Jorge David

Structural Studies of the Water Hexamer

In this paper we report the geometries and properties of 24 structural isomers located on the MP2/6-311++g** potential energy surface of the water hexamer. At least 15 structural patterns are located within 3 kcal/mol of the most stable conformation, leading to a very complex potential energy surface, several isomers having significant contributions. A quadratic correlation between the distance from the proton to the center of the hydrogen bond with the distance between oxygen atoms for all clusters is reported. MP2/6-311++g** and CCSD(T)/aug-cc-pvdz//MP2/6-311++g** predict different stabilization orderings but are in good agreement for binding energies. Compact structures are energetically favored by electronic energies with zero point energy corrections, while noncompact cyclic structures are preferred when temperature and entropy are accounted for.

Structural Characterization of the (Methanol)4 Potential Energy Surface

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.

Microsolvation of dimethylphosphate: a molecular model for the interaction of cell membranes with water

We present an exhaustive stochastic search of the quantum conformational spaces of the (CH(3)O)(2)PO(2)(-) + nH(2)O (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.

Structures, energies, and bonding in the water heptamer

In this paper we report the geometries and properties of 38 distinct geometrical motifs located on the B3LYP/6-31+G(d), MP2/6-311++G(d, p) potential energy surfaces of the water heptamer. Binding energies of up to 45 kcal/mol are calculated. All motifs fall within 10 kcal/mol of the most stable conformation, with at least 13 structural patterns located no more than 3 kcal/mol above, leading to a very complex potential energy surface, populated by a multitude of motifs each one allowing large numbers of conformations. Cluster stability does not seem to be correlated with the number of hydrogen bonds. Compact structures are energetically favored by electronic energies with zero-point energy corrections, while more open structures are preferred when temperature and entropy are accounted for. The molecular interactions holding the clusters as discrete units lead to large binding energies but are not strong enough to cause significant changes in the geometries of the interacting monomers. Our results indicate that bonding in the water heptamers can be considered as largely non-shared interactions with contributions from intermediate character of increasing covalency.

Structure and Reactivity of the 1Au6Pt Clusters

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 ≈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.

The Jahn-Teller effect: a case of incomplete theory for d4 complexes?

We present relativistic calculations at the four-component Dirac-DFT level for the geometries of the series of group 9 monoanionic hexafluorides MF(6)(-), M = Co, Rh, Ir. Highly correlated four-component relativistic CCSD(T) energies were also calculated for the optimized geometries. Spin-orbit coupling effects influence the geometrical preferences for molecular structures: relativistic calculations predict ground states with octahedral symmetries O(h)* for all hexafluorides in this study, while at the nonrelativistic limit, a structural deviation toward D(4h) ground state symmetries is predicted. Our findings suggest that relativistic effects have an important role in molecular structure preferences for the title hexafluorides.

Hydrophobic meddling in small water clusters

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.

Structural characterization of the (MeSH) 4 potential energy surface

AbstractA random walk on the PES for (MeSH)4 clusters produced 50 structural isomers held together by hydrogen-bonding networks according to calculations performed at the B3LYP/6–311++G** and MP2/6–311++G** levels. The geometric motifs observed are somewhat similar to those encountered for the methanol tetramer, but the interactions responsible for cluster stabilization are quite different in origin. Cluster stabilization is not related to the number of hydrogen bonds. Two distinct, well-defined types of hydrogen bonds scattered over a wide range of distances are predicted. FigureTwo distinct types of hydrogen bonds are predicted for the Methanethiol tetramers

Hydrogen bonding in the binary water/ammonia complex

A detailed study of the interactions leading to stabilization of the NH_{3}H_{2}O_{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++Gd, 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, ΔH^°_{298} -41.75 kJ/mol, is in excellent agreement with the experimental enthalpy of adsorption of NH_{3} into surface water, reported to be-41 ± 5 kJ/mol.

Structure and reactivity of the (1)Au6Pt clusters.

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 ≈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.