Examinando por Autor "Ramirez, Frank"

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    Ítem
    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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    Ítem
    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.