Quantum-chemical analysis of the cluster models of coordination of imidazoline nitroxides on the silica surface

The results of ESR-spectroscopic and quantum-chemical studies of the coordination of 2,2,4,5,5-pentamethyl-3-imidazoline-N-oxyl and 2-phenyl-2,4,5,5-tetramethyl-3-imidazoline-N-oxyl with the surface acid sites (AS) of silica are analyzed. Cluster models of AS, corresponding to one- and two-point coordination and accepted in radiospectroscopy on the basis ofg-factors and constants of hyperfine splitting at the N nuclei in the resulting donor-acceptor complexes, are discussed. Within the framework of the unrestricted Hartree-Fock method using the MNDO approach, a comparative quantum-chemical analysis of the structural, spin, electrostatic, energy, and radiospectroscopic characteristics of the coordination of the model cluster AS to the imidazoline nitroxide probes is performed. For the two-center adsorption, three types of AS structures are considered, together with the paramagnetic surface complexes they form. A consistent semi-quantitative interpretation of the whole set of features found experimentally for the coordination of imidazoline nitroxides to the surface AS on silica is given. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1081–1086, June, 1998.     

Quantum-chemical cluster models for the Si(111)/SiO2 interface

The MPDP method is used to construct cluster models for the Si(111)/SiO₂ interface. The crystallochemical environment of the clusters is included by a method based on the satisfaction of the unusual cluster boundary valence by hydrogen atoms, whose locations, as far as possible, are fixed by the stoichiometry of the charge distribution on the atoms of silicon and oxygen in the model fragment. In these cluster models for the Si(111)/SiO₂ interface, considering only normal atomic displacements relative to their ideal positions, there appears a 7 Å thick transition layer encompassing four surface atomic planes of the silicon substrate and two successive Si-O and O-Si links from the SiO₂ film.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 268–275, May–June, 1990.The authors thank V. A. Chaplanov for stimulating our interest in this problem and for helpful discussions.     

Quantum-chemical investigation into the geometric and energetic characteristics of kaolinite cluster models

A method of constructing molecular models for layered aluminosilicate crystals is put forward. The geometric and energetic characteristics of cluster models are calculated for the elementary kaolinite crystal layer using the MNDO approximation.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 3, pp. 214–218, May–June, 1993.     

Polycyclohexanes: Quantum-chemical models

The steric and electronic structures of organic polymeric structures based on cyclohexane and dodecahydrophenalene were studied in terms of the density functional theory (B3LYP/6-31G*). The examined systems were found to be structurally stable, and their geometric parameters did not differ very strongly from those typical of diamond-like reference structure. Only dodecahydrophenalene-based systems turned out to be auxetic.     

Anion coordination and cation packing in oxides

It is shown by considering a few examples that oxide structures are usefully described in terms of their cation packings and the coordination of the anions by these cations. This, together with a consideration of nonbonded repulsions between the atoms, leads simply to a rationalization of some crystal structures and coordination numbers, and to an understanding of the volume changes in certain phase transitions.     

Valence change in rare earth cluster oxides

Rare earth clusters of europium, thullium and ytterbium were generated by gas aggregation technique and probed by photoionization mass spectrometry. Their relative intensities in mass spectra have shown that their stabilities are governed by compact geometrical structures. The addition of oxygen gas in the nucleation region was used to produce the reactive nucleation. Several stages of oxidation were observed as a function of oxygen pressure up to saturation. For the maximal degree of oxidation the observed oxide ion compositions enable one to follow the valence of metal atom in its oxide as cluster size increases. This exhibits a divalent to trivalent valence change with cluster size. Moreover it emerges from the data that the divalent to trivalent transition for Tm, Yb, and Eu occurs at different size values.     

Acid-base properties of single-phase aluminum oxides

It was found that the surface of single-phase aluminum oxides contains terminal, bridging, and hydrogen-bonded hydroxyl groups, which differ in both concentration and position of related absorption bands. A transition from -Al₂O₃ to -Al₂O₃ does not change the number of absorption bands, but the intensities of these bands decrease. The total concentration of Lewis acid sites in single-phase oxides increases from 2.5 to 5.34 µmol/m² for -Al₂O₃ and -Al₂O₃, respectively. As distinct from other species, -Al₂O₃ contains strong Lewis acid sites ((CO) = 2238 cm^–1). The total concentration of basic sites in aluminum oxides prepared by boehmite dehydration at 600, 800, and 1000°C decreases from 4.86 to 3.72 µmol/m².Translated from Kinetika i Kataliz, Vol. 46, No. 1, 2005, pp. 141–146. Original Russian Text Copyright © 2005 by Kulko, Ivanova, Budneva, Paukshtis.     

Dependence of excitation energy on the coordination of oxygen atoms in cluster models of MgO surface

Cluster models of the MgO surface have been calculated by a semi empirical method. The energies of transitions between the surface states localized on ions of definite coordination are dependent on their coordination numbers. The lower transition energies correspond to a lower coordination number of oxygen ions.

---

, . , , , . .

     

Quantum-chemical calculation on models of hydroxy-containing zeolite fragments

Fragments of the zeolite structure have been calculated by the EHT method. The results are in agreement with spectral data on structural hydroxy groups of zeolites.

---

PMX . .

     

Quantum-chemical investigation of the silicon and carbon coordination bond in their isostructural compounds

Ab initio calculations within the minimal basis set of STO -4LGTO have been carried out on molecules of type H₃MX (M = Si, C; X = H, NH₂, OH, F). The influence of the MH₃-group inversion on the electronic structure of these compounds has been investigated and illustrated by MO electron density maps. The ability of the central atom to form an additional bond has been estimated with the help of calculations on the complexes of these molecules with the hydrogen negative ion. The complexes of type [H₄SiX]^? have been found to be more stable than their unbonded components. The [H₄CX]^? complex formation has not proved to be advantageous.     

Quantum-chemical examination of the factors rule in terms of simplest cluster calculation approaches

CNDO/BW examination of the factors rule in terms of the cluster approach has revealed its validity not only for the adsorption energy and the frequency shift of the stretching vibrations in the formation of hydrogen bonds but also for the variations of the hydrogen atom charge and the OH bond length.

---

CNDO/BW . () , OH .

     

Quantum-chemical interpretation of regioselective coordination in the series of 2-aminoazole—metal complexes

Relative thermodynamic stabilities and competitive bonding (regioselective coordination) of Pearson acids MCl₂ (hard BeCl₂, intermediate ZnCl₂, and soft HgCl₂) with aminohetero-cycles L (2-aminooxazole, 2-aminothiazole, and 2-aminoimidazole) have been calculated by the quantum-chemical MNDO method. It follows from the results of the calculation that in tetrahedral MCl₂·2L complexes the coordination bond is formed by the endocyclic pyridine-type nitrogen atom independent of the type of ligands.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2378–2380, December, 1995.This work was financially supported by the Russian Foundation for Basic Research (Project Nos. 94-03-09731 and 93-03-18692).     

Qualitative quantum-chemical models for surface OH groups of oxides

The dependence of the acidity and frequency characteristics of surface OH groups on the number and electronegativity of their bonded nonoxygen atoms has been analyzed in terms of the Del Re localized molecule orbitals method. The model suggested is applied, in particular, to interpret the nonmonotonous dependence of the stretching vibration frequencies for single-coordinated OH groups on the electronegativity of nonacid atoms for a great variety of oxides.

---

Pe OH- . , , OH- .

     

Surface nanostructures based on tantalum and aluminum oxides

Generalized results from investigations into the processes of formation of tantalum and aluminum oxide nanostructures and their multilayered systems produced by the method of molecular lamination on a (100) silicon surface and aluminum are reported. Conditions for the layer-by-layer growth of oxide structures and multilayered low-dimensional systems with alternating zones of the said oxides are determined. Dielectric characteristics of the synthesized nanostructures are evaluated.     

Size-dependent barriers for reaction of aluminum cluster ions with oxygen

Reactions of cooled, size-selected aluminum cluster ions (Al_n⁺, n = 1–8) with oxygen have been studied at collision energies from 0.15 to 10.0 eV (center-of-mass) under single-collision conditions. With the exception of the atomic ion, all size clusters undergo exoergic reactions which result in extensive fragmentation of the metal cluster framework. Significant energy barriers are found for reaction of all clusters except the dimer. The barrier height increases with cluster size from Al₃⁺ to Al₇⁺, then drops for Al₈⁺.     

Evolution of cluster models in quantum chemical analysis of spectroscopic characteristics of surface structures on oxides at the IOC RAS

The author’s review integrates and analyzes the results of quantum chemical studies on a common topic carried out at the Institute of Organic Chemistry of the RAS (IOC RAS). The cluster approaches to quantum chemical analysis of adsorption and heterogeneous catalysis processes on various oxides proposed and developed at the IOC RAS are characterized. The approaches comprise the construction of covalent and also neutral and partially charged ionic cluster models of surface active sites and their complexes with molecules and free radicals. Examples of chemically acceptable cluster models specially designed for the review and calculated by density functional theory (DFT) with the B3LYP functional and the 6-311G(d) basis set clearly demonstrate the efficiency of using such models to retrieve structure-chemical information from radiospectroscopic, ultraviolet, and infrared spectra of simple molecules and free radicals adsorbed on oxide systems that are widely used in practice.