Examinando por Autor "Hadad, C. Z."

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    Hydrophobic meddling in small water clusters
    (SPRINGER, 2013-08-01) Hadad, C. Z.; Restrepo, Albeiro; Jenkins, Samantha; Ramirez, Frank; David, Jorge; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)
    What would be the effects on the nature of hydrogen bonds, on the energies, and on the overall structural possibilities of replacing some hydrogen atoms by small hydrophobic groups in small water networks? Aiming at investigating this question, we performed an exhaustive search of the conformational space of the (Methanol)2(Water)3 representative model system, characterized the results, and made key comparative analysis with pentameric pure water clusters. The potential energy surface yielded a global minimum structural motif consisting of several puckered ring-like cyclic isomers very close in energy to each other. They are followed by other structural motifs, which, contrary to conventional belief, would also contribute to the properties of a macroscopic sample of this composition. We found that the C-H O interactions play a subordinate structural role and preferably accommodate to the established O-H O based structures. In comparison with the pure (H2O)5 case, we showed that (1) the same basic structural motifs and in a similar hierarchy energy order are obtained, but with a richer structural isomerism; (2) in general, the bonding is reinforced by the increase in the electrostatic and in the "degree of covalency" of the hydrogen-bonding components. Therefore, at least for this small cluster size, methyl groups slightly affect the structural isomerism and reinforce the hydrogen bonding. Additionally, we identified general factors of instability of the more unstable structures. © Springer-Verlag Berlin Heidelberg 2013.
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    Microsolvation of dimethylphosphate: A molecular model for the interaction of cell membranes with water
    (ROYAL SOC CHEMISTRY, 2013-01-01) Ibargueen, Cesar; Manrique-Moreno, Marcela; Hadad, C. Z.; David, Jorge; Restrepo, Albeiro; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)
    We present an exhaustive stochastic search of the quantum conformational spaces of the (CH3O)2PO2 - + nH 2O (n = 1,2,3) systems. We uncover structural, conformational and energetic features of the problem. As in the isolated species, clusters containing the gauche-gauche (gg) conformation of dimethylphosphate (DMP -) are energetically preferred, however, contributions from hydrated gauche-anti (ga) and anti-anti (aa) monomers cannot be neglected because such structures are quite common and because they are close in energy to those containing the gg monomer. At least seven distinct types of O?H-O-H contacts lead to DMP- ? water interactions that are always stabilizing, but not strong enough to induce significant changes in the geometries of either DMP- or water units. Our results lead us to postulate DMP- to be a suitable model to study explicit and detailed aspects of microsolvation of cell membranes. This journal is © 2013 the Owner Societies.
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    Structural studies of the water pentamer
    (ELSEVIER SCIENCE BV, 2011-05-09) Ramirez, Frank; Hadad, C. Z.; Guerra, Doris; David, Jorge; Restrepo, Albeiro; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)
    A computational study of the water pentamer gas phase conformational space is reported in this Letter. Forty-four stationary points distributed among 12 structural patterns were located at the MP2/6-311++G(d,p) level. At least 5 geometrical motifs (25 structures) are predicted within 3 kcal/mol of the most stable conformation at the CCSD(T)/aug-cc-pVTZ//MP2/6-311++G(d,p) level. We show evidence that dipole-dipole interactions are at play in stabilizing the clusters. Electron densities and their Laplacians at the hydrogen bond critical points were found to be linearly correlated with relative energies for all clusters. Logarithmic relationships were found for the [req, ?(rc)] and [req,?2?( rc)] pairs in all hydrogen bonds. © 2011 Elsevier B.V. All rights reserved.
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    Structure and reactivity of the (1)Au6Pt clusters
    (AMER CHEMICAL SOC, 2010-10-07) David, Jorge; Guerra, Doris; Hadad, C. Z.; Restrepo, Albeiro; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)
    In this paper we report the geometries, properties, and reactivity descriptors of 12 structural isomers located on the MP2/SDDALL potential energy surface of the (1)Au(6)Pt binary clusters. A nonplanar, D(3d) symmetry, cyclohexane chairlike structure is predicted to be the global minimum. Binding energies per atom in the range approximately 44-51 kcal/mol account for very stable clusters. The relative stability of the clusters is directly related to all global and local reactivity descriptors. All structures are predicted to have large electron affinities. The chemical environment of the Pt atom on the structures plays a central role in the resulting relative stabilities and global and local reactivities. Our results show that more peripheral Pt atoms are more likely to be involved in electron-accepting processes.