Papers by David Alejandro Hernandez
Structure of perfluoroalkane fluids at the liquid-vapor interface was studied by molecular dynami... more Structure of perfluoroalkane fluids at the liquid-vapor interface was studied by molecular dynamics simulations at different temperatures. It was observed that fluids formed a freezing layer at the liquid-vapor interface with a strong structure which it was more pronounced at low temperatures, close to the melting point. Whereas molecules did not have any preference orientation in the bulk fluid they developed a monolayer of molecules aligned perpendicular at the interface. Similar results have been observed for alkanes fluids in the same interface, however, the structure of those molecules is weaker than that reported for perfluoroalkanes. As the temperature increased the layer melted until it vanished. Finally, thermodynamic studies were also conducted in terms of surface tensions and good agreement with experimental data was found.
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Journal of Chemical Physics
Computer simulations for several alkane fluids were carried out to study thermodynamics and struc... more Computer simulations for several alkane fluids were carried out to study thermodynamics and structural behavior of the molecules at the liquid-vapor interface. Three different models were used to simulate the fluids, one of them was proposed in this work and we obtained a slightly better agreement than the other models with experimental data. The fluid structure at the interface was analyzed at temperatures close to the melting point using the new model and it was found that molecules at the free surface present more order than those at the bulk liquid phase. By calculating the order of the hydrocarbon chains a strong structure of molecules was observed at the interface than those in bulk, moreover, some of those molecules at the interface were aligned perpendicular to the interface. Previous simulations report stronger structures at the interface by the formation of a monolayer of alkane chains, however, those simulations started at very low temperatures and they did not reproduce thermodynamic properties such as the interfacial tension correctly. The model proposed in the present work not only presents good agreement with surface tension data but also shows evidence that the fluid structured as experiments indicated at temperatures close to the melting temperature.
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Quantum chemical calculations using gradient-corrected density functional theory at the BP86 leve... more Quantum chemical calculations using gradient-corrected density functional theory at the BP86 level in conjunction with TZ2P basis sets have been carried out for the multimetallocenes CpMnCp, where M = Be, Mg, Ca, and Zn with n = 2−5. The equilibrium geometries and energetics with respect to loss of one metal atom are theoretically predicted. The nature of the metal−ligand interactions between the Mn2+ and (Cp-)2 moieties was investigated with energy decomposition analysis (EDA). The calculations predict that the CpMnCp species with n > 2 are thermodynamically unstable with respect to loss of one metal atom except for the beryllium compounds. The beryllocenes exhibit unusual stabilities in the gas phase for the whole series CpBenCp up to n = 5. The calculations suggest that the energy for loss of one metal atom from CpBe2Cp is significantly higher than from CpZn2Cp. The energy for the metal extrusion reaction of CpBe3Cp is much less endothermic than for CpBe2Cp but it is still more endothermic than the reaction of CpZn2Cp. The thermodynamic stability of the higher members CpBe4Cp and CpBe5Cp toward loss of one metal atom is only slightly less than for CpBe3Cp, while the other multimetallocenes, CpM3Cp, CpM4Cp, and CpM5Cp (M = Mg, Ca, Zn), possess little extra stabilization with respect to the dimetallocenes. The calculated reaction energies which include the heats of sublimation of the metals indicate that CpBe2Cp might become isolated in the condensed phase, while the prospect for CpCa2Cp and CpMg2Cp and for the higher members CpM3Cp, CpM4Cp, and CpM5Cp is less likely. The analysis of the metal−ligand bonding in CpMnCp using the EDA method suggests that the interactions between Mn2+ and (Cp-)2 have a larger electrostatic than covalent character. The beryllocenes are more covalently bonded than the other multimetallocenes. The orbital interactions in the lower members of CpMnCp come mainly from π orbitals, but the σ contribution continuously increases when n becomes larger and eventually may become stronger than the π contributions, which become weaker in the higher members of the series.
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This work describes the synthesis and characterization of Pt/MgO (Pt/Mg(OH)2, calcined Pt/MgO and... more This work describes the synthesis and characterization of Pt/MgO (Pt/Mg(OH)2, calcined Pt/MgO and agglomerated Pt/MgO ) catalysts. The supports of the catalysts were obtained by thehydrolysis-precipitation method and the metallic phase of platinum was incorporated by impregnation. The characterization of the synthesized materials was made by means of DRX,FTIR, TGA-DTA and TPR techniques. The catalysts were applied to the hydrosilylation ofPhC≡CH with three different silanes (Ph3SiH, Ph2MeSiH and PhMe2SiH) as a model reaction.The activity of the proposed catalysts Pt/Mg(OH)2, calcined Pt/MgO and agglomerated Pt/MgOhad a reaction yield of 95 %. When the catalysts Pt/Mg(OH)2 and agglomerated Pt/MgO were used in the reaction with Ph3SiH, the selectivity to the trans isomer was favored (90%). Arelationship was found between the increase in the reaction time and the increase of the kineticvolume of the silane radicals. The present results were compared to the previously obtained results using as a catalyst Pt/MgO obtained by the sol-gel method.
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Papers by David Alejandro Hernandez