Examinando por Autor "David, J."

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    Hydrogen bonding in the binary water/ammonia complex
    (IOS Press, 2014-01-01) Guerra, D.; David, J.; Restrepo, A.; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)
    A detailed study of the interactions leading to stabilization of the NH3(H2O){n =1,2} clusters is presented in this work. The Potential Energy Surface for the trimers was explored using an adapted version of the simulated annealing optimization procedure that produced cluster candidate structures that were further optimized, refined, and characterized at the MP2/6-311++G(d, p) level. Our results indicate that hydrogen bonding of the N-H type is stronger and more covalent than of the O-H type. We provide evidence that suggests that the topological complexity of the electron distributions is directly correlated with cluster stability and that most of the stabilization energy originates in electrostatic and exchange contributions. Our calculated trimerization enthalpy, H° 298-41.75 kJ/mol, is in excellent agreement with the experimental enthalpy of adsorption of NH3 into surface water, reported to be-(41 ± 5) kJ/mol. © 2014-IOS Press.
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    Mechanical properties study for new hypothetical crystalline phases of ReB2: A computational approach using density functional theory
    (Elsevier, 2016-09-01) Marín-Suárez, M.; Vélez, M.E.; David, J.; Arroyave-Franco, M.; Marín-Suárez, M.; Vélez, M.E.; David, J.; Arroyave-Franco, M.; Universidad EAFIT. Departamento de Ciencias; Lógica y Computación
    Rhenium diboride (ReB2) in its P6(3)/mmc crystalline structure, is widely known as a super-hard material, and has been studied many times using the Density Functional Theory (DFT) approach. In this work the same chemical composition was studied in three additional unreported hypothetical crystallographic phases by means of DFT with the hybrid functional approach, and the elastic constants of each phase were calculated. The elastic behavior of ReB2 was analyzed by means of elastic moduli calculations. Additionally, the velocities of the elastic waves of each phase were calculated, along with the Debye's temperatures, also elastic anisotropy is studied. Semi-empirical and empirical models of hardness were used to estimate qualitatively which phases are or are not hard. It has been determined that the elastic moduli of two out of the three hypothetical phases are desirable and the elastic waves move very slow (< 2 km/s) in one of them. These results and the analysis of the bond critical points (bcp) of each phase allow us to conclude that one of them is soft while the other two are hard. The synthesized phase of ReB(2)P6(3)/mmc was studied in order to compare and confirm the results. (C) 2016 Elsevier B.V. All rights reserved.
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    Mechanical properties study for new hypothetical crystalline phases of ReB2: A computational approach using density functional theory
    (Elsevier, 2016-09-01) Marín-Suárez, M.; Vélez, M.E.; David, J.; Arroyave-Franco, M.; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)
    Rhenium diboride (ReB2) in its P6(3)/mmc crystalline structure, is widely known as a super-hard material, and has been studied many times using the Density Functional Theory (DFT) approach. In this work the same chemical composition was studied in three additional unreported hypothetical crystallographic phases by means of DFT with the hybrid functional approach, and the elastic constants of each phase were calculated. The elastic behavior of ReB2 was analyzed by means of elastic moduli calculations. Additionally, the velocities of the elastic waves of each phase were calculated, along with the Debye's temperatures, also elastic anisotropy is studied. Semi-empirical and empirical models of hardness were used to estimate qualitatively which phases are or are not hard. It has been determined that the elastic moduli of two out of the three hypothetical phases are desirable and the elastic waves move very slow (< 2 km/s) in one of them. These results and the analysis of the bond critical points (bcp) of each phase allow us to conclude that one of them is soft while the other two are hard. The synthesized phase of ReB(2)P6(3)/mmc was studied in order to compare and confirm the results. (C) 2016 Elsevier B.V. All rights reserved.
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    Microsolvation of methylmercury: structures, energies, bonding and NMR constants (Hg-199, C-13 and O-17)
    (ROYAL SOC CHEMISTRY, 2016-01-21) Flórez, E.; Maldonado, A.F.; Aucar, G.A.; David, J.; Restrepo, A.; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)
    Hartree-Fock (HF) and second order perturbation theory (MP2) calculations within the scalar and full relativistic frames were carried out in order to determine the equilibrium geometries and interaction energies between cationic methylmercury (CH3Hg+) and up to three water molecules. A total of nine structures were obtained. Bonding properties were analyzed using the Quantum Theory of Atoms In Molecules (QTAIM). The analyses of the topology of electron densities reveal that all structures exhibit a partially covalent Hg center dot center dot center dot O interaction between methylmercury and one water molecule. Consideration of additional water molecules suggests that they solvate the (CH3Hg center dot center dot center dot OH2)(+) unit. Nuclear magnetic shielding constants sigma(Hg-199), sigma(C-13) and sigma(O-17), as well as indirect spin-spin coupling constants J(Hg-199-C-13), J(Hg-199-O-17) and J(C-13-O-17), were calculated for each one of the geometries. Thermodynamic stability and the values of NMR constants correlate with the ability of the system to directly coordinate oxygen atoms of water molecules to the mercury atom in methylmercury and with the formation of hydrogen bonds among solvating water molecules. Relativistic effects account for 11% on sigma(C-13) and 14% on sigma(O-17), which is due to the presence of Hg (heavy atom on light atom, HALA effect), while the relativistic effects on sigma(Hg-199) are close to 50% (heavy atom on heavy atom itself, HAHA effect). J-coupling constants are highly influenced by relativity when mercury is involved as in J(Hg-199-C-13) and J(Hg-199-O-17). On the other hand, our results show that the values of NMR constants for carbon and oxygen, atoms which are connected through mercury (C-Hg center dot center dot center dot O), are highly correlated and are greatly influenced by the presence of water molecules. Water molecules introduce additional electronic effects to the relativistic effects due to the mercury atom.