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Videos de Conceptos Relacionados

Catalysis02:50

Catalysis

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The presence of a catalyst affects the rate of a chemical reaction. A catalyst is a substance that can increase the reaction rate without being consumed during the process. A basic comprehension of a catalysts’ role during chemical reactions can be understood from the concept of reaction mechanisms and energy diagrams.
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¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

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The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene...
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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
1.7K
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
1.5K
Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation02:24

Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation

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Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
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Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO2 Reduction
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Acoplamiento C-C en catalizadores heterogéneos basados en un solo átomo

Xiaoyan Zhang1,2,3, Zaicheng Sun2, Bin Wang4

  • 1Department of Chemical Engineering and Department of Chemistry, University of Kansas , Lawrence, Kansas 66045, United States.

Journal of the American Chemical Society
|December 22, 2017
PubMed
Resumen
Este resumen es generado por máquina.

Este estudio introduce un nuevo catalizador de nanopartículas basado en TiO2 con átomos de paladio individuales (Pd1/TiO2) para reacciones eficientes de acoplamiento Sonogashira C-C. Este catalizador heterogéneo ofrece una alta actividad y una fácil separación, superando las limitaciones de los catalizadores homogéneos tradicionales.

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Área de la Ciencia:

  • Catálisis heterogénea
  • Catalización de las nanopartículas
  • Química orgánica de los metales

Sus antecedentes:

  • La catálisis de acoplamiento C-C homogéneo enfrenta desafíos con la separación y el costo del catalizador.
  • El desarrollo de catalizadores heterogéneos eficientes es crucial para la síntesis química sostenible.
  • El dióxido de titanio (TiO2) es un material versátil para el soporte de catalizadores.

Objetivo del estudio:

  • Desarrollar un catalizador heterogéneo altamente activo y reutilizable para las reacciones de acoplamiento C-C de Sonogashira.
  • Para investigar el rendimiento catalítico de un solo átomo de paladio en nanopartículas de TiO2.
  • Para elucidar el mecanismo de reacción utilizando cálculos de la teoría funcional de la densidad (DFT).

Principales métodos:

  • Síntesis de catalizadores de nanopartículas basados en TiO2 que anclan átomos de Pd dispersos por separado (Pd1/TiO2).
  • Prueba de la actividad y la selectividad del catalizador en más de 10 reacciones de acoplamiento C-C de Sonogashira.
  • Realizar cálculos DFT para comprender el mecanismo de reacción a nivel atómico.

Principales resultados:

  • Pd1/TiO2 demostró una alta actividad y selectividad para más de 10 reacciones de acoplamiento de Sonogashira.
  • Se logró una tasa de rotación de 51,0 moléculas de difenilacetileno por átomo de Pd por minuto a 60 °C.
  • Se observó una baja barrera de activación aparente de 28,9 kJ/mol, sin costo de separación del catalizador.

Conclusiones:

  • El paladio monoatómico en nanopartículas de TiO2 (Pd1/TiO2) es un catalizador heterogéneo eficaz para el acoplamiento C-C de Sonogashira.
  • El catalizador facilita la formación eficiente de enlaces C-C con una fácil separación y reutilización.
  • Los cálculos de DFT revelaron un mecanismo que involucra el fenil intermedio adsorbido en Pd1 y el fenilacetilenil en TiO2.