Analysis of the segmented contraction of basis functions using density matrix theory

A particular formulation based on density matrix (DM) theory at the Hartree-Fock level of theory and the description of the atomic orbitals as integral transforms is introduced. This formulation leads to a continuous representation of the density matrices as functions of a generator coordinate and to the possibility of plotting either the continuous or discrete density matrices as functions of the exponents of primitive Gaussian basis functions. The analysis of these diagrams provides useful information allowing: (a) the determination of the most important primitives for a given orbital, (b) the core-valence separation, and (c) support for the development of contracted basis sets by the segmented method. (C) 2006 Wiley Periodicals, Inc.27151822182

Electronic and structural properties of SnxTi1-xO2 solid solutions: a periodic DFT study

The structural and electronic properties of selected compositions of SnxTi1-xO2 solid solutions (x = 0, 1/24, 1/16, 1/12, 1/8, 1/6, 1/4, 1/2, 3/4, 5/6, 7/8, 11/12, 15/16, 23/24 and 1) were investigated by means of periodic density functional theory (DFT) calculations at B3LYP level. The calculations show that the corresponding lattice parameters vary non-linearly with composition, supporting positive deviations from Vegard's law in the SnxTi1-xO2 system. Our results also account for the fact that chemical decomposition in SnxTi1-xO2 system is dominated by composition fluctuations along [0 0 1] direction. A nearly continuous evolution of the direct band gap and the Fermi level with the growing value of x is predicted. Ti 3d states dominate the lower portion of the conduction band of SnxTi1-xO2 solid solutions. Sn substitution for Ti in TiO2 increases the oxidation-reduction potential of the oxide as well as it renders the lowest energy transition to be indirect. These two effects can be the key factors controlling the rate for the photogenerated electron-hole recombination. These theoretical results are capable to explain the enhancement of photoactivity in SnxTi1-xO2 solid solutions. (C) 2003 Elsevier B.V All rights reserved.854173114515

Sulfide and sulfoxide oxidations by mono- and diperoxo complexes of molybdenum. A density functional study

The molecular mechanism for the oxidation of sulfides to sulfoxides and subsequent oxidation to sulfones by diperoxo, MoO(O-2)(2)(OPH3) (I), and monoperoxo, MoO2(O-2)(OPH3) (II), complexes of molybdenum was studied using density functional calculations at the b3lyp level and the transition state theory. Complexes I and II were both found to be active species. Sulfide oxidation by I or II shows similar activation free energy values of 18.5 and 20.9 kcal/mol, respectively, whereas sulfoxides are oxidized by I (DeltaGdouble dagger = 20.6 kcal/mol) rather than by II (DeltaGdouble dagger = 30.3 kcal/mol). Calculated kinetic and thermodynamic parameters account for the spontaneous overoxidation of sulfides to sulfones as has been experimentally observed. The charge decomposition analysis (CDA) of the calculated transition structures of sulfide and sulfoxide oxidations revealed that I and II are stronger electrophilic oxidants toward sulfides than they are toward sulfoxides.68155870587

Why do peroxomolybdenum complexes chemoselectively oxidize the sulfur centers of unsaturated sulfides and sulfoxides? A DFT analysis

The reasons why oxo-peroxo molybdenum complexes chemoselectively oxidize unsaturated sulfides to the corresponding sulfoxides and these to sulfones without any epoxidation of the electron-rich double bond were elucidated by transition state theory and density functional calculations at the B3LYP level. For the diperoxo model complex with the structure MoO(η(2)-O-2)(2)OPH3 and for allyl methyl sulfide as a model of unsaturated sulfides, the calculations show that the oxygen transfer process from the peroxomolybdenum complex to the sulfur center requires a lower free activation energy (&UDelta; G&DDAG; = 19.0 kcalmol(-1)) than the attack on the double bond moiety (&UDelta; G&DDAG;= 26.4 kcal mol(-1)) of the unsaturated sulfide. Subsequent oxidation of allyl methyl sulfoxide at the sulfinyl group to yield the corresponding sulfone also requires a lower activation energy (&UDelta; G&DDAG; = 21.5 kcal mol(-1)) than the corresponding epoxidation process (&UDelta; G&DDAG; = 27.7 kcal mol(-1)), yielding 2,3-epoxypropyl methyl sulfoxide, These results unambiguously account for these complexes' excellent chemoselectivity towards the sulfur groups in unsaturated sulfides and sulfoxides, as has been observed experimentally. On the basis of SCRF calculations, the level of chemoselectivity is predicted to diminish with increasing solvent polarity. Charge decomposition analysis and the orbital interaction model reveal that unsaturated sulfides with reaction sites carrying lone pair and π electrons behave as nucleophiles toward the electrophilic peroxo oxygen group. The origin of the chemoselectivity is ascribed to the fact that the electronic state related to the sulfur lone pair (HOMO) lies 1.2 eV above that associated with the π electrons (HOMO-1). © Wiley-VCH Verlag GmbH & Co.112406241

The use of the generator coordinate method for designing basis set. Application to oxo-diperoxo molybdenum complexes

The molecular and electronic structures of MoO(O(2))(2) (1), MoO(O(2))(2)(OPY) (2) and MOO(O(2))(2)(OPY)(H(2)O) (3) complexes were investigated at the Hartree-Fock and density functional method (B3LYP) calculation levels. The generator coordinate method (GCM) has been used to design basis sets that properly represent the electronic density on the Mo and O atoms for all electron calculations, while a variant of the GCM method has been employed to design a valence basis set for the Mo atom for the pseudopotential calculations. Compound 1 adopts a distorted tetragonal pyramid structure, where the four peroxo oxygen atoms are located in the same plane, which is perpendicular to the axis defined by the Mo and oxo oxygen atoms. An analysis based upon the geometrical and electronic parameters and the vibrational frequencies renders that 1 can be described as two peroxide fragments bonded to the MoO moiety. 2 and 3 complexes are bipyramidal pentagonal structures, with the OPy ligand occupying a quasi-equatorial position in the same plane as the two peroxo triangles while the H(2)O ligand is situated trans to the oxo group in 3. A comparison between theoretical and experimental results for the geometry and vibrational frequencies of 3 complex shows good agreement. The relationship between the reactivity of 1, 2 and 3 complexes and their coordination number has been established by analyzing the values of the vibrational frequencies, frontier molecular orbitals and the values of electron affinities. (C) 2002 Elsevier Science B.V. All rights reserved.58925126

Periodic study on the structural and electronic properties of bulk, oxidized and reduced SnO2(110) surfaces and the interaction with O-2

The structural and electronic properties of bulk and both oxidized and reduced SnO2(110) surfaces as well as the adsorption process of O-2 on the reduced surface have been investigated by periodic DFT calculations at B3LYP level. The lattice parameters, charge distribution, density of states and band structure are reported for the bulk and surfaces. Surface relaxation effects have been explicitly taken into account by optimizing slab models of nine and seven atomic layers representing the oxidized and reduced surfaces, respectively. The conductivity behavior of the reduced SnO2(110) surface is explained by a distribution of the electrons in the electronic states in the band gap induced by oxygen vacancies. Three types of adsorption approaches of O-2 on the four-fold tin at the reduced SuO(2)(110) surface have been considered. The most exothermic channel corresponds to the adsorption of O-2 parallel to the surface and to the four-fold tin row, and it is believed to be associated with the formation of a peroxo O-2(2-) species. The chemisorption of O-2 on reduced SnO2(110) surface causes a significant depopulation of states along the band gap and it is shown to trap the electrons in the chemisorbed complex producing an electron-depleted space-charge layer in the inner surface region of the material in agreement with some experimental evidences. (C) 2002 Elsevier Science B.V. All rights reserved.5114169940842

Recent research developments in SnO2-based varistors

This paper discusses some advances in research conducted on SnO2-based electroceramics. The addition of different dopants, as well as several thermal treatments in oxidizing and inert atmospheres, were found to influence the microstructure and electrical properties of SnO2-based varistor ceramics. Measurements taken by impedance spectroscopy revealed variations in the height and width of the potential barrier resulting from the atmosphere in which thermal treatments were performed. High nonlinear coefficient values, which are characteristic of high-voltage and commercial ZnO varistors, were obtained for these SnO2-based systems. All the systems developed here have potentially promising varistor applications. (C) 2004 Elsevier B.V. All rights reserved.9011

Evaluation of bulk and surfaces absorption edge energy of sol-gel-dip-coating SnO2 thin films

The absorption edge and the bandgap transition of sol-gel-dip-coating SnO2 thin films, deposited on quartz substrates, are evaluated from optical absorption data and temperature dependent photoconductivity spectra. Structural properties of these films help the interpretation of bandgap transition nature, since the obtained nanosized dimensions of crystallites are determinant on dominant growth direction and, thus, absorption energy. Electronic properties of the bulk and (110) and (101) surfaces are also presented, calculated by means of density functional theory applied to periodic calculations at B3LYP hybrid functional level. Experimentally obtained absorption edge is compared to the calculated energy band diagrams of bulk and (110) and (101) surfaces. The overall calculated electronic properties in conjunction with structural and electro-optical experimental data suggest that the nature of the bandgap transition is related to a combined effect of bulk and (101) surface, which presents direct bandgap transition