Elizabeth Florez

Stochastic Search of the Quantum Conformational Space of Small Lithium and Bimetallic Lithium-Sodium Clusters

In this paper we report the results obtained by an implementation and application of the simulated annealing optimization procedure to the exploration of the conformational space of small neutral and charged lithium clusters (Li(n)(q), n = 5, 6, 7; q = 0, +/-1) and of the bimetallic lithium/sodium clusters (Li5Na) in their lowest spin states. Our methodology eliminates the structure guessing procedure in the process of generating cluster configurations. We evaluate the quantum energy, typically with the Hartree-Fock Hamiltonian, of randomly generated points in the conformational space and use a modified Metropolis test in the annealing algorithm to generate candidate structures for atomic clusters. The structures are further optimized by analytical methods (gradient following) at the Møller-Plesset second order perturbation theory level (MP2), in conjunction with basis sets including polarization functions with and without diffuse functions. High accuracy ab initio energies at the coupled clusters level, with single, double, and triple substitutions from the Hartree-Fock determinant (CCSD(T)), on the MP2 geometries were calculated and used to establish the relative stability of the isomers within each potential energy surface. Various cluster properties were computed and compared to existing values in order to validate our methods. Our results show excellent agreement with previous experimental and theoretical reports. Even at these small sizes, evidence for 10 new structures never reported before for the lithium clusters and four new structures for the bimetallic clusters is presented.

Topological Analysis of the Fukui Function

In this work an alternative to the analysis of the Fukui function will be presented and compared with the traditional condensed function. The topological analysis allows us to define basins corresponding to different regions of the space, and the numerical integration of the density over those volumes gives a number amenable of a chemical interpretation in line with the Fukui function applications. Various examples are shown, a series of small molecules, a couple of clusters, and aromatic molecules. They are discussed in comparison with other methodologies and with the experimental evidence.

Hydrogenation Reactions on Au/TiC(001): Effects of AuC Interactions on the Dissociation of H2

Density functional calculations carried out for realistic models evidence that Au particles supported on TiC(001) are very active towards H2 dissociation. The molecular mechanisms show that the support is not a mere spectator but plays a major role in the catalyzed reaction and acts as a reservoir of atomic H, making this system an excellent candidate as a catalyst for the hydrogenation of olefins and hydrodesulfurization reactions.

Why is quercetin a better antioxidant than taxifolin? Theoretical study of mechanisms involving activated forms

The stronger antioxidant capacity of the flavonoid quercetin (Q) compared with taxifolin (dihydroquercetin, T) has been the subject of previous experimental and theoretical studies. Theoretical work has focused on the analysis of hydrogen bond dissociation energies (BDE) of the OH phenolic groups, but consider mechanisms that only involve the transfer of one hydrogen atom. In the present work we consider other mechanisms involving a second hydrogen transfer in reactions with free radicals. The relative stability of the radicals formed after the first hydrogen transfer reaction is considered in discussing the antioxidant activity of Q and T. In terms of global and local theoretical reactivity descriptors, we propose that the radical arising from Q should be more persistent in the environment and with the capability to react with a second radical by hydrogen transfer, proton transfer and electron transfer mechanisms. These mechanisms could be responsible of the stronger antioxidant capacity of Q.

An orbital and electron density analysis of weak interactions in ethanol-water, methanol-water, ethanol and methanol small clusters

A computational study of (ethanol)(n)-water, n = 1 to 5 heteroclusters was carried out employing the B3LYP∕6-31+G(d) approach. The molecular (MO) and atomic (AO) orbital analysis and the topological study of the electron density provided results that were successfully correlated. Results were compared with those obtained for (ethanol)(n), (methanol)(n), n = 1 to 6 clusters and (methanol)(n)-water, n = 1 to 5 heteroclusters. These systems showed the same trends observed in the (ethanol)(n)-water, n = 1 to 5 heteroclusters such as an O---O distance of 5 Å to which the O-H---O hydrogen bonds (HBs) can have significant influence on the constituent monomers. The HOMO of the hetero(clusters) is less stable than the HOMO of the isolated alcohol monomer as the hetero(cluster) size increases, that destabilization is higher for linear geometries than for cyclic geometries. Changes of the occupancy and energy of the AO are correlated with the strength of O-H---O and C-H---O HBs as well as with the proton donor and/or acceptor character of the involved molecules. In summary, the current MO and AO analysis provides alternative ways to characterize HBs. However, this analysis cannot be applied to the study of H---H interactions observed in the molecular graphs.

Phosphorus recovery through struvite precipitation from wastewater: effect of the competitive ions

AbstractIn wastewater treatment plants (WWTP), struvite precipitation occurs spontaneously under conditions which are influenced by factors such as: concentration of Mg2+, and , pH, temperature, and competitive ions. These parameters are often difficult to control and, spontaneous precipitation of struvite creates operational problems in WWTP. Struvite is also a potentially marketable product as a fertilizer alternative. For these two reasons it is important to study the principles of struvite precipitation, and evaluate the parameters that control this process. In this study, the influence of the ions Ca2+ and Al3+ in struvite precipitation process was evaluated using three different molar ratios of Mg2+:Ca2+ and Mg2+:Al3+. The products were characterized by thermogravimetric analysis, X-ray diffraction, and scanning electron micrograph. The results showed that calcium interfered in struvite precipitation only when this is in equal or greater molar ratios to magnesium, while aluminum inhibited completely...

Adsorption and diffusion of Au atoms on the (001) surface of Ti, Zr, Hf, V, Nb, Ta, and Mo carbides.

The adsorption of atomic Au on the (001) surface of TiC, ZrC, HfC, VC, NbC, TaC, and delta-MoC and the mechanism of diffusion of this adatom through the surface have been studied in terms of a periodic density functional theory based approach. In all the cases, the Au adsorption energies are in the range of 1.90-2.35 eV. The moderately large adsorption energies allow the Au diffusion before desorption could take place. For TiC(001), ZrC(001), and HfC(001), atomic Au is adsorbed directly on top of C atoms and diffusion takes place along the diagonal of the squares formed by M-C-M-C atoms with the transition state located above the hollow sites. For the rest of transition metal carbides the situation is less simple with the appearance of more than one stable adsorption site, as for NbC and TaC, of a small energy barrier for diffusion around the most stable adsorption site and of a more complex diffusion pathway. The small energy barrier for diffusion around the most stable site will result in a highly mobile Au species which could be observed in scanning tunnel microscope experiments. After depositing Au on metal-carbide surfaces, there is a noticeable charge transfer from the substrate to the adsorbed Au atom. The electronic perturbations on Au increase when going from TiC to ZrC or TaC. Our results indicate that metal carbides should be better supports for the chemical activation of Au than metal oxides.

Potential Energy Surfaces of WC6 Clusters in Different Spin States

Stochastic explorations of the structural possibilities of neutral WC6 clusters in several spin states lead to very rich and complex potential energy surfaces, with geometries quite different from those of pure carbon clusters at the PBE0/def2-TZVP level. The global minimum is predicted to be a triplet-state semicyclic C6 conformation having every carbon in direct coordination to the W atom. Interaction energies are comparable to those of C7 clusters, revealing very strong W-C bonding. Our results suggest that C-C interactions in the clusters should be considered as intermediate between single and double bonds.

Spin–Orbit Coupling Effects in AumPtn Clusters (m + n = 4)

A study of AumPtn(m + n = 4) clusters with and without spin-orbit (SO) coupling using scalar relativistic (SR) and two component methods with the ZORA Hamiltonian was carried out. We employed the PW91 functional in conjunction with the all-electron TZ2P basis set. This paper offers a detailed analysis of the SO effects on the cluster geometries, on the LUMO-HOMO gap, on the charge distribution, and on the relative energies for each relativistic method. In general, SO coupling led to an energetic rearrangement of the species, to changes in geometries and structural preferences, to changes in the structural identity of the global minimum for the Au3Pt, AuPt3 and Pt4 cases, and to a reduction of relative energies among the clusters, an effect that appears stronger as the amount of Pt increases.

Unusual solvation through both p-orbital lobes of a carbene carbon

As a result of a configurational space search done to explain the experimental evidence of transient specific solvation of singlet fluorocarbene amide with tetrahydrofuran, we found that the most stable structures consist in a group in which each oxygen of two tetrahydrofuran molecules act as electron donor to its respective empty p-orbital lobe of the carbene carbon atom, located at each side of the carbene molecular plane. This kind of species, which to our knowledge has not been reported before, explains very well the particular experimental characteristics observed for the transient solvation of this system. We postulate that the simultaneous interaction to both p-orbital lobes seems to confer a special stability to the solvation complexes, because this situation moves away the systems from the proximity of the corresponding transition states for the ylide products. Additionally, we present an analysis of other solvation complexes and a study of the nature of the involved interactions.