Estudio teórico de la interacciones de los brasinoesteroides con el medio biológico

Tesis doctoral inédita. Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química Física Aplicada. Fecha de lectura: 28-09-201

Unusual hydrogen bond patterns contributing to supramolecular assembly: conformational study, Hirshfeld surface analysis and density functional calculations of a new steroid derivative

A structural and conformational study of 3β-acetoxy-17-chloro-16-formyl-5α-androstan-16-ene has been carried out by using X-ray analysis and M06-2X density functional calculations. The compound crystallizes with three independent molecules in the asymmetric unit. Natural Bond Order and Atoms in Molecules methods were used for a better understanding of the key factors that determine the stability of this steroidal molecule, particularly the role of C-H...Cl intramolecular interactions. A detailed investigation of C-H...Cl and C-H...O intermolecular interactions, in addition to the most important van der Waals contribution, are presented by means of Hirshfeld surface analysis. The crystal packing exhibits an unusual intra- and intermolecular hydrogen bond pattern, and shows the importance of non-classical interactions in the construction of the supramolecular assembly. Excellent agreement between the theoretical and experimental data is found.Ministerio de Economía y Competitividad (MINECO)ICTP/CLA

Metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides

In the present review advances in the metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides, mainly in the last decade, will be presented and discussed. An overview on the structure, preparation, dimerization and related reactions as well as the relevant aspects in the cycloaddition chemistry of nitrile oxides (including mechanistic aspects) have also been considered

On the stability of noble-metal nanoclusters protected with thiolate ligands

Noble metal nanoclusters (NCs) protected with thiolate ligands have been of interest because of their long-term stability that makes them suitable as building blocks for diverse assembled systems with emergent and improved functions. Despite the advances in synthesis and characterization, the mechanisms that contribute to their stability are still poorly understood. In this article, we review the different criteria that have been used to explain the experimental stability of NCs with a well-defined number of atoms that are protected with thiolate ligands. We discuss why these criteria are not enough to explain the stability. We conclude that there are other physical factors that should be included when explaining the stability of these systems and could be important for the discovery of new noble-metal NCs

DFT Study of the Adsorption and SERS of Pyridine on M₁₀N₁₀ (M, N = Cu, Ag) Tetrahedral Clusters

This work presents a theoretical detailed analysis of the surface-enhanced Raman spectroscopy (SERS) of the pyridine–M10N10 (M, N = Ag, Cu) tetrahedral (Td) clusters considering two binding positions: vertex (V) and surface (S). In addition to the well-known monometallic Td structure, we added two different bimetallic Ag–Cu compositions, named Td1 and Td2 geometries. Density functional methodology with the use of BP86 and CAM-B3LYP exchange-correlation functionals (XCs) and LANL2DZ pseudopotential has been employed for analyzing the electronic structure and geometries, the chemical static (CHEM), and resonant Raman mechanisms (RR): charge transfer RR–CT and intracluster excitation RR–CR. The static CHEM mechanism shows an increase in the enhancement factors (EFs) of Py–V concerning Py–S positions, which can also be distinguished by the averaged adsorption energies and bond polarizabilities. The static SERS response for Cu–Py–V junction is from 5 to 10 times greater than Ag–Py–V EFs and up to 28 times greater than Py–S complexes. For the static Raman, we found that the analyses of ν8a and ν1 normal modes are related to the EF changes and allow us to distinguish V from S complexes. The TDDFT calculations show striking differences between BP86 and CAM-B3LYP XCs analyzed spectra, and CAM-B3LYP granted a clear distinction between V and S for the location of CT-type transitions. In addition, important differences were obtained from the analysis of the charge transfer excitations between both XCs. Resonant Raman calculations evidenced significant enhancements for RR–CT and RR–CR as compared to the static enhancements, and RR–CT can be distinguished from the RR–CR mechanism, while specific normal modes help to differentiate the vertex from the surface Py-junction. Bimetallic Ag–Cu nanostructures represent promising choices for SERS substrates, showing EFs higher than those of monometallic Ag

Stability and Electronic Charge Compensation of [Ag_{44–<i>x</i>}Au_<i>x</i>(SR)₃₀]^4– Clusters

The stability and electronic properties of [Ag44–xAux(SR)30]4– clusters with x = 0, 12, 20, and 32, and nine thiolate ligands with aromatic and aliphatic groups are investigated using density functional theory. We found differences in the energy gaps, atomic geometries, and charge distributions that depend on both the metallic composition and ligand nature. Completely different behavior for aromatic and aliphatic ligands is found when we analyze the charge density. For aromatic ligands, a charge balance between the metallic core and the sulfur atoms is observed, such that a kind of charge compensation is achieved. This charge compensation is better when x = 0 and x = 12, corresponding to clusters that have been experimentally obtained. On the other hand, aliphatic ligands do not show such charge compensation, also in agreement with the fact that these clusters have not been reported. This charge balance between metals and sulfur atoms might be useful to explain the stability of these clusters and to propose the synthesis of new clusters with aromatic ligands and different substituents, as the chiral ligands that we explore here

Electronic structure and charge compensation in AuxAg25-xSR181− (x = 0, 12, 13, 25), AuAg12Au12SR181−, and AgAu12Ag12SR181− clusters

The stability and electronic properties of AuxAg25-xSR181− (x = 0, 12, 13, 25), AuAg12Au12SR181−, and AgAu12Ag12SR181− clusters with aromatic and aliphatic ligand groups are investigated using density functional theory. We find differences in atomic geometries, energy gaps, and charge distributions, depending on the metal atoms in the outer shells composing staple units. Clusters with gold staple units show larger gaps than silver ones. The analysis of the total and partial density of states shows that HOMO and LUMO states are mainly from the inner shell metal atoms belonging to icosahedral structures, which characterize such clusters’ atomic arrangement. We also find that sulfur atoms in ligands linked to silver and gold capping units behave differently. In contrast to other known clusters, charge distributions of this cluster size are different, where a charge compensation is not present, allowing their synthesis with a plethora of ligands. These results confirm the use of ligands with different natures to obtain clusters of this particular size experimentally