Examinando por Materia "Conformational space"
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Ítem Structural characterization of the (methanol)4 potential energy surface(AMER CHEMICAL SOC, 2009-09-24) David, Jorge; Guerra, Doris; Restrepo, Albeiro; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)In this paper, we report the geometries and properties of the structural isomers obtained from a random walk of the potential energy surface (PES) of the methanol tetramer. Thirty-three structures were obtained after B3LYP/6-31+g* optimization of 94 candidate structures generated from a stochastic search of the PM3 conformational space. The random search was carried out using a recently proposed modified Metropolis acceptance test in the simulated annealing (SA) procedure. Corrections for the basis set superposition error (BSSE) show improvements on the binding energies of the clusters in an average of approximately 2.0 kcal/mol, while geometries are predicted to be less sensitive to BSSE corrections. MP2/aug-cc-pvdz calculations on representative structures did not change the geometries but predicted better binding energies. Highly correlated CCSD(T) energies were calculated on the B3LYP and MP2 stationary points and used to establish relative stabilities. We report several new conformations and group the structures into six distinct geometrical motifs. Only the cyclic tetramers with four primary hydrogen bonds in the same plane are predicted to have significant populations. Secondary hydrogen bonds, those for which the donated proton comes from an alkyl group, lead to a rich conformational space.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.Ítem Structure, stability and bonding in the 1Au 10 clusters(ELSEVIER SCIENCE BV, 2012-06-29) David, Jorge; Guerra, Doris; Restrepo, Albeiro; Universidad EAFIT. Departamento de Ciencias Básicas; Electromagnetismo Aplicado (Gema)A stochastic exploration of the quantum conformational space for the 1Au 10 system using a modified Metropolis acceptance test afforded 15 stable configurations in the MP2/SDDALL potential energy surface. The global minimum is predicted to be a 3D structure with D2 d symmetry. Topological analyses of the electron densities suggest that bonding appears to be of intermediate character, with substantial contributions from both covalent and closed shell interactions and that there is a direct correlation between the topological complexity of the electron density and cluster stability. Evidence regarding the nature of the interactions is gathered from many sources, including the total number of delocalized electrons (nde), a novel covalency index. Localization indices and condensed Fukui functions predict higher electron populations on peripheral, lowly coordinated atoms. © 2012 Elsevier B.V. All rights reserved.