Computational modeling of amorphous silica. 3. Modeling the initial structures. Silica gel

A qualitative algorithm for construction of large clusters of silica gel model structures is used. According to this algorithm and in line with our classification of dispersive silicas, silica gel is regarded as cyclosilica, which is characterized by loose packing of the cycles of silicon-oxygen tetrahedra. A model of silica gel structure is proposed, where the primary structural fragments are hollow frameworks with a network surface. The units of the surface network are formed by nonplanar [SiO₄] chain cycles of various sizes. This structural model makes it possible to explain the peculiarities of the vibration spectra of the framework, hydroxyl, and aqueous components of silica gels. Institute of Surface Chemistry, Ukrainian Academy of Sciences. Russian University of Peoples' Friendship. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 3, pp. 27–34, May–June, 1994. Translated by O. Kharlamova     

Computational modeling of amorphous silica. 2. Modeling the initial structures. Aerosil

A qualitative algorithm for constructing large clusters of aerosil model structures is used. According to this algorithm and the classification of amorphous silicas, aerosil is classified with tectosilicas, which are characterized by close packing of silicon-oxygen tetrahedra. Two quantitative algorithms for constructing large close-packed clusters are proposed. The structures of the clusters having from 10 to 24 silicon atoms, completely optimized by quantum chemical methods, were obtained. Small, medium, and large clusters for modeling the local and collective properties of atomic and functional group packing in the aerosil structure are distinguished. Institute of Surface Chemistry, Ukrainian Academy of Sciences. Russian University of Peoples' Friendship. Translated fromZhurnal Struktumoi Khimii, Vol. 35, No. 3, pp. 16–26, May–June, 1994. Translated by O. Kharlamova     

A new algorithm for the variational solution of the vibronic problem for polyatomic molecules

A new effective algorithm for solving the complete vibronic problem by the variational method is proposed. The algorithm reduces the variational matrix by successively including the shift and entanglement of normal coordinates and using recurrent formulas for determining the eigenvectors of the matrix. No additional approximations are used. The method is faster than those used previously by two or more orders of magnitude. This work was supported by the Russian Fundamental Research Fund (93-02-3405). K. A. Timiryazev Moscow Agricultural Academy. Translated fromZhurmal Strukturnoi Khimii, Vol. 35, No. 6, pp. 23–30, November–December, 1994. Translated by O. Kharlamova     

Computational modeling of amorphous silica. 1. Modeling the starting structures. A general conception

This paper presents concepts underlying computational modeling of dispersive silicas. An object for modeling is polyatomic nanometer-sized clusters. A chemical software package adapted for the PC AT 386(486)/387 is utilized. A structure-sensitive method of vibration spectroscopy is suggested to verify the results. Ultradispersive materials are considered to be an adequate object to establish feedback between computational and real experiments. The program is realized on a set of dispersive silicas; an algorithm for construction of large clusters is proposed, which is based on the concept of radical difference of dispersive silicas. From analysis of vibration spectra we suggest that aerosils, silica gels, and aerogels should be regarded as amorphous tecto-, cyclo-, and polysilicas, respectively, in line with Liebau's classification of crystal silicates. The category of silica is determined by its production procedure. Institute of Surface Chemistry, Ukrainian Academy of Sciences. Russian University of Peoples' Friendship. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 2, pp. 74–84, March–April, 1994. Translated by O. Kharlamova     

Synthesis and structure of the product of bis-condensation of 4-methyl-2,6-diformylphenol with dimethyl N′,N′-hydrazine diacetate

Bis-condensation of 4-methyl-2,6-diformylphenol with dimethyl N′,N′-hydrazine diacetate gave product I. The structure of I was determined by X-ray analysis. The conformations of the two side chains of dimethyl N′,N′-hydrazine diacetate are significantly different. The position of one chain is fixed by the intramolecular H-bond of O−H.…N type. Due to its conformation, compound I is partly ready for complex formation with d-metals. Institute of Applied Physics, Academy of Sciences, Moldova Republic. Institute of Chemistry, Academy of Sciences, Moldova Republic. Translated fromZhurnal Struktumoi Khimii, Vol. 35, No. 3, pp. 91–98, May–June, 1994. Translated by L. Smolina     

Structural inhomogeneity types of perovskite-like superconductors. Relation to growth conditions

Literature data and our studies are used to consider three types of structural inhomogeneity in perovskite-like superconductors. Their irregular mixed-layered structures differ in the nature and distribution of impurity layers. A three-dimensional diagram is suggested for each type of defect. A great variety of postgrowth conditions are discussed to choose optimal conditions for obtaining perfect single crystals with the desired content of oxygen and T_s. St. Petersburg State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 5, pp. 15–22, September–October, 1994. Translated by O. Kharlamova     

Isomerization kinetics of carbon clusters 2. Transformation of tricyclic structures to fullerenes

A quantitative model of isomerization of carbon clusters is proposed, and possible ways of transformation of tricyclic structures to fullerenes are considered in order to account for the spontaneous transformation of the polycyclic clusters arising during the condensation of the carbon vapor. The isomerization process is calculated thermodynamically using the modified classical interatomic potential, the relative effeciencies of different mechanisms of the fullerene formation are estimated, and the main regularities of this process are established. The results of the simulation are consistent with the experimental data. The model can be employed in the calculation of long isomerization chains of complex molecules. The accuracy of the quantitative estimation of the kinetics is determined by the errors in the calculations of the bond energies of the isomers. Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences. Institute of Computation Technologies, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 671–681, July–August, 1996. Translated by I. Izvekova     

Simultaneous electron diffraction and mass spectrometric studies of molecular structure of manganese trifluoride

It is shown that two geometrical models of the MnF₃ molecule equally fit experimental electron diffraction data: a pyramidal model of C_3v symmetry and a planar model with nonequivalent F-F distances of C_2v symmetry. Ivanovo Chemical Technological Institute. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 4. pp. 61–67, July–August, 1994. Translated by L. Smolina     

Topology of polyhedral clathrate frameworks. 2. Frameworks constructed of polyhedral stories

The main features of the most widespread class of polyhedra clathrate frameworks formed of polyhedral stories are investigated. An algorithm for constructing such frameworks is suggested. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 1, pp. 123–130, January–February, 1996. Translated by L. Smolina     

Regularization method and molecular integrals with singular exponential functions

The regularization principle, which is based on the concept of linearly independent singular functions, makes it possible to calculate many important types of molecular matrix elements arising in the variational LCAO-MO-SCF scheme. This is done using a direct approach that employs reduction of these elements to finite sums of convergent and divergent one-electron integrals. A universal algorithm is developed to calculate two-center one-electron molecular integrals involving both singular and ordinary Slater functions. The numerical stability of the algorithms and the accuracy of the integral calculation are analyzed, and numerical estimates are given. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Struktunoi Khimii, Vol. 35, No. 2, pp. 3–11, March–April, 1994. Translated by L. Chernomorskaya     

Investigation of structure and energetics of β-diketonates. III. Conformational study of copper(II) dipivaloylmethanate. Determination of the internal rotation barrier oftert-butyl groups

Ivanovo State University. Ivanovo Chemical Engineering Institute. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 4, pp. 135–138, July–August, 1994.     

The notion of structure in modern chemistry. II. Structural variety of condensed phases. Generalized crystal chemistry

The structural variety of condensed phases with crystalline (c) or quasicrystalline (q) order or complete/partial disorder (d) is represented as a triangular diagram. The c, q, or d characteristic refers to one of the three dimensions so that the type of condensed phase is denoted by three symbols (ccc is the ideal crystal; cqd is a phase where disorder is combined with crystalline and quasicrystalline orders). Comparative studies of various condensed phases with data extrapolation to other states is termed generalized crystal chemistry. M. V. Lomonosov Moscow State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 3, pp. 99–102, May–June, 1994. Translated by L. Smolina     

The shape of vibronic absorption bands and the structure of dye dimers

The shape of absorption bands of rhodamine 6G (R6G) and methylene blue (MB) dimers is compared with calculations by the Hemenger theory based on the model of the degenerate ground state (DGS). An interpretation of the origin of long-wave maxima in the spectra of the dimers is proposed from which it follows that their structure belongs to the “sandwich” type with a parallel position of axes of the monomers forming a dimer. Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 1, pp. 49–52, January–February, 1994. Translated by L. Chernomorskaya     

Electronic structure analysis of 4-chlorophenylisoxazoles and 4-chloroisoxazolines by35Cl NQR spectra and quantum chemical calculations

Electronic structure of substituted phenylisoxazoles and substituted isoxazolines is examined by³⁵Cl NQR and MINDO/3 calculations. It is shown that the chlorine atom in the 4-position of the isoxazole ring serves to transmit the influence from the phenyl ring in the 3-position to that in the 5-position. The influence is transferred via both the σ- and π-bond systems. Tomsk State Pedagogical Institute. Institute of Chemistry, A. Mitskevich University. Institute of Biophysics, Health Ministry of Russia. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 2, pp. 61–68, March–April, 1994. Translated by L. Smolina     

Structural transition to electrolyte-water solvent and changes in the molecular dynamics of water and properties of solutions

A model of concentration transition “ions and complexes in a water structure → ionic and ionicaqueous clusters → polymer structures of salt and crystal hydrate melts” is suggested. The appearance of cluster nanostructures outside the first zone of a waterlike structure is regarded as a general characteristic of solutions. The characteristics of solutions, phase equilibria, and salts of complex composition are interpreted based on this model. Investigation of the complex dielectric constant of electrolytic solutions in the SHF and EHF bands (7–119 GHz) at high concentrations showed that there are two dispersion regions in which the relaxation times differ by a factor of 5–10. Relaxation processes are separated, the numbers of molecules in hydration shells are calculated, and relaxation times are determined for bulky tetrahedral water with hydration shells of ions, for clusters, and for ionicaqueous polymer chains. It is shown that the two structure subsystems of water molecules in concentrated solutions may be described using the limited rotator/generalized diffusion molecular model. Translated fromZhumal Strukturnoi Khimii, Vol. 39, No. 5, pp. 851–863, September–October, 1998.     

Preparation of Alumina Aerogel Films by Low Temperature CO2 Supercritical Drying Process

Alumina aerogel thin films were formed by a new synthesis route. Sols were prepared by the Yoldas process. Gels were formed by sol evaporation in a few hours. Films were prepared by dip coating glass or alumina substrates into both the sols and the gels. Aerogel films with special morphology were produced for the first time by exchanging the film solvent with acetone after the dip coating, followed by supercritical drying. The morphology of the films, studied by SEM and TEM, consists of fiber-like network of round chains (≈0.1 μm thick), and pores (0.1–0.5 μm in diameter). It is shown that the fibers contain a homogeneous arrangement of sol particles, 2–4 nm in size. Formation of this microstructure can be attributed to phase separation in the alumina-water-acetone system in a 2D film geometry. A conceptual model for the film development is proposed.     

Classification of heavy-metal fluorides. fluorides with fluorite-like cation arrangement

The main feature of fluorite structure type is four families of crystallographic planes with dense trigon cation nets which are mutually oriented as the faces of a regular octahedron. A complicated cation composition may cause lowering of general symmetry to tetragonal (only one four-fold axis remains in the octahedral complex), rhombohedral (one three-fold axis), orthorhombic, and monoclinic. The geometric parameters of the cation sublattice remain constant. They are close to the parameters of the ideal face-centered cubic cation sublattice of fluorite (the measure of similarity is 1.00–0.81). This is true for all analyzed fluoride structures and some U⁴⁺, Th⁴⁺, Zr, and Ln oxyfluorides. Thus, the concept is confirmed that the crystalline state is determined by systems of parallel planes filled by dense heavy-atom nets. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Institute of Crystallography, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 35, No. 1, pp. 71–80, January–February, 1994. Translated by T. Yudanova     

The use of reductive amination for separation of M- and G-chains of laminarans

A method for the separation of M- and G-chains of laminarans (reserve β-glucans of brown algae) is proposed. G-chains were converted to the corresponding pyridylamino derivatives by treatment with 2-aminopyridine and dimethylamine-borane complex followed by the separation of derivatized and underivatized chains by ion-exchange chromatography on Spheron S-1000. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1813–1817, October, 1993.     

Effect of terminal groups on the relative stability of linear mercury clusters

The problem of stabilizing the linear clusters of mercury by terminal groups is discussed in terms of the MNDO method for the following model and real compounds: Hg_n (I), Hg_nCl₂ (II) and Hg_n(AlCl₄)₂ (III) with n=2–4. It is shown that the terminal acceptor groups stabilize the linear mercury chains. It is established that in systems (II) and (III), unlike in (I), the highest occupied molecular orbitals are bonding. Mordoviya State University. Institute of Organoelement Compounds, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 4, pp. 623–629, July–August, 1995. Translated by I. Izvekova     

Decomposition of electronic properties and calculation of solvation energies for surface-active molecules

For ions and polar molecules located near a surface, we derived and analyzed the general formulas to calculate their solvation energies. St. Petersburg State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 6, pp. 3–12, November-December, 1994. Translated by A. Arbuznikov