14,425 research outputs found
Phosphoramidite Gold(I)-Catalyzed Diastereo- and Enantioselective Synthesis of 3,4-Substituted Pyrrolidines
In this article the utility of phosphoramidite ligands in enantioselective AuI catalysis was explored in the development of highly diastereo- and enantioselective AuI-catalyzed cycloadditions of allenenes. A Au^I-catalyzed synthesis of 3,4-disubstituted pyrrolidines and γ-lactams is described. This reaction proceeds through the enantioselective AuI-catalyzed cyclization of allenenes to form a carbocationic intermediate that is trapped by an exogenous nucleophile, resulting in the highly diastereoselective construction of three contiguous stereogenic centers. A computational study (DFT) was also performed to gain some insight into the underlying mechanisms of these cycloadditions. The utility of this new methodology was demonstrated through the formal synthesis of (−)-isocynometrine
Marginal Fermi liquid behavior from 2d Coulomb interaction
A full, nonperturbative renormalization group analysis of interacting
electrons in a graphite layer is performed, in order to investigate the
deviations from Fermi liquid theory that have been observed in the experimental
measures of a linear quasiparticle decay rate in graphite. The electrons are
coupled through Coulomb interactions, which remain unscreened due to the
semimetallic character of the layer. We show that the model flows towards the
noninteracting fixed-point for the whole range of couplings, with logarithmic
corrections which signal the marginal character of the interaction separating
Fermi liquid and non-Fermi liquid regimes.Comment: 7 pages, 2 Postscript figure
Modelos matemáticos a partir del modelo nomológico–deductivo de la explicación científica
Mostraremos a continuación la posibilidad de generar modelos matemáticos simples a partir de la explicación de un hecho físico. El marco teórico de partida es el de la explicación científica con la estructura del modelo nomológico deductivo. El uso de modelos matemáticos en este marco genera herramientas didácticas de distinto tipo, en este articulo desarrollamos brevemente el diseño de proyectos de investigación para los alumnos. El docente puede generar y luego utilizar estos proyectos de distintos modos, por ejemplo, como actividad de cierre de un curso, o también para generar una discontinuidad en el transcurso de la cursada, como actividad en paralelo que ocupe algún momento de las clases, etc
On algebraic classification of quasi-exactly solvable matrix models
We suggest a generalization of the Lie algebraic approach for constructing
quasi-exactly solvable one-dimensional Schroedinger equations which is due to
Shifman and Turbiner in order to include into consideration matrix models. This
generalization is based on representations of Lie algebras by first-order
matrix differential operators. We have classified inequivalent representations
of the Lie algebras of the dimension up to three by first-order matrix
differential operators in one variable. Next we describe invariant
finite-dimensional subspaces of the representation spaces of the one-,
two-dimensional Lie algebras and of the algebra sl(2,R). These results enable
constructing multi-parameter families of first- and second-order quasi-exactly
solvable models. In particular, we have obtained two classes of quasi-exactly
solvable matrix Schroedinger equations.Comment: LaTeX-file, 16 pages, submitted to J.Phys.A: Math.Ge
Spectroscopic analysis of peculiar stars. : II. HD 192913 and HD 224801
High dispersion spectra of HD 192913 and HD 224801, two objects belonging to the so called "Si - λ4200" group have been analysed. Both stars show differences in the line intensities of several elements. Hd 192913 contains iron in three stages of ionization, shows C II and has V, Cr and Zr intensified. HD 224801 shows only normal Fe and no V, Cr and Zr. The only abnormalities of this object are the enhancement of Si, Mn and some rare-earths. The paper will be published in full somewhere else.Asociación Argentina de Astronomí
Spectroscopic analysis of peculiar stars. : II. HD 192913 and HD 224801
High dispersion spectra of HD 192913 and HD 224801, two objects belonging to the so called "Si - λ4200" group have been analysed. Both stars show differences in the line intensities of several elements. Hd 192913 contains iron in three stages of ionization, shows C II and has V, Cr and Zr intensified. HD 224801 shows only normal Fe and no V, Cr and Zr. The only abnormalities of this object are the enhancement of Si, Mn and some rare-earths. The paper will be published in full somewhere else.Asociación Argentina de Astronomí
Spectroscopic analysis of peculiar stars. : II. HD 192913 and HD 224801
High dispersion spectra of HD 192913 and HD 224801, two objects belonging to the so called "Si - λ4200" group have been analysed. Both stars show differences in the line intensities of several elements. Hd 192913 contains iron in three stages of ionization, shows C II and has V, Cr and Zr intensified. HD 224801 shows only normal Fe and no V, Cr and Zr. The only abnormalities of this object are the enhancement of Si, Mn and some rare-earths. The paper will be published in full somewhere else.Asociación Argentina de Astronomí
The anatomy of urban social networks and its implications in the searchability problem
The appearance of large geolocated communication datasets has recently
increased our understanding of how social networks relate to their physical
space. However, many recurrently reported properties, such as the spatial
clustering of network communities, have not yet been systematically tested at
different scales. In this work we analyze the social network structure of over
25 million phone users from three countries at three different scales: country,
provinces and cities. We consistently find that this last urban scenario
presents significant differences to common knowledge about social networks.
First, the emergence of a giant component in the network seems to be controlled
by whether or not the network spans over the entire urban border, almost
independently of the population or geographic extension of the city. Second,
urban communities are much less geographically clustered than expected. These
two findings shed new light on the widely-studied searchability in
self-organized networks. By exhaustive simulation of decentralized search
strategies we conclude that urban networks are searchable not through
geographical proximity as their country-wide counterparts, but through an
homophily-driven community structure
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