Juvencio Robles

Nucleation and growth of cobalt onto different substrates: Part I. Underpotential deposition onto a gold electrode

Abstract The formation and stripping of a cobalt adlayer were studied by electrochemical techniques at underpotential conditions, monitoring the deposition/dissolution onto/from a polycrystalline gold electrode. The kinetics of the former process fit a model where adsorption and diffusion-controlled two-dimensional instantaneous nucleation of cobalt occurs simultaneously onto the gold electrode surface. The latter involves a complex mechanism where adsorption/desorption competes with two processes, an instantaneous nucleation and growth of two-dimensional holes under charge transfer control and a progressive one. However, it was found that the relative contribution of the instantaneous nucleation was the most important independently of the applied potential.

Nucleation and growth of cobalt onto different substrates: Part II. The upd-opd transition onto a gold electrode

A study of the electrochemical deposition of cobalt was carried out using electrochemical techniques. The formation kinetics and growth of cobalt nuclei onto a polycrystalline gold electrode were studied employing an aqueous 10 � 2 M CoCl2 1MN H 4Cl solution (pH 9.5). We obtain the potentiostatic j � /t plots when the potential step jumps from a potential value in the underpotential deposition zone to a final potential in the opd region. It was found that these current density transients can be described through a kinetic mechanism that involves three different contributions: (a) a Langmuir type adsorption process, (b) 2D diffusion-controlled instantaneous nucleation and (c) 3D nucleation limited by a mass transfer reaction. In order to describe the contribution due to 3D growth we test two different approaches by Scharifker and Mostany (J. Electroanal. Chem. 177 (1984) 13) and Heerman and Tarallo (J. Electroanal. Chem. 470 (1997) 70), the values of the experimental parameters A (nucleation rate constant), N0 (number of active nucleation sites) and D (diffusivity of the depositing ions) obtained in the two cases are quite close, however if the influence of the adsorption and 2D nucleation and growth processes is not considered, A and N0 are overestimated and underestimated, respectively. # 2003 Elsevier Science B.V. All rights reserved.

Tuning aromaticity in trigonal alkaline earth metal clusters and their alkali metal salts

In this work, we analyze the geometry and electronic structure of the [XnM3]n−2 species (M = Be, Mg, and Ca; X = Li, Na, and K; n = 0, 1, and 2), with special emphasis on the electron delocalization properties and aromaticity of the cyclo‐[M3]2− unit. The cyclo‐[M3]2− ring is held together through a three‐center two‐electron bond of σ‐character. Interestingly, the interaction of these small clusters with alkali metals stabilizes the cyclo‐[M3]2− ring and leads to a change from σ‐aromaticity in the bound state of the cyclo‐[M3]2− to π‐aromaticity in the XM3− and X2M3 metallic clusters. Our results also show that the aromaticity of the cyclo‐[M3]2− unit in the X2M3 metallic clusters depends on the nature of X and M. Moreover, we explored the possibility for tuning the aromaticity by simply moving X perpendicularly to the center of the M3 ring. The Na2Mg3, Li2Mg3, and X2Ca3 clusters undergo drastic aromaticity alterations when changing the distance from X to the center of the M3 ring, whereas X2Be3 and K2Mg3 keep its aromaticity relatively constant along this process.

Coordination and Haptotropic Migration of Cr(CO)3 in Polycyclic Aromatic Hydrocarbons: The Effect of the Size and the Curvature of the Substrate

In this work, we study the reaction mechanism of the tricarbonylchromium complex haptotropic rearrangement between two six-membered rings arranged like in naphthalene of four polycyclic aromatic hydrocarbons (PAHs). It has been found that the reaction mechanism of this haptotropic migration can either occur in a single step or stepwise depending on the interaction between the orbitals of the Cr(CO)3 and the PAH fragments. Our results show that the size of the cyclic system favors the metal migration whereas the curvature of the organic substrate tends to slow down the rearrangement. We discuss the key factors that help to explain this behavior through orbital and energy decomposition analysis.

Analysis of the effect of changing the a0 parameter of the Becke3-LYP hybrid functional on the transition state geometries and energy barriers in a series of prototypical reactions

A series of eleven gas-phase chemical reactions have been examined to assess the dependence of transition state geometries and energy barriers, as well as energy differences between reactants and products, on the a0 B3LYP functional parameter. Throughout the study we have changed the a0 parameter from 0.1 to 0.9 and for the ac and ax parameters we have followed the relationships ax = 1 − a0 and ac = ax. By comparing with the QCISD transition state geometries and energy barriers, our systematic study allows us to identify the influence of the a0 parameter in the reactions studied. In general, B3LYP calculations with the original parameters underestimate energy barriers, this trend being corrected when the a0 parameter increases. Our study also shows that the fraction of Hartree–Fock exchange needed to predict accurate barrier heights differs from the optimal fraction needed to predict thermochemical properties and geometries.

Theoretical and Experimental Study of Cobalt Nucleation and Growth onto Gold Substrate with Different Crystallinity

Cobalt electrodeposition onto both polycrystalline and monocrystalline (111) gold electrodes was studied using electrochemical techniques. Current density transients were recorded in these systems starting with the applied potential in the underpotential region and then jumping to different potential values in the overpotential zone. From the analysis of the experimental J-t curves, it was found that the process of cobalt deposition involves a 2D-3D nucleation growth transition in both cases. We also found that a mechanism comprising the simultaneous presence of three different contributions namely, Langmuir-type adsorption processes and 2D and 3D nucleation, both mass-transfer-controlled nucleation processes, can adequately describe the experimental evidence. This model predicts a greater nucleation rate and larger number of active nucleation sites for polycrystalline gold substrate compared with the Au(lll) substrate for each applied potential. Moreover, from a theoretical quantum study, we analyzed the reactivity of the bare Au( 111) surface determining that there exists a distribution of active reduction sites inherent to this substrate. The existence of these sites can be considered as one of several previous steps required for the formation of active nucleation sites. We found a ratio of 2.5 X 10 4 active reduction sites for each active nucleation site obtained from experimental results.

Quadrupolar moment calculations and mesomorphic character of model dimeric liquid crystals

Abstract Model dimeric compounds CH3(CH2)nCOOROOC(CH2)pCOOROOC(CH2)nCH3, where (R=C6H4C(CH3)HCC6H4), of semiflexible liquid crystal polymers (LCP) have been studied by molecular modeling techniques. First, a conformational analysis of the molecules was made in order to find the lowest energy conformation structure. In a second stage, geometry optimization calculations of these conformers were performed at the quantum mechanics semiempirical PM3 level. Some molecular parameters (electric dipolar and quadrupolar moments) were obtained at the improved ab initio Hartree–Fock HF/6-31G(d)//PM3 level and are related with the mesomorphic character of these systems. It is found that the trace of the quadrupole moment tensor correlates with the spacer ((CH2)p-linkage) length, and this can be an indication of the trend that this magnitude exhibits in aromatic liquid crystal systems.

Unraveling the Origin of the Relative Stabilities of Group 14 M2N22+ (M, N = C, Si, Ge, Sn, and Pb) Isomer Clusters

We analyze the molecular structure, relative stability, and aromaticity of the lowest-lying isomers of group 14 M2N2(2+) (M and N = C, Si, and Ge) clusters. We use the gradient embedded genetic algorithm to make an exhaustive search for all possible isomers. Group 14 M2N2(2+) clusters are isoelectronic with the previously studied group 13 M2N2(2-) (M and N = B, Al, and Ga) clusters that includes Al4(2-), the archetypal all-metal aromatic molecule. In the two groups of clusters, the cyclic isomers present both σ- and π-aromaticity. However, at variance with group 13 M2N2(2-) clusters, the linear isomer of group 14 M2N2(2+) is the most stable for two of the clusters (C2Si2(2+) and C2Ge2(2+)) , and it is isoenergetic with the cyclic D(4h) isomer in the case of C4(2+). Energy decomposition analyses of the lowest-lying isomers and the calculated magnetic- and electronic-based aromaticity criteria of the cyclic isomers help to understand the nature of the bonding and the origin of the stability of the global minima. Finally, for completeness, we have also analyzed the structure and stability of the heavier Sn and Pb group 14 M2N2(2+) analogues.

Synthesis of azepino[4,5-b]indol-4-ones via MCR/free radical cyclization and in vitro–in silico studies as 5-Ht6R ligands

A series of nine new 3-acetamide-azepino[4,5-b]indol-4-ones were synthesized in two steps: (i) multicomponent reaction (Ugi-4CR/SN2) and (ii) free radical-mediated cyclization. These compounds, along with their tetrazole-based analogs, were studied in vitro to assess their binding with the 5-hydroxytryptamine type 6 receptor (5-Ht6R). The 3-acetamide-azepino[4,5-b]indol-4-ones displayed moderate affinity, whereas the 3-tetrazolylmethyl-azepino[4,5-b]indol-4-ones affinity values are lower. However, one of the 3-acetamide-azepino[4,5-b]indol-4-ones exhibited a hit value of Ki (211.2nM) to the 5-Ht6R. Minimal-energy structures of one cis-amide and its tetrazole-based analog (calculated by means of the Density Functional Theory) were analyzed to assess some interesting bioisosterism aspects. Interactions and binding energies between all products and the 5-Ht6R were calculated through in silico molecular couplings. Finally, a QSAR model was proposed using the results of the in vitro assays.

Spirochromone-chalcone conjugates as antitubercular agents: synthesis, bio evaluation and molecular modeling studies

A new series of spirochromone annulated chalcone conjugates were synthesized and evaluated for their antitubercular activity against Mycobacterium tuberculosis H37Rv strain. These compounds were subjected to molecular modeling studies using docking and chemoinformatics based approaches. The docking simulations were performed against a range of known receptors for chalcone derived compounds to reveal MTB phosphotyrosine phosphatase B [MtbPtpB] protein as the most probable target based on the high binding affinity scores. Five compounds exhibit significant inhibition, showing minimum inhibitory concentration values i.e. MIC values ranging from 3.13–12.5 μg mL−1. Further analysis of the synthesized compounds with known and in-house developed chemoinformatics tools unequivocally established their potential as anti-tubercular compounds. QSAR modeling revealed a quantitative relationship between biological activities and frontier molecular orbital energies of synthesized compounds. The predictive model can be employed further for virtual screening of new compounds in this series.