Computational modeling of amorphous silica. 4. Modeling the initial structures. Aerogel

A qualitative classification algorithm for constructing large clusters of aerogel model structures is used. According to this algorithm, aerogel is classified with polysilicas characterized by a packing of long chains of silicon-oxygen tetrahedra. A model of the aerogel structure is suggested, according to which aerogel is described as a set of cross-linked chains forming double or more complex ribbons. This model is in agreement with the conclusions drawn from analysis of the vibration spectra of its components. Institute of Surface Chemistry, Ukrainian Academy of Sciences. Russian University of Peoples' Friendship. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 3, pp. 35–38, May–June, 1994. Translated by O. Kharlamova     

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     

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     

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     

Matrix method of conformational optimization. Application to cyclic and polyhedral water clusters

A matrix method for conformational optimization of quasi-one-dimensional molecular structures is suggested. The method is used to determine cyclic and polyhedral configurations of water clusters with the maximal number of stable trans-conformations of molecular pairs forming hydrogen bonds. Institute of Earth Cryosphere, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 107–115, January–February, 1996 Translated by L. Smolina     

Conformational combinatorial analysis of polyhedral water clusters

A topological model of polyhedral water clusters that takes into account the conformational type of hydrogen bond is suggested. The polyhedral structures are classified according to the number of transand cis-conformations. The structural features of clusters with the maximal number of more stable trans-conformations are studied. Institute of Earth Cryosphere, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 98–106, January–February, 1996 Translated by L. Smolina     

Interactions between cyclic carbon clusters

This work reports on the results of MINDO/3 calculations of the structures and energies of monocyclic and bicyclic carbon clusters. Interactions between the rings and between the cyclic and linear clusters are considered. For the most typical reactions (cyclization of linear clusters and insertion of chains into rings and formation of bicyclic structures), the energy barriers are estimated. The bond energies of the bicyclic structures depend on the parities of the initial rings. The most stable configurations with bond energies of the order of 5 eV result from coupling of the odd-membered rings. It is shown that typical condensation conditions correspond to the absorption of the linear clusters by the cyclic structures. Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences. Institute of Computational Technologies, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp. 991–997, November–December, 1995. Translated from I. Izvekova     

Effect of the Structure of Benzo- and Dibenzocrown Ethers on Their Reaction with the Aerosil Surface

The adsorption of 4-substituted benzo- and 5,5′-disubstituted dibenzocrown ethers from benzene by aerosil A-300 was studied. The ethers included a total of 45 compounds with diphenyl oxide, diphenyl, and diphenyl sulfide fragments. Analysis of the Henry coefficients (K_H) and the extent of desorption, the IR spectra of the adsorbed compounds, and the relationship between their structure and adsorption capacity indicated that all structural fragments of the crown ethers studied interact to some extent with aerosil surface fragments. The number of oxygen atoms in the polyether fragment and the size of the macrocycle are the predominant factors. A large, nonadditive increase in K_H is observed in going from pentadentate to hexadentate crown ethers. __________ Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 41, No. 4, pp. 247–251, July–August, 2005.     

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     

Comparative crystal chemistry of heavy metal fluorides and complex niobates and tantalates in terms of a new crystalline state concept

In many crystal structures of Nb (Ta) fluorides and complex oxides analyzed earlier, cationic sublattices of F, I, and AA types were found (face- and body-centered cubic and hexagonal monolayer sublattices, respectively). The density of anion packing in the cationic sublattice (N_a/N_c — the ration of the number of anions to that of cations) is close to two; this means that the mean valence of a cation is about two in fluorides and about four in oxides. These and other structural features are explained by the tendency of the atomic systems of the structures toward fewer degrees of freedom. The review of the structures suggests that there are stability regions of structures both in the field of cationic sublattice parameters and in the field of compositions. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 5, pp. 907–915, September–October, 1996. Translated by L. Smolina     

Stretching vibrations and structure of (HF) n (n=4–8) clusters

IR spectra of 24 structural isomers of (HF) _n (n=4–8) clusters were calculated in the framework of semiempirical theory of polyatomic molecule vibrations. Based on the results obtained and available experimental data it is proposed that (HF) _n associates comprising 3–5-membered cycles with attached monomeric HF units are present in molecular beams and gas phase.Ab initio calculations performed by the SCF method show the existence of local minima corresponding to such structures on the potential energy surface of (HF) _n clusters (n=4–6). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 435–443, March, 1997.     

Effect of the structure of copper(II) complexes, adsorbed on the surface of SiO2, on their catalytic activity in ozone decomposition

We describe the composition, structure, and catalytic activity in the reaction of ozone decomposition for copper(II) complexes with acido ligands and immobilized Schiff’s bases (propyl benzaldimine derivatives) that are anchored on silica (silica gel, aerosil). We demonstrate methods for controlling their catalytic activity. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 55–60, January–February, 2006.     

Structure of binary sulfides withD 2h 16 symmetry

CAS-PAN analysis for revealing close-packed cristallographic planes is performed to investigate the structures of a series of binary chalcogenides containing heavy cations and having D _2h ¹⁶ , symmetry with a short spacing perpendicular to the mirror plane. For chalcogenides with complex compositions, complex relationships between planes are typically observed. Absolute ordering of atoms by mirror planes generally coexists with ordering along the other two directions. By analogy with simple sulfides, these structures have cationic and anionic matrices of AA′ type, that is, one-layer hexagonal packing of trigon nets with a base plane split into two planes and with a doubled number of sites with respect to the number of atoms. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 2, pp. 324–334, March–April, 2000.     

Isomerization kinetics of carbon clusters 1. Potential of interatomic interaction

Formation of fullerenes — spherical carbon clusters C₆₀ and clusters of other sizes — during condensation of carbon vapors has not yet received theoretical explanation. Recent experimental works concerned with cluster formation in carbon vapors have established that during condensation carbon atoms form rings and then polycyclic clusters, which are precursors of fullerenes. Theoretical investigation of the spontaneous formation of fullerenes from polycyclic rings calls for a simple model of the potential of interatomic interaction of carbon, which would allow fast calculations of bond energies and statistical sums of the clusters. We use the modified Brenner potential, which was developed for hydrocarbon molecules. The parameters of the potential are refitted according to the results of quantum chemical calculations. Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences. Institute of Computation Technologies, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No.4, pp. 664–670, July–August, 1996. Translated by I. Izvekova     

On the percolative character of the liquid-glass phase transition

Mutual arrangement of atomic cells (defined as Voronoi polyhedra) in a large volume is studied by percolation analysis on the Delaunay network in terms of molecular dynamic models of Lennard-Jones liquids, crystals, and amorphous solids. Characteristics of percolative clusters as well as percolation thresholds proved to be almost the same for all the phases. From this it is inferred that the liquid-glass phase transition is not due to percolation over regions with large free volume. Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences. Korean Institute of Advanced Science and Technology. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp. 473–480, May–June, 1995 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.     

Structure and some properties of small water clusters

The energies and structures of many small water clusters (H₂O)_n (n=8-26) are calculated using the atom-atom potential functions suggested earlier. For each n, several stable configurations were found that differ in the number of H-bonds and in the topology of the graphs formed by such bonds. The clusters in which the molecules lie at the vertices of convex polyhedra have the lowest-energy but other configurations may have close or even lower energies. For the most stable clusters, the energy dependence on n is close to linear. At 300 K, the mean energies of the clusters behave similarly. Monte-Carlo simulations showed that the clusters undergo pseudomelting at approximately 200 K. Puebla Autonomous University, Puebla, Mexico, Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences. Institute of Mathematical Problems of Biology, Russian Academy of Sciences. Institute of Physical Chemistry, Russian Academy of Sciences Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 6 pp. 113–121, November–December, 1994. Translated by L. Smolina     

Fragment calculation of electronic structures of polyatomic molecules in the ground state. II. A method of delocalized states of fragments

A fragment method is proposed to calculate the electronic structures of polyatomic molecules in the ground state. Localization and delocalization of the electronic states of molecular fragments are calculated simultaneously. The compact formulation of the method allows algorithmically efficient calculations of the electronic structures of interacting molecular fragments as well as of the whole molecules. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 3, pp. 395–400, May–June, 1995. Translated by I. Izvekova     

Tunnel-exchange states of square-planar bielectron transfer clusters

The tunnel states of square-planar bielectron transfer dⁿ-dⁿ-dⁿ⁺¹-dⁿ⁺¹ (n=0–4) clusters are considered. The nature of the ground spin state is revealed in the limiting case of a strong double exchange. It is shown that the magnetic properties of these systems radically differ from those of clusters with one migrating electron (hole) and tetrameric tetrahedral bielectron transfer clusters. Moldova State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 5, pp. 826–833, September–October, 1996. Translated by I. Izvekova