The wide synthetic versatility of five membered rings containing phosphorus

The chemistry of cyclopentadiene rings has been widely studied. This review article deals with a similar chemistry of new compounds containing from 1 to 5 phosphorus atoms on the ring substituting the carbon atoms. The neutral rings containing one, two and three phosphorus atoms can be used as building blocks for the synthesis of new organic compounds containing phosphorus. These rings plus the anionic ones also show great potential as ligands in coordination chemistry. The aim of this article is to show how important this new area is and how diverse the chemistry related to a single type of ring can be.

Estudos da eletrodeposição de metais em regime de subtensão

This work reviews recent studies of underpotential deposition (UPD) of several metals on Pt and Au substrates performed in the Grupo de Materiais Eletroquímicos e Métodos Eletroanalíticos (IQSC -- USP, São Carlos). The UPD Cu, Cd and Pb on Pt were analysed in terms of their influence in the oxygen evolution reaction. Partial blockage of surface active sites, promoted by Pb ad-atoms, resulted in a change from water to hydrogen peroxide as the final product. The Ag UPD on Pt and Au substrates was also discussed in this work. A detailed model of charge calculation for Ag monolayer was developed and confirmed by the rotating ring-disk data. The partial charge transfer in UPD studies was analysed in the Cd/Pt and Cd/Au systems and a value of 0.5 was found for the adsorption electrovalence of Cd ad-ions. The Sn/Pt UPD systems were studied from the point of view of the valences of metallic ions in solution. The deposition from Sn(IV) generates a full monolayer with a maximum occupation of approximately 40% of the surface active sites (340 µC cm-2) plus 105 µC cm-2 of Hads (half monolayer). Changing the metallic ion for Sn(II), it was possible to deposit a full monolayer (210 µC cm-2) without any detectable Hads. Finally, the effect of anions was discussed in the Zn/Pt and Zn/Au systems. Here, the hydrogen evolution reaction (her) and the hydrogen adsorption/desorption were used in order to investigate the maximum coverage of the surface with Zn ad-atoms. The full monolayer, characterised by the complete absence of Hads, was achieved only in 0.5 M HF solutions.

O uso de sílica gel organofuncionalizada como agente sequestrante para metais

An overview about the homogeneous and heterogeneous methods of synthesizing silylating agents and applications of the organofunctionalized silica-gel samples was explored. The pendant molecules attached covalently to the inorganic surface displayed important properties to act as sequestrating agents for metals in aqueous and non-aqueous solutions. The large variety of basic centres anchored on organic molecules improve the capacity in adsorbing cations. The increase in adsorption is dependent on the number of basic atoms disposed on the pendant molecules on the surface. The combination of acidic and basicity properties favours the selectivity, such is exemplified by the thiol modified silicas towards mercury (II) cations.

Sobre a aplicabilidade da termogravimetria para a estimativa de parâmetros termoquímicos de compostos de coordenação: os elementos lantanídicos

By using thermochemical data reported for a series of chelates of the type [Ln(thd)3], thd = 2, 2, 6, 6 ­ tetramethyl- 3,5-heptanedione and Ln = La, Pr, Nd, Sm, Gd, Tb, Ho, Er, Tm and Yb, empirical correlations were found involving thermochemical parameters (e.g. dissociation enthalpy) and the thermodynamic temperatures of the beginning of thermal degradation of the chelates, t i. It is shown that t i values are of capital importance in the study of this all class of coordination compounds. Among others, the empirical equation is obtained: r3+ = (-0,013.Z + 1,36)/0,005, that relates the lanthanide cation radius (pm) with the atomic number of the element. The remarkable fact is that this equation is achieved by using thermogravimetric and calorimetric parameters. Is also shown that t i values are related with the P(M) function values, which are very close related with the energy difference, deltaE, between the lowest electronic energy level of the f n s²d¹ configuration and the lowest energy level of the f n+1s² configuration in the neutral gaseous atoms.

Estudo da adsorção de hidrogênio e sulfeto na superfície de paládio: aspectos experimentais (eletroquímica) e teóricos (ab initio e Teoria do Funcional da Densidade)

The adsorption of H and S2- species on Pd (100) has been studied with ab initio, density-functional calculations and electrochemical methods. A cluster of five Pd atoms with a frozen geometry described the surface. The computational calculations were performed through the GAUSSIAN94 program, and the basis functions adapted to a pseudo-potential obtained by using the Generator Coordinate Method adapted to the this program. Using the cyclic voltammetry technique through a Model 283 Potentiostat/Galvanostat E.G.&G-PAR obtained the electrochemical results. The calculated chemisorption geometry has a Pd-H distance of 1.55Å, and the potential energy surface was calculated using the Becke3P86//(GCM/DFT/SBK) methodology. The adsorption of S2- ions on Pd surface obtained both through comparison between the experimental and theoretical results, at MP2 level, suggest a S2- absorption into the metallic cluster. The produced Pd-(S2-) system was show to be very stable under the employed experimental conditions. The paper has shows the powerful aid of computational methods to interpret adsorption experimental data.

Adsorção de xantatos sobre pirita

This paper presents a study of adsorption of xanthate with alkyl chain of two (C2XK), four (C4XK) and eight (C8XK) atoms of carbon, on pyrite from Santa Catarina, Brazil. The results showed that pyrite surface changes from hydrophilic to hydrophobic when xanthate is adsorbed increasing the contact angle to 35º for C2XK, and to 90º for C4XK and C8XK. The rate of flotation of pyrite particles after adsorption increases with the increase of the number of carbon atoms in the alkyl chain in agreement with the results of contact angle measurements.

Colapso da equipartição da energia

This paper discuss the consequences of the equipartition principle when used to calculate the heat capacity of atoms and molecules, a discussion that appeared at the end of XIX century and beginning of the XX century. Classical molecular thermodynamics prediction of the heat capacity is introduced, followed by a presentation of the degrees of freedom of a system. The historical discussion that appeared at the time, by Dulong, Petit, Maxwell, Boltzmann, Rayleigh and Kelvin is discussed afterwards. The necessity of a new theory is also presented as a direct consequence of the equipartition principle collapse.

Borohidreto complexos de cobre (I) contendo difosfinas: caracterização espectroscópica e comportamento térmico

Tetrahydroborate complexes of copper (I) with bidentate phosphines, [Cu(eta²-BH4)(dppm)] (1), [Cu(eta²-BH4)(dppe)] (2), [Cu(eta²-BH4)(cis-dppet)] (3) and [Cu(eta²-BH4)(dppb)] (4) (dppm = bis(diphenylphosphino)methane; dppe = 1,2-bis(diphenylphosphino) ethane; cis-dppet = 1,2-cis(diphenylphosphino)ethene; dppb = 1,4-bis(diphenylphosphino)butane) were prepared and characterized by elemental analysis, infrared spectroscopy, NMR and thermogravimetric analysis. The IR data for 1-4 showed bands typical of a bidentate coordination of BH4 group to the copper atom and the 31P{¹H} NMR spectra indicated that the phosphorous atoms are chelating the metal centre. The thermal behavior of the compounds was investigated and suggested that their thermal stability is influenced by the phosphines. Their thermal stability decreased as follows: [Cu(eta²-BH4)(dppe)] (2) > [Cu(eta²-BH4)(dppm)] (1) > [Cu(eta²-BH4)(dppb)] (4) > [Cu(eta²-BH4)(cis-dppet)] (3). According to thermal analysis and X-ray diffraction patterns all compounds decomposed giving Cu(BO2)2, CuO, CuO2 and Cu as final products.

Correlação entre a estrutura atômica superficial e o processo de adsorção-dessorção reversível de hidrogênio em eletrodos monocristalinos Pt(111), Pt(100) e Pt(110)

Platinum is widely used as electrode in electrocatalytic processes, however the use of polycrystalline electrodes introduces a series of variables in the electrochemical system due to the aleatory contribution of all the crystallographic orientations with different surface packing of atoms. Single crystal platinum electrodes of low Miller index present surface structure of high regularity and serve as model to establish a correlation among the macroscopic and microscopic properties of the electrochemical interface. Therefore, the main aim of this work is the study of the voltammetric profiles of the reversible adsorption-desorption of hydrogen on Pt(100), Pt(110) and Pt(111), in order to correlate the electrochemical properties of each different orientation with the surface atomic structure.

Redução de amidas por boranos

Despite the fact that boranes are frequently used in amide reductions, the reaction mechanisms of the involved are note well known. This work presents the results of a bibliographic search on probable amide reduction mechanisms and an analysis of the existing literature. Steric and electronic effects were considered in light of reactivity since it could be concluded that the formation of intermediates and products depends mainly on the substitution patterns of both the boron and nitrogen atoms. Otherwise, results described in the literature for the reactions of boranes, sodium borohydride, lithium aluminum hydride, alkylboranes or haloboranes with others functional groups such as carboxylic acids, esters, ketones and alkenes were analysed with the aim to obtain something about the N-substituted amide reactions employing boranes.

Estudo da geometria da uréia por métodos ab initio e simulação computacional de líquidos

A study was carried out on the urea geometries using ab initio calculation and Monte Carlo computational simulation of liquids. The ab initio calculated results showed that urea has a non-planar conformation in the gas phase in which the hydrogen atoms are out of the plane formed by the heavy atoms. Free energies associated to the rotation of the amino groups of urea in water were obtained using the Monte Carlo method in which the thermodynamic perturbation theory is implemented. The magnitude of the free energy obtained from this simulation did not permit us to conclude that urea is non-planar in water.

Compostos organolantanídeos contendo o ligante pirazinamida: síntese, caracterização e atividade catalítica na polimerização do etileno

In this work we report the synthesis of some organolanthanide compounds which were identified as LnCl2Cp(PzA)2, Ln = Nd, Sm, Eu and Tb, Cp = cyclopentadienyl and PzA = pirazinamide, by elemental analyses, complexometric titration with EDTA, thermal analyses and IR spectra. Thermal analysis and infrared spectra indicated that the coordination of the pyrazinamide to the lanthanide ions was made by the O atom of the carbonyl group and by one or both N atoms of the pyrazinamide ring. This class of compound showed catalytic activity of ca. 4.0 to 6.4 kgPE molLn-1 h-1 bar-1, in ethylene polymerization, using methylaluminoxane as cocatalyst. The resulting polyethylene presented low crystallinity (20%).

Estudo da estrutura e da solvatação do HNP-3, um antibiótico natural, por dinâmica molecular

The structure and hydration of the HNP-3 have been derived from molecular dynamics data using root mean square deviation, radial and energy distributions. Three antiparallel beta sheets were found to be preserved. 15 intramolecular hydrogen bonds were identified together with 36 hydrogen bonds on the backbone and 35 on the side chain atoms. From the point of view of the hydration dynamics, the analysis shows a high solvent accessibility of the monomer and attractive interactions with water molecules.

O uso de pseudopotenciais e modelos HF/MP2/DFT na previsão de freqüências vibracionais em complexos metálicos

Four different pseudopotentials and three methodologies were employed in the calculation of the geometry and the frequencies of metal complexes like [M(NH3)2X2] [X=halogen, M=Zn, Cd], and [Hg(NH3)2]Cl2. The vibrational assignments were carefully checked and compared to the theoretically calculated ones. Graphical procedures were employed to estimate family errors and their average behavior. The calculated results show the SBK-X basis set with the best results for the geometries and calculated frequencies, for individual species and statistical results. Its use is recommend, mainly if the neighborhood atoms are described with similar pseudopotentials. Excellent results were also obtained with the Hay and Wadt pseudopotential.

Propriedades eletrônicas e estruturais do PbTiO3: teoria do funcional de densidade aplicada a modelos periódicos

Calculations based on density functional theory at the B3LYP hybrid functional level applied to periodic models have been performed to characterize the structural and electronic properties of PbTiO3. Two different slab terminations (PbO and TiO2) have been considered to obtain and discuss the results of band structure, density of states, charge distribution on bulk and surface relaxation. It is observed that the relaxation processes are most prominent for the Ti and Pb surface atoms. The electron density maps confirm the partial covalent character of the Ti-O bonds. The calculated optical band gap and other results are in agreement with experimental data.