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¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

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,...
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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

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...
P-N junction01:11

P-N junction

A p-n junction is formed when p-type and n-type semiconductor materials are joined together. At the interface of the p-n junction, holes from the p-side and electrons from the n-side begin to diffuse into the opposite sides due to the concentration gradient. This diffusion of carriers leads to a region around the junction where there are no free charge carriers, known as the depletion region. The charge density within the depletion region for the n-side and p-side can be described by the...
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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

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...
Metal-Semiconductor Junctions01:24

Metal-Semiconductor Junctions

The contact of metal and semiconductor can lead to the formation of a junction with either Schottky or Ohmic behavior.
Schottky Barriers
Schottky barriers arise when a metal with a work function (Φm) contacts a semiconductor with a different work function (Φs). Initially, electrons transfer until the Fermi levels of the metal and semiconductor align at equilibrium. For instance, if Φm > Φs, the semiconductor Fermi level is higher than the metal's before contact. The semiconductor's...

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Single-Molecule F&ouml;rster Resonance Energy Transfer Methods for Real-Time Investigation of the Holliday Junction Resolution by GEN1
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Published on: September 18, 2019

Rotaciones frustradas en las uniones de una sola molécula.

Young S Park1, Jonathan R Widawsky, Maria Kamenetska

  • 1Department of Chemistry, Columbia University, New York, New York, USA.

Journal of the American Chemical Society
|September 3, 2009
PubMed
Resumen
Este resumen es generado por máquina.

La orientación molecular de los enlaces oro-calcógeno influye en el transporte de electrones en las moléculas conjugadas. Una mayor superposición entre electrodos de oro, pares aislados de calcógeno y sistemas pi aromáticos mejora la conductividad de las uniones moleculares.

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Área de la Ciencia:

  • La electrónica molecular es la electrónica molecular.
  • Química orgánica es la química orgánica.
  • Ciencias de la superficie Ciencias de la superficie.

Sus antecedentes:

  • Comprender el transporte de carga en las uniones moleculares es crucial para el desarrollo de nuevos dispositivos electrónicos.
  • La interfaz entre los electrodos y las moléculas orgánicas tiene un impacto significativo en el rendimiento del dispositivo.

Objetivo del estudio:

  • Investigar cómo la orientación de los enlaces oro-calcógeno afecta el transporte de electrones a través de moléculas conjugadas.
  • Aclarar el papel de los pares solitarios de calcógeno y los sistemas pi aromáticos en la conductividad molecular.

Principales métodos:

  • Mediciones de conductividad de uniones moleculares diseñadas específicamente.
  • Comparación de la conductividad entre diferentes arquitecturas moleculares (por ejemplo, 1,4-bis(metiltio) benceno frente a tetrahidrobenzodiotiofeno).

Principales resultados:

  • La orientación de los enlaces oro-azufre (Au-S) y oro-selenio (Au-Se) en relación con el sistema pi aromático controla explícitamente el transporte de electrones.
  • Las vías de conducción involucran a los pares de pones de calcógeno, conectando electrodos de oro al sistema pi aromático.
  • El aumento de la superposición entre estos componentes conduce a una mayor conductividad.

Conclusiones:

  • La orientación molecular en las interfaces electrodo-molécula es un factor clave en el control del transporte de carga.
  • Las interacciones de pares solitarios de calcógeno con sistemas aromáticos son críticas para el transporte eficiente de electrones en las uniones moleculares.
  • Este trabajo proporciona ideas fundamentales para el diseño de dispositivos electrónicos moleculares de alto rendimiento.