Investigating the Nature of Noble Gas-Copper Bonds by the Quantum Theory of Atoms in Molecules

We investigated noble gas copper bonds in linear complexes represented by the NgCuX general formula in which Ng and X stand for a noble gas (neon, argon, krypton, or xenon) and a halogen (fluorine, chlorine or bromine), respectively, by coupled cluster methods and modified cc-pVQZ basis sets. The quantum theory of atoms in molecules (QTAIM) shows a linear relation between the dissociation energy or noble gas-copper bonds and the amount of electronic charge transferred mainly from the noble gas to copper during complexation. Large changes in the QTAIM quadrupole moments of copper and noble gases resulting from this bonding and a comparison between NgCuX and NgNaCl systems indicate that these noble gas-copper bonds should be better interpreted as predominantly covalent. Finally, QTAIM atomic dipoles of noble gases in NgNaCl systems agree satisfactorily with atomic dipoles given by a simple model for these NgNa van der Waals bonds.

Estudo teórico da solvatação de ânions em solventes orgânicos

O presente trabalho relata a parametrização do modelo de contínuo IEF-PCM para o estudo da solvatação de ânions monovalentes em acetonitrila e N,N-dimetilformamida. Nós propomos duas metodologias de parametrização do modelo de contínuo polarizável para este estudo teórico. A primeira proposta é baseada unicamente no cálculo do termo eletrostático da energia livre e na equiparação deste termo com a energia livre de solvatação experimental. Na segunda proposta de metodologia, a parametrização do modelo de contínuo polarizável foi realizada com o auxílio das simulações de dinâmica molecular e dentro desta segunda proposta nós calculamos todos os termos que contribuem para a energia livre de solvatação, a energia de cavitação. a energia eletrostática e a energia de van der Waals. Com a aplicação da primeira metodologia, é obtido um erro médio absoluto de 2,1 kcal/mol nas energias de solvatação dos ânions em acetonitrila enquanto que o erro médio absoluto calculado para energias de solvatação dos ânions em N,N-dimetilformamida é 2,8 kcal/mol. A aplicação da segunda metodologia resulta num erro médio absoluto de 1,4 kcal/mol nas energias de solvatação dos ânions em acetonitrila e um erro médio absoluto de 1,2 kcal/mol nas energias de solvatação calculadas para os ânions em N,N-dimetilformamida. Estes resultados indicam que o modelo de contínuo polarizável IEF-PCM é adequado para o estudo dos efeitos termodinâmicos nas soluções de ânions nos solventes acetonitrila e N,N-dimetilformamida.

Mapping of adhesion forces on soil minerals in air and water by atomic force spectroscopy (AFS)

The adhesion force between an atomic force microscope (AFM) tip and sample surfaces, mica and quartz substrates, was measured in air and water. The force curves show that the adhesion has a strong dependence on both the surface roughness and the environmental conditions surrounding the sample. The variability of the adhesion force was examined in a series of measurements taken at the same point, as well as at different places on the sample surface. The adhesion maps obtained from the distribution of the measured forces indicated regions contaminated by either organic compounds or adsorbed water. Using simple mathematical expressions we could quantitatively predict the adhesion force behavior in both air and water. The experimental results are in good agreement with theoretical calculations, where the adhesion forces in air and water were mostly associated with capillary and van der Waals forces, respectively. A small long-range repulsive force is also observed in water due to the overlapping electrical double-layers formed on both the tip and sample surfaces.

Nanostructured hybrid films containing nanophosphor: Fabrication and electronic spectral properties

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Estudo teórico QTAIM e DFT dos compostos de coordenação: efeito quelato, titanocenos e ligação química

This work is a study of coordination compounds by quantum theory of atoms in molecules (QTAIM), based on the topological analysis of the electron density of molecular systems, both theoretically and experimentally obtained. The coordination chemistry topics which were studied are the chelate effect, bent titanocene and chemical bond in coordination complexes. The chelate effect was investigated according to topological and thermodynamic parameters. The exchange of monodentate ligands on polydentate ligands from same transition metal increases the stability of the complex both from entropy and enthalpy contributions. In some cases, the latter had a higher contribution to the stability of the complex in comparison with entropy. This enthalpic contribution is explained according to topological analysis of the M-ligand bonds where polidentate complex had higher values of electron density of bond critical point, Laplacian of electron density of bond critical point and delocalization index (number of shared electrons between two atoms). In the second chapter, was studied bent titanocenes with bulky cyclopentadienyl derivative π-ligand. The topological study showed the presence of secondary interactions between the atoms of π-ligands or between atoms of π-ligand and -ligand. It was found that, in the case of titanocenes with small difference in point group symmetry and with bulky ligands, there was an nearly linear relationship between stability and delocalization index involving the ring carbon atoms (Cp) and the titanium. However, the titanocene stability is not only related to the interaction between Ti and C atoms of Cp ring, but secondary interactions also play important role on the stability of voluminous titanocenes. The third chapter deals with the chemical bond in coordination compounds by means of QTAIM. The quantum theory of atoms in molecules so far classifies bonds and chemical interactions in two categories: closed shell interaction (ionic bond, hydrogen bond, van der Waals interaction, etc) and shared interaction (covalent bond). Based on topological parameters such as electron density, Laplacian of electron density, delocalization index, among others, was classified the chemical bond in coordination compounds as an intermediate between closed shell and shared interactions

Obtenção de dispersões de complexos polieletrolíticos à base de quitosana e poli(ácido metacrílico) e análise de adsorção de albumina bovina sérica

Dispersions composed of polyelectrolyte complexes based on chitosan and poly(methacrylic acid), PMAA, were obtained by the dropping method and template polymerization. The effect of molecular weight of PMAA and ionic strength on the formation of chitosan/poly(methacrylic acid), CS/PMAA, complexes was evaluated using the dropping method. The increase in molecular weight of PMAA inhibited the formation of insoluble complexes, while the increase in ionic strength first favored the formation of the complex followed by inhibiting it at higher concentrations. The polyelectrolyte complexation was strongly dependent on macromolecular dimensions, both in terms of molecular weight and of coil expansion/contraction driven by polyelectrolyte effect. The resultant particles from dropping method and template polymerization were characterized as having regions with different charge densities: chitosan predominating in the core and poly(methacrylic acid) at the surface, the particles being negatively charged, as a consequence. Albumin was adsorbed on templatepolymerized CS/PMAA complexes (after crosslinking with glutardialdehyde) and pH was controlled in order to obtain two conditions: (i) adsorption of positively charged albumin, and (ii) adsorption of albumin at its isoelectric point. Adsorption isotherms and zeta potential measurements showed that albumin adsorption was controlled by hydrogen bonding/van der Waals interactions and that brushlike structures may enhance adsorption of albumin on these particles

Using genetic algorithm to design protein sequence

In this work, genetic algorithms concepts along with a rotamer library for proteins side chains are used to optimize the tertiary structure of the hydrophobic core of Cytochrome b(562) starting from the known PDB structure of its backbone which is kept fixed while the side chains of the hydrophobic core are allowed to adopt the conformations present in the rotamer library. The atoms of the side chains forming the core interact via van der Waals energy. Besides the prediction of the native core structure, it is also suggested a set of different amino acid sequences for this core. Comparison between these new cores and the native are made in terms of their volumes, van der Waals energies values and the numbers of contacts made by the side chains forming the cores. This paper proves that genetic algorithms area efficient to design new sequence for the protein core. (C) 2007 Elsevier B.V. All rights reserved.

Theoretical Models for Surface Forces and Adhesion and Their Measurement Using Atomic Force Microscopy

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Crystal structure of isotibolone: a major degradation product of tibolone

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

High precision calculation of multipolar dynamic polarizabilities and two- and three-body dispersion coefficients of atomic hydrogen

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Light-induced relief gratings and a mechanism of metastable light-induced expansion in chalcogenide glasses

We report on a metastable light-induced volume expansion in Ge25+xGa10-xS65 glasses under irradiation with band gap (UV) light, which can result in recording of relief gratings on their surface in the case of irradiation with two interfering beams. We propose a mechanism for the expansion, which is based on the light-induced change in the polarizability of secondary (van der Waals type) bonds and the effect of this change on primary (covalent type) bonds of the glass. The effect is suggested to be due to an interference of electrons, which belong to a chalcogen atom and participate in the formation of secondary and primary bonds, respectively. We suggest that a minimum point of the Lennard-Jones potential, which corresponds to the equilibrium position of a chalcogen atom is shifted in the course of irradiation to a larger interatomic distance. This shift causes a volume expansion and allows a diffusion of chalcogen atoms into the irradiated area. We show that light-induced polymerization of the glass network is an important attribute of the light-induced volume expansion.

Structure and luminescence of Eu3+-doped class I siloxane-poly(ethylene glycol) hybrids

Transparent, flexible, and luminescent EU3+-doped siloxane-poly(ethylene glycol) (PEG) nanocomposites have been obtained by the sol-gel process. The inorganic (siloxane) and organic PEG phases are usually linked by weak bonds (hydrogen bonds or van der Waals forces), and small-angle X-ray scattering (SAXS) measurements suggest that the structure of these materials consists of fractal siloxane aggregates embedded in the PEG matrix. For low Eu3+ contents, n = 300 and n = 80, the aggregates are small and isolated and their fractal dimensions are 2.1 and 1.7, respectively. These values are close to those expected for gelation mechanisms consisting of reaction-limited cluster-cluster aggregation (RLCCA) and diffusion-limited cluster-cluster aggregation (DLCCA). For high Eu3+ content, SAYS results are consistent with a two-level structure: a primary level of siloxane aggregates and a second level, much larger, formed by the coalescence of the primary ones. The observed increase in the glass transition temperature for increasing Eu3+ content is consistent with the structural model derived from SAXS measurements. Extended X-ray absorption fine structure (EXAFS) and luminescence spectroscopy measurements indicate that under the experimental conditions utilized here Eu3+ ions do not strongly interact with the polymeric phase.

Motional heterogeneities in siloxane/poly(ethylene glycol) ormolyte nanocomposites studied by C-13 solid-state exchange NMR

C-13 exchange solid-state NMR methods were used to study two families of siloxane/poly-(ethylene glycol) hybrid materials: Types I and II, where the polymer chains interact with the inorganic phase through physical (hydrogen bonds or van der Waals forces) or chemical (covalent bonds) interactions, respectively. These methods were employed to analyze the effects of the interactions between the organic and inorganic phases on the polymer dynamics in the milliseconds to seconds time scale, which occurs at temperatures below the motional narrowing of the NMR line width and around the polymer glass transition. Motional heterogeneities associated with these interactions and evidence of both small and large amplitude motions were directly observed for both types of hybrids. The results revealed that the hindrance to the slow molecular motions of the polymer chains due to the siloxane structures depends on the chain length and the nature of the interaction between the organic and inorganic phases.

Structural Refinement and Photoluminescence Properties of MnWO4 Nanorods Obtained by Microwave-Hydrothermal Synthesis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Structure and growth mechanism of CuO plates obtained by microwave-hydrothermal without surfactants

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)