New [60]fullerene-steroid conjugates (4-6) have been synthesized by 1,3-dipolar cycloaddition and Bingel-Hirsch cyclopropanation reactions from suitably functionalized epiandrosterone and [60]fullerene. Since a new stereocenter is created in the formation of the Prato monoaduct, two different diastereomers were isolated by HPLC (4, 5) whose absolute configurations were assigned according to the highly reliable "sector rule" on fullerenes. A further reaction of the malonate-containing diastereomer 5 with a second C60 molecule has afforded dumbbell fullerene 6 in which the two fullerene units are covalently connected through an epiandrosterone moiety. The new compounds have been spectroscopically characterized and their redox potentials, determined by cyclic voltametry, reveal three reversible reduction waves for hybrids 4 and 5, whereas these signals are split in dumbbell 6. Theoretical calculations at semiempirical (AM1) and single point B3LYP/6-31G(d) levels have predicted the most stable conformations for the hybrid compounds (4-6), showing the importance of the chlorine atom on the D ring of the steroid. Furthermore, TDDFT calculations have allowed assignments of the experimentally determined circular dichroism (CD) of the [60]fullerene-steroid hybrids based on the sign and position of the Cotton effects, despite the exceptionally large systems under study.

Pyrrolil-silicon compounds were investigated by different theoretical approaches.Model monomers consisted in pyrrole ring N-substituted with silylmethoxy and silylhydroxy end-groups through propyl chain spacer, designated as PySi and PySiOH. Geometrical, vibrational and electronic properties, as well as chemical reactivity, are discussed and compared with Pyrrole (Py) and N-propylpyrrole (N-PrPy) that were studied in parallel for reference purposes and methods validation. The electronic distribution between PySi and PySiOH importantly differs, being the former electron donor, as Py and N-PrPy. Conversely, PySiOH presents donor-acceptor character with LUMO energy level localized on the silanol end-group.Global and local reactivity descriptors predict PySiOH more reactive than PySi with two preferential reactive sites: electron-rich Py ring and electron-deficient silanol group. Based on experimental studies, oligomers of PySiOH linked α−α' via Py rings (α−α'Py n SiOH, n = 2,3) were considered as model molecules of hydrolyzed PySi. The most stable structures were derived from randomly generated α−α'Py n SiOH that were optimized at semiempirical AM1 and refined with M05-2X/6-31G(d,p). Conformational analysis of dimer and trimer structures points to stability enhanced by molecular packing. Nonetheless, NBO and RDG results indicate that oligomers stability is dictated by the cooperative contribution of hydrogen bonding between silanol end-groups and dispersive vdW interactions between silanol and the π system of Py ring.The latter interaction resulting from electron delocalization induced by electron deficient silanol group seems to determine the smaller gap energy of T-shaped OH-π arrangements. The theoretical findings support the peculiar chemical behavior revealed by experiment.

Structural and conformational study of a new steroid derivative using X-ray and density functional calculations.

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An analysis of the electron density of different conformers of the 1-butyl-3-methylimidazolium chloride (bmimCl) ionic liquid by using DFT through the BVP86 density functional has been obtained within the framework of Bader's atom in molecules (AIM), localized orbital locator (LOL), natural bond orbital (NBO), and deformed atoms in molecules (DAM). We also present an analysis of the reduced density gradients that deliver the non-covalent interaction regions and allow to understand the nature of intermolecular interactions. The most polar conformer can be characterized as ionic by AIM, LOL, and DAM methods while the most stable and the least polar shows shared-type interactions. The NBO method allows to comprehend what causes the stabilization of the most stable conformer based on analysis of the second-order perturbative energy and the charge transferred among the natural orbitals involved in the interaction.

This work presents a theoretical detailed analysis of the surface-enhanced Raman spectroscopy (SERS) of the pyridine− M 10 N 10 (M, N = Ag, Cu) tetrahedral (Td) clusters considering two binding positions: vertex (V) and surface (S). In addition to the wellknown 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 ν 8a and ν 1 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.

PACS 61.46.Bc -Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate) PACS 68.43.Hn -Structure of assemblies of adsorbates (two-and three-dimensional clustering) Abstract -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.

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