Fabrício R. Sensato

Static simulation of bulk and selected surfaces of anatase TiO2

A theoretical investigation has been carried out to characterize bulk and selected surfaces of anatase TiO2. The calculations are performed using a B3LYP hybrid functional and 6-31G basis set within the periodic density functional approximation. Optimization procedures have been employed to determine the equilibrium geometry of the crystal and slab surface models. The compressibility, band structure, and the bulk and surface charge distributions are reported. The surface relative energies are identified to follow the sequence: (001)<(101)<(100)≪(110)⋘(111), from the most stable surface to the least stable one. Relaxation of (0 0 1) and (1 0 1) surfaces are moderate, with no displacements exceeding ≈0.19 A. The theoretical results are compared with previous theoretical studies and available experimental data.

Periodic study on the structural and electronic properties of bulk, oxidized and reduced SnO2(1 1 0) surfaces and the interaction with O2

The structural and electronic properties of bulk and both oxidized and reduced SnO2(1 1 0) surfaces as well as the adsorption process of O2 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(1 1 0) 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 O2 on the four-fold tin at the reduced SnO2(1 1 0) surface have been considered. The most exothermic channel corresponds to the adsorption of O2 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 O2 on reduced SnO2(1 1 0) 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. 2002 Elsevier Science B.V. All rights reserved.

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

The structural and electronic properties of selected compositions of Sn xTi1−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 TiO 2 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.

The origin of photoluminescence in amorphous lead titanate

We discuss the nature of visible photoluminescence at room temperature in amorphous lead titanate in the light of the results of recent experimental and theoretical calculations. Experimental results obtained by XANES and EXAFS revealed that amorphous lead titanate is composed of a Ti-O network having fivefold Ti coordination and NBO-type (non-bridging oxygen) defects. These defects can modify the electronic structure of amorphous compounds. Our calculation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the lead titanate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous lead titanate. The results of our theoretical calculations of amorphous lead titanate indicate that the formation of fivefold coordination in the amorphous system may introduce delocalized electronic levels in the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital). A comparison of the experimental and theoretical results of amorphous compounds suggests the possibility of a radiative recombination (electron-hole pairs), which may be responsible for the emission of photoluminescence.

Olefin Epoxidation by Molybdenum Peroxo Compound: Molecular Mechanism Characterized by the Electron Localization Function and Catastrophe Theory

The oxygen atom transfer reaction from the Mimoun-type complex MoO(η(2)-O(2))(2)OPH(3) to ethylene C(2)H(4) affording oxirane C(2)H(4)O has been investigated within the framework of the Bonding Evolution Theory in which the corresponding molecular mechanism is characterized by the topological analysis of the electron localization function (ELF) and Thoms catastrophe theory (CT). Topological analysis of ELF and electron density analysis reveals that all Mo-O bonds in MoO(η(2)-O(2))(2)OPH(3) and MoO(2)(η(2)-O(2))OPH(3) belong to closed-shell type interactions though negative values of total energy densities E(e)(r(BCP)) imply some covalent contribution. The peroxo O(i)-O(j) bonds are characterized as charge-shift or protocovalent species in which pairs of monosynaptic basins V(3)(O(i)), V(3)(O(j)) with a small electron population of ~0.25e each, are localized between core basins C(O(i)), C(O(j)). The oxygen transfer reaction from molybdenum diperoxo complex MoO(η(2)-O(2))(2)OPH(3) to C(2)H(4) system can be described by the following consecutive chemical events: (a) protocovalent peroxo O(2)-O(1) bond breaking, (b) reduction of the double C(1)=C(2) bond to single C(1)-C(2) bond in ethylene, (c) displacement of oxygen O(1) with two nonbonding basins, V(i=1,2)(O(1)), (d) increase of a number of the nonbonding basins to three (V(i=1,2,4)(O(1))); (e) reorganization and reduction in the number of nonbonding basis to two basins (V(i=1,4)(O(1))) resembling the ELF-topology of the nonbonding electron density in oxirane, (e) formation of the first O(1)-C(2) bond in oxirane, (f) C(2)-O(1)-C(2) ring closure, (g) formation of singular nonbonding basin V(O(2)) in new Mo=O(2) bond. The oxygen atom is transferred as an anionic moiety carrying a rather small electronic charge ranging from 0.5 to 0.7e.

Theoretical study of the structure and stability of NbxOy and NbxOy+ (x=1–3; y=2–5, 7, 8) clusters

Abstract Geometric, thermodynamic and electronic properties of cluster neutrals Nb x O y and cations Nb x O y + ( x =1–3; y =2–5, 7, 8) have been characterized theoretically. A DFT calculation using a hybrid combination of B3LYP with contracted Huzinaga basis sets. Numerical results of the relative stabilities, ionization potentials and band gaps of different clusters are in agreement with experiment. Analysis of dissociation channels supports the more stable building blocks as formed by NbO 2 , NbO 2 + , NbO 3 and NbO 3 + stoichiometries. The net atomic charges suggest that oxygen donor molecules can interact more favorably on central niobium atoms of cluster cations, while the interaction with oxygen acceptor molecules is more favorable on the terminal oxygen atoms of neutral clusters. A topological analysis of the electron localization function gradient field indicates that the clusters may be described as having a strong ionic interaction between Nb and O atoms.

Oxygen Atom Transfer Reactions from Mimoun Complexes to Sulfides and Sulfoxides. A Bonding Evolution Theory Analysis

In this research, a comprehensive theoretical investigation has been conducted on oxygen atom transfer (OAT) reactions from Mimoun complexes to sulfides and sulfoxides. The joint use of the electron localization function (ELF) and Thoms catastrophe theory (CT) provides a powerful tool to analyze the evolution of chemical events along a reaction pathway. The progress of the reaction has been monitored by structural stability domains from ELF topology while the changes between them are controlled by turning points derived from CT which reveal that the reaction mechanism can be separated in several steps: first, a rupture of the peroxo O1-O2 bond, then a rearrangement of lone pairs of the sulfur atom occurs and subsequently the formation of S-O1 bond. The OAT process involving the oxidation of sulfides and sulfoxides is found to be an asynchronous process where O1-O2 bond breaking and S-O1 bond formation processes do not occur simultaneously. Nucleophilic/electrophilic characters of both dimethyl sulfide and dimethyl sulfoxide, respectively, are sufficiently described by our results, which hold the key to unprecedented insight into the mapping of electrons that compose the bonds while the bonds change.

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

Abstract 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.

Rheological properties of tin oxide suspensions

Abstract The rheological properties of tin oxide slurries were studied experimentally and theoretically. The deflocculants used were ammonium polyacrilate (PAA) and the copolymer poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (PVB-PVA-PVAc), in water and ethanol, respectively. The amount of deflocculant was optimized for different solid contents by means of viscosity measurements. In spite of the high stability of PVB-dispersed slurries, a high solid concentration was not obtained. On the other hand, a slurry with a 56.4 vol.% of solids was attained when PAA was used. A theoretical study of the adsorption of PAA in its dissociated (basic solution) and non-dissociated (acidic solution) forms on SnO2 (110) is presented. This analysis was made by means of the PM3 method using a large cluster Sn15O28 for the surface model. The calculated adsorption energy is larger for the ionized PAA than for the non-ionized form, indicating that alkaline slurries favor PAA adsorption on the SnO2 surface.

Sulfonamide–metal complexes endowed with potent anti-Trypanosoma cruzi activity

Abstract In this article, we describe that mononuclear complexes composed of (5-chloro-2-hydroxybenzylidene)aminobenzenesulfonamides (L1–3) of general formula (L2(M)2H2O, where M is Co, Cu, Zn, Ni or Mn) reduced epimastigote proliferation and were found cidal for trypomastigotes of Trypanosoma cruzi Y strain. Complexes C5 and C11 have IC50 of 2.7 ± 0.27 and 4.8 ± 0.47 µM, respectively, for trypomastigotes, when the positive control Nifurtimox, which is also an approved drug for Chagas disease, showed IC50 of 2.7 ± 0.25 µM. We tested whether these complexes inhibit the enzyme T. cruzi trypanothione reductase or acting as DNA binders. While none of these complexes inhibited trypanothione reductase, we observed some degree of DNA binding, albeit less pronounced than observed for cisplatin in this assay. Unfortunately, most of these complexes were also toxic for mouse splenocytes. Along with the present studies, we discuss a number of interesting structure–activity relationships and chemical features for these metal complexes, including computational calculations.