon of conjugation and the spin density distribution of the unpaired electron in CH₂CCH are studied at the qualitative level. Characteristics of the electronic structure of CH₂CCH and its parent molecules CH₃-C≡CH and CH₂=C=CH₂ are compared. With the use of the rigid rotator-anharmonic oscillator model the thermodynamic properties of the propargyl radical and enthalpies of bond cleavage in propyne and allene are calculated in the temperature range 298-1500 K. The relationship between the electronic and thermodynamic properties of CH₂CCH is considered and its conjugation energy is calculated.

Within density functional theory the experimentally observed stereoeffects of the ligand environment in low-spin bis-chelates of Ni(II), Pd(II), and Pt(II) with aromatic azomethines is modeled. It is shown that complexes with the MN₂O₂ coordination core are characterized by the trans -configuration and in complexes with MN₂S₂ or MN₂Se₂ cores the stabilization of the cis -configuration occurs. The relationship is found between the composition of metal cycles and their conformation (an inflection along the donor atom line), the degree of steric hindrances in the cis -configuration due to the interligand interaction of R substituents at azomethine nitrogen atoms and the relative stability of cis - and trans -isomers of the complex.

In the work a prediction method based on modified Randic indices to estimate the enthalpies of vaporization under standard conditions D_vap H ⁰(298.2) is proposed for esters with different structures and numbers of ester groups. It is shown that the proposed method enables the prediction of enthalpies of vaporization of esters with an accuracy as good as experimental.

Based on density functional theory with regard to the dispersion interaction the crystal structure and electronic properties of C₄H₈N₁₂O₆ and C₄H₈N₁₀Cl₂O₈ are studied. Atomic structural parameters, bond populations, atomic charges, energy and spatial electron distributions are calculated. Differences in the studied characteristics caused by the non-equivalence of atoms are shown. A partially covalent nature of anion-cation bonds is revealed. The cationic nature of the lower unoccupied states is established, which results in a small band gap of ~1.7 eV as compared to other nitrates and perchlorates.

Ab initio calculations are employed to investigate nitrogen inversion as a configuration change that can supply an extremely useful switchable control mechanism for some complex systems. In this paper, the design of a new artificial rotary molecular machine based on nitrogen inversion is discussed. The introduced design of a molecular rotator is based on the reciprocating motion of a substituent due to the inversion phenomenon, leading to the rotary motion in the molecule. Since simple secondary amines easily face the inversion process at room temperature, aziridine is selected as the initial driver for the molecular motion. The most obvious finding from this study is that, following the displacement of the substituent attached to the aziridine nitrogen atom, two rotary motions occurr in the molecule, one clockwise and another counterclockwise with a 39.52 to 150.09° angle domain.

By the low-gradient Čzochralski method, both undoped and copper ion activated Li_{2-2 x}Mg_{2+ x}(MoO₄)₃ crystals are grown. The charge state and structural position of impurity copper ions are determined using EPR. Investigations of the luminescence properties reveal that luminescence with a maximum at λ = 520 nm is observed for Li_{2-2 x}Mg_{2+ x}(MoO₄)₃ crystals. A decrease in the temperature increases the intensity of this luminescence. It is found that the doping of Li_{2-2 x}Mg_{2+ x}(MoO₄)₃ crystals with copper ions also increases the luminescence intensity with a maximum at λ = 520 nm. It is supposed that cation vacancies, which provide the charge compensation when copper ions substitute for lithium ions, are responsible for the luminescence.

4-Amino-1,2,4-triazolium hexafluoridoniobate(V) and hexafluoridotantalate(V) (C₂H₅N₄)MF₆ (M = Nb, Ta) crystallizing in the monoclinic system (space group P 2₁/ n ) are synthesized for the first time and their crystal structures and spectroscopic features are studied by single crystal X-ray diffraction and ¹H and ¹⁹F NMR spectroscopy. The crystal structures of isostructural (C₂H₅N₄)MF₆ compounds are formed of octahedral complex [MF₆]^- anions (M = Nb, Ta) and monoprotonated heterocyclic 4-amino-1,2,4-triazolium cations (C₂H₅N₄)⁺ organized in a three-dimensional structure via N-H⋯F and N-H⋯N hydrogen bonds. The character and types of ion motions in the fluoride sublattice of (C₂H₅N₄)MF₆ are determined in a wide temperature range.

An iron(II) complex with tris(pyrazol-1-yl)methane (HC(Pz)₃) of the composition [Fe(HC(Pz)₃))₂]xEu(dipic)₂(Hdipic)]x2H₂O, containing an outer-sphere complex anion, is synthesized. The compound is examined by single crystal and powder X-ray diffraction, UV-Vis and IR spectroscopies, and static magnetic susceptibility. The investigation of the temperature dependence χ(T) in the temperature range 80-470 K reveals that the polycrystalline phase of the compound undergoes spin-crossover ¹А₁ <=> ⁵Т₂. The transition is accompanied by thermochromism (color change pink <=> white).

New biligand complexes of iron(III) are synthesized based on 3,4,5-tri(tetradecyloxy)benzoyloxy-4-salicylidene-N'-ethyl-N-ethylenediamine azomethine with the outer sphere NO3, PF6, Cl^-, BF4, Cl4, and CNS^- anions. All the target compounds are characterized by gel exclusion chromatography, elemental analysis, and electron, IR, and NMR spectroscopy. The presence of complex-forming ions is confirmed by FT-IR spectra in the far region. The formation of biligand polychelate complexes with an octahedral packing of the iron ion is observed. Phase transitions in the resulting coordination compounds are studied by differential scanning calorimetry and optical polarizing thermomicroscopy. The presence of several polymorphic crystalline modifications, as well as mesophases, is established. Mesomorphic properties are found for complexes with chloride and tetrafluoroborate anions.