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Caffeic acid (C9;H8.;O4;) and its conjugate base C9;H7;O4; - (anionic form-known as caffeate) were analyzed computationally through the use of quantum chemistry to assess their intrinsic global and local reactivity using the tools of conceptual density functional theory. The anionic form was found to be better at coordinating the silver cation than caffeic acid thus suggesting the use of caffeate as a complexation agent. The complexation capability of caffeate was compared with that of some of the most common ligand agents used to coordinate silver cations. Local reactivity descriptors allowed identification of the preferred sites on caffeate for silver cation coordination thus generating a plausible silver complex. All silver complexes were analyzed thermodynamically considering interaction energies in both gas and aqueous phases; the complexation free energy in aqueous phase was also determined. These results suggest that more attention be paid to the caffeate anion and its derivatives because this work has shed new light on the behavior of this anion in the recovery of silver cations that could be exploited in silver mining processes in a environmentally friendly way. © Springer-Verlag 2012.

Martinez-Araya J.I.,Pedro de Valdivia University | Salgado-Moran G.,Andres Bello University | Glossman-Mitnik D.,CIMAV
Journal of Physical Chemistry B | Year: 2013

A density functional theory study of eight oxicams was carried out in order to determine their global and local reactivities. These types of reactivities were measured by means of global and local reactivity descriptors coming from the conceptual density functional theory. Net electrophilicity as a global reactivity descriptor and local hypersoftness as a local reactivity descriptor were the used tools to distinguish reactivity and selectivity among these oxicams. Globally, isoxicam presents the highest electron donating capacity; meanwhile, the highest electron accepting capacity is exhibited by droxicam. Locally, two oxicams present neither nucleophilic nor electrophilic relevant reactivity in their peripheral pyridine ring, droxicam and tenoxicam, so that their more reactive zones are found on the respective fused rings. Oxicams have been divided into two subgroups in order to facilitate the local analysis of reactivity. One group is characterized because their most important condensed values for local hypersoftnes are well-separated: 4-meloxicam, lornoxicam, meloxicam, and normeloxicam. Meanwhile, the opposite situation is found in droxicam, isoxicam, piroxicam, and tenoxicam. As a whole, the nucleophilic characteristic noticeably predominates in these eight oxicams instead of an electrophilic behavior, thus meaning a greater tendency to donate electrons rather than withdrawing them; a consequence of this behavior implies a favorable interaction with a hypothetical receptor bearing one or more electron acceptor functional groups rather than electron donor functional groups; this would imply a maximization of this interaction from the covalent point of view. © 2013 American Chemical Society.

Martinez-Araya J.I.,Pedro de Valdivia University
Journal of Molecular Modeling | Year: 2013

At present, there are two levels of approximation to compute the dual descriptor (DD). The first uses the total electronic density of the original system along with the electronic densities of the system with one more electron and one less electron, but this procedure is time consuming and normal termination of computation of total electronic densities is not guaranteed. The second level of approximation uses only the electronic densities of frontier molecular orbitals, HOMO and LUMO, to avoid the former approximation; however, the orbital relaxation implicitly included in the first level of approximation is absent in the second, thus risking an incorrect interpretation of local reactivity. Between the lowest occupied molecular orbital (LOMO) and the highest unoccupied molecular orbital (HUMO), a framework to provide an expression of the DD in terms of the electronic densities of all molecular orbitals (except HUMO and LOMO) has been proposed to be implemented by programmers as a computational code. This methodology implies another level of approximation located between the conventional approximation methods mentioned above. In this study, working equations have been oriented toward molecular closed- and open-shell systems. In addition, the mathematical expression for a closed-shell system was applied to acetylene in order to assess the capability of this approach to generate the DD. © 2012 Springer-Verlag Berlin Heidelberg.

Martinez-Araya J.I.,Pedro de Valdivia University
Journal of Molecular Modeling | Year: 2013

The intrinsic reactivity of cyanide when interacting with a silver cation was rationalized using the dual descriptor (DD) as a complement to the molecular electrostatic potential (MEP) in order to predict interactions at the local level. It was found that DD accurately explains covalent interactions that cannot be explained by MEP, which focuses on essentially ionic interactions. This allowed the rationalization of the reaction mechanism that yields silver cyanide in the gas phase. Other similar reaction mechanisms involving a silver cation interacting with water, ammonia, and thiosulfate were also explained by the combination of MEP and DD. This analysis provides another example of the usefulness of DD as a tool for gaining a deeper understanding of any reaction mechanism that is mainly governed by covalent interactions. [Figure not available: see fulltext.] © 2012 Springer-Verlag.

Martinez-Araya J.I.,Pedro de Valdivia University | Martinez-Araya J.I.,Centro Para La Investigacion Interdisciplinaria Avanzada En Ciencias Of Los Materiales | Quijada R.,Centro Para La Investigacion Interdisciplinaria Avanzada En Ciencias Of Los Materiales | Quijada R.,University of Chile | And 2 more authors.
Journal of Physical Chemistry C | Year: 2012

A density functional theory study of the ethylene polymerization mechanism catalyzed by metallocene methyl cations based on group IVB (Ti, Zr, and Hf) is presented. The concept called reaction force was applied in order to decompose the activation energy into two parts with the aim of distinguishing the predominance of structural or electronic effects within intervals along the reaction coordinate on each step of the polymerization process. This has implied an alternative rational analysis of elementary chemical reactions associated with the polymerization mechanism under the assumption that the Cossée-Arlman's mechanism is operating along the whole process. Three reaction models representing elementary chemical reactions of the polymerization process (initiation, propagation, and termination) were used through molecular quantum mechanical calculations. The simplest of metallocene methyl cations (built up by two cyclopentadienyl groups and one methyl group linked to the metal) was used as a catalytic molecular model. Since the main goal was focused on getting information of intrinsic global reactivity of catalytic systems, both solvent and co-catalyst were not modeled in the present work. As a result, energy profiles indicate that ethylene polymerization reactions are better catalyzed by the respective metallocene cation based on Ti, whereas the catalysts based on Zr and Hf present similar characteristics among them, thus supporting experimental results where the molecular weight of polyethylene obtained by means of metallocene cation based on Ti approximately doubles the molecular weights of polyethylene catalyzed by metallocenes based on Zr and Hf. However propagation and termination steps are better catalyzed by metallocene complexes based on Zr and Hf, thus masking the influence of initiation step upon the molecular weight of the obtained polymer and providing more importance to termination step rather than propagation step. This would help to better understand differences presented in the average molecular weight of a same polymer obtained with each of these catalytic systems. As some key steps of the polymerization process would be more favored with one type of metallocene rather than other one, the use of reaction force would help to better understand how to modify energetic barriers and or global changes in energy by perturbating geometrical or electronic structure of catalytic systems. The latter suggests that the ideal polymerization process should be carried out with different catalytic systems depending on the step of polymerization and not with a unique catalyst as has been commonly performed up to now. That might lead to the obtention of a more wide range of new polymeric materials. © 2012 American Chemical Society.

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