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NbN and Nb-Si-N films have been deposited by magnetron sputtering of the Nb and Si targets on silicon wafers at various powers supplied to the Nb target. The films have been investigated by an atomic force microscope, X-ray diffraction, X-ray photoelectron spectroscopy, nanoindentaion and microindentation. The NbN films were nanostructured, and the Nb-Si-N films represented an aggregation of δ-NbN nanocrystallites embedded into the amorphous SiN matrix (nc-δ-NbN/a-SiN). The annealing of the films in vacuum showed that their intensive oxidation occurred at annealing temperature higher than 600°C. To explain the experimental results on the Nb-Si-N films, first-principles molecular dynamics simulations of the NbN(001)/SiN heterostructures have been carried out. [ABSTRACT FROM AUTHOR]

Computational protocols that successfully predict standard reduction potentials of N-heterocyclic compounds in dimethyl formamide and their standard oxidation potentials in acetonitrile were developed. Different solvation models were verified in conjunction with the MPWB1K/6-31 + G(d) level of density functional theory. For reduction potentials calculations, the PCM(UA0) and SMD(Bondi) models were used to compute solvation energies of neutral forms and anion-radical forms, respectively. For oxidation potential calculations, the best results were obtained by a combination of SMD(UAHF) and PCM(Bondi) models to compute solvation energies of neutral forms and cation-radical forms, respectively. The mean absolute deviations (MAD) and root mean square errors (RMSE) of the current theoretical models for reduction potentials were found to be 0.09 V and 0.10, respectively, and for oxidation potentials MAD = 0.12 V and RMSE = 0.16. [ABSTRACT FROM AUTHOR]

High level ab initio studies demonstrate substantial conformational flexibility of amino groups of nucleic acid bases. This flexibility is important for biological functions of DNA. Existing force field models of molecular mechanics do not describe this phenomenon due to a lack of quantitative experimental data necessary for an adjustment of empirical parameters. We have performed extensive calculations of nucleic acid bases at the MP2/6-31G(d,p) level of ab initio theory for broad set of amino group configurations. Two-dimensional maps of energy and geometrical characteristics as functions of two amino hydrogen torsions have been constructed. We approximate the maps by polynomial expressions, which can be used in molecular mechanics calculations. Detailed considerations of these maps enable us to propose a method for determination of numerical coefficients in the developed formulae using restricted sets of points obtained via higher-level calculations. [ABSTRACT FROM AUTHOR]

The potential functions of internal rotation around the C $$_{sp^2 }$$ -S bond in the C6H5S(O)CH3 and C6H5S(O)CF3 molecules were obtained by ab initio MP2(full)/6-31+G* calculations. The stationary points were identified by solving the vibrational problems. The structures in which the plane of the C $$_{sp^2 }$$ -S-C $$_{sp^3 }$$ bonds is approximately perpendicular to the benzene ring plane correspond to the energy minimum. The barriers to rotation around the C $$_{sp^2 }$$ -S bond, corrected for the zero-point vibration energy, are 21.29 [C6H5S(O)CH3] and 28.98 [C6H5S(O)CF3] kJ mol−1. The bond angles (deg) are as follows: 95.7 (CSC), 107.1 ( C $$_{sp^2 }$$ SO), 106.3 ( C $$_{sp^3 }$$ SO) in C6H5S(O)CH3; 93.5 (CSC), 108.2 ( C $$_{sp^2 }$$ SO), 105.2 ( C $$_{sp^3 }$$ SO) in C6H5S(O)CF3. The bond lengths are as follows (Å): 1.520 (S=O), 1.804 ( C $$_{sp^2 }$$ -S), 1.810 ( C $$_{sp^3 }$$ -S) in C6H5S(O)CH3; 1.507 (S=O), 1.799 ( C $$_{sp^2 }$$ -S), 1.870 ( C $$_{sp^3 }$$ -S) in C6H5S(O)CF3. According to the results of NBO calculations, the formally double S=O bond consists of a strongly polarized covalent σ bond (S→O) and an almost ionic bond. An increase in the S=O bond multiplicity relative to a single bond is mainly due to hyperconjugation by the mechanism n(O)→σ*( C $$_{sp^2 }$$ -S) and n(O)→σ*( C $$_{sp^3 }$$ -S) and, to a lesser extent, by interaction of the oxygen lone electron pairs with the Rydberg orbitals of the S atoms, characterized by a large contribution of the d component. [ABSTRACT FROM AUTHOR]

The MINDO/3 semiempirical approximation in SCF MO LCAO has been used to examine whether a cluster scheme can be used to calculate the electronic structures for aluminum oxides. Conditions are defined under which the hypervalent environment of the aluminum atom is stable under geometry optimization. A univalent pseudoatom model is compared with an uncharged ionic one to estimate the geometrical optimization. A univalent pseudoatom model is compared with an uncharged ionic one to estimate the geometrical and energy characteristics for clusters representing three-coordinated, four-coordinated, and six-coordinated Al. [ABSTRACT FROM AUTHOR]

In an analysis of the activity and selectivity of five-membered aromatic heterocycles in electrophilic substitution, we demonstrated the unsuitability of static reactivity indices obtained from calculations of isolated molecules. This unsuitability is related to nonobservance of the non-cross-over rule for the potential curves of these reactions. [ABSTRACT FROM AUTHOR]

An analysis of the mechanism for the effect of substituents and heteroatoms on the direction of electrophilic attack in monosubstituted benzene, furan, thiophene, and pyrrole was carried out within the framework of the Swain-Lupton, Yukawa- Tsuno, and Dewar-Grisdale methods. The variation in the resonance factor is in good accord with the activity series for the heterocycles in electrophilic substitution reactions, while description of the selectivity of electrophilic attack requires consideration of both the resonance and inductive effects of the substituent and of the heteroatom. [ABSTRACT FROM AUTHOR]

We have investigated the mechanism of gas-phase chlorination of acetylene using an ab initio quantum chemical method in a 3-21G° basis. We show that formation of cis and trans addition products may be connected with the appearance of carbene in the reaction medium, formed upon addition of a chlorine molecule to one of the acetylene carbon atoms. We have found that intermediate carbene may also be a source for the appearance of radicals in the reaction medium. [ABSTRACT FROM AUTHOR]

The potential functions of internal rotation around the C $$_{sp^2 }$$ -S bond in the C6H5S(O)CH3 and C6H5S(O)CF3 molecules were obtained by ab initio MP2(full)/6-31+G* calculations. The stationary points were identified by solving the vibrational problems. The structures in which the plane of the C $$_{sp^2 }$$ -S-C $$_{sp^3 }$$ bonds is approximately perpendicular to the benzene ring plane correspond to the energy minimum. The barriers to rotation around the C $$_{sp^2 }$$ -S bond, corrected for the zero-point vibration energy, are 21.29 [C6H5S(O)CH3] and 28.98 [C6H5S(O)CF3] kJ mol−1. The bond angles (deg) are as follows: 95.7 (CSC), 107.1 ( C $$_{sp^2 }$$ SO), 106.3 ( C $$_{sp^3 }$$ SO) in C6H5S(O)CH3; 93.5 (CSC), 108.2 ( C $$_{sp^2 }$$ SO), 105.2 ( C $$_{sp^3 }$$ SO) in C6H5S(O)CF3. The bond lengths are as follows (Å): 1.520 (S=O), 1.804 ( C $$_{sp^2 }$$ -S), 1.810 ( C $$_{sp^3 }$$ -S) in C6H5S(O)CH3; 1.507 (S=O), 1.799 ( C $$_{sp^2 }$$ -S), 1.870 ( C $$_{sp^3 }$$ -S) in C6H5S(O)CF3. According to the results of NBO calculations, the formally double S=O bond consists of a strongly polarized covalent σ bond (S→O) and an almost ionic bond. An increase in the S=O bond multiplicity relative to a single bond is mainly due to hyperconjugation by the mechanism n(O)→σ*( C $$_{sp^2 }$$ -S) and n(O)→σ*( C $$_{sp^3 }$$ -S) and, to a lesser extent, by interaction of the oxygen lone electron pairs with the Rydberg orbitals of the S atoms, characterized by a large contribution of the d component. [ABSTRACT FROM AUTHOR]