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Videos de Conceptos Relacionados

Common Ion Effect03:24

Common Ion Effect

Compared with pure water, the solubility of an ionic compound is less in aqueous solutions containing a common ion (one also produced by dissolution of the ionic compound). This is an example of a phenomenon known as the common ion effect, which is a consequence of the law of mass action that may be explained using Le Châtelier’s principle. Consider the dissolution of silver iodide:
Metal-Ligand Bonds02:51

Metal-Ligand Bonds

The hemoglobin in the blood, the chlorophyll in green plants, vitamin B-12, and the catalyst used in the manufacture of polyethylene all contain coordination compounds. Ions of the metals, especially the transition metals, are likely to form complexes.
In these complexes, transition metals form coordinate covalent bonds, a kind of Lewis acid-base interaction in which both of the electrons in the bond are contributed by a donor (Lewis base) to an electron acceptor (Lewis acid). The Lewis acid in...
Ligand-gated Ion Channels01:19

Ligand-gated Ion Channels

Ligand-gated ion channels are transmembrane proteins with a channel for ions to pass through and a binding site for a ligand. The channel opens only when a ligand attaches to the binding site.
Three Subfamilies of Ligand-gated Ion Channels
Ligand-gated ion channels fall into three subfamilies. The 'Cys-loop' includes the nicotinic acetylcholine receptors, γ-aminobutyric acid (GABA), glycine, and 5-hydroxytryptamine receptors. The second one is the 'Pore-loop' channels that include the...
Indirect-Acting Cholinergic Agonists: Chemistry and Structure-Activity Relationship01:29

Indirect-Acting Cholinergic Agonists: Chemistry and Structure-Activity Relationship

Indirect-acting cholinergic agonists are agents that interact with the acetylcholinesterase enzyme in the synaptic cleft, preventing the breakdown of acetylcholine into choline and acetate. Consequently, the concentration of acetylcholine in the synaptic cleft increases. These agonists can be classified into reversible and irreversible inhibitors based on their duration of action.
Reversible inhibitors display short to medium durations of action. Short-acting agents include simple alcohols with...
Complexation Equilibria: The Chelate Effect01:19

Complexation Equilibria: The Chelate Effect

In complexation reactions, metal atoms or cations interact with ligands to form donor-acceptor adducts called metal complexes. Ligands that bind through one donor site are monodentate, ligands with two donor sites are bidentate, and those with more than two donor sites are polydentate ligands. For example, ethylene diamine is a bidentate ligand that binds through two nitrogen donor atoms, forming a five-membered ring. EDTA is a polydentate ligand that binds through four oxygen and two nitrogen...
Colloidal precipitates01:09

Colloidal precipitates

The high insolubility of some precipitates can result in an unfavorable relative supersaturation. This can lead to colloidal particles with a large surface-to-mass ratio, where adsorption is promoted. For instance, in the precipitation of silver chloride, silver ions are adsorbed on the surface of the colloidal particles, forming a primary layer. This layer attracts ions of opposite charge (such as nitrate ions), forming a diffuse secondary layer of adsorbed ions. This electric double layer...

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Ion Mobility-Mass Spectrometry Techniques for Determining the Structure and Mechanisms of Metal Ion Recognition and Redox Activity of Metal Binding Oligopeptides
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El catión de litio como catalizador de la polimerización radical

Timothy Clark1

  • 1Computer-Chemie-Centrum der Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany. clark@chemie.uni-erlangen.de

Journal of the American Chemical Society
|August 24, 2006
PubMed
Resumen
Este resumen es generado por máquina.

Los cationes de litio desnudos catalizan la polimerización de las olefinas promoviendo la adición radical sobre la abstracción de hidrógeno. Este efecto catalítico, impulsado por la complejación de iones metálicos, permite un crecimiento eficiente de la cadena en disolventes no polares.

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Área de la Ciencia:

  • Química Física es la química física.
  • La ciencia de los polímeros es la ciencia de los polímeros.
  • Química computacional es la química computacional.

Sus antecedentes:

  • La polimerización radical de las olefinas a menudo se ve obstaculizada por la abstracción competitiva del átomo de hidrógeno.
  • Los iones metálicos pueden influir en las vías de reacción en la síntesis orgánica.
  • Trabajos experimentales anteriores sugirieron que las sales de litio podrían promover la polimerización de alquenos.

Objetivo del estudio:

  • Para investigar el papel catalítico de los cationes de litio "desnudos" en la polimerización radical de las olefinas.
  • Para dilucidar el mecanismo por el cual los cationes de litio influyen en la adición radical frente a la abstracción de hidrógeno.
  • Para apoyar computacionalmente las observaciones experimentales de la polimerización de alquenos promovida por litio.

Principales métodos:

  • Cálculos de la teoría de la densidad-funcional (DFT).
  • Cálculos de química cuántica desde el principio (QCISD y CBS-RAD).
  • Modelado computacional de vías de reacción y barreras energéticas.

Principales resultados:

  • La complejación de cationes de litio favorece la adición de radicales alquilo a las olefinas sobre la abstracción de hidrógeno alílico.
  • La adición de dioxígeno triplet a las olefinas es termoneutral con una barrera baja cuando se compleja con litio.
  • La abstracción del átomo de hidrógeno también es favorecida por la complejación del litio, pero menos que la adición.

Conclusiones:

  • Los cationes de litio "desnudos" actúan como catalizadores para la polimerización de los radicales olefinos al promover selectivamente la propagación de la cadena.
  • La complejación de oxígeno mediada por litio facilita el inicio de la polimerización.
  • Los hallazgos computacionales se alinean con la evidencia experimental para la polimerización de alquenos promovida por litio.