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Videos de Conceptos Relacionados

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...
1.6K
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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

1.4K
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...
1.4K
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

1.4K
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 Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

1.4K
In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...
1.4K
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

2.9K
The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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Acoplamiento orden-ión en cristales líquidos nemáticos

Rajratan Basu1

  • 1United States Naval Academy, Department of Physics, Soft Matter and Nanomaterials Laboratory, The , Annapolis, Maryland 21402, USA.

Physical review. E
|December 23, 2025
PubMed
Resumen

Se estudiaron cuantitativamente las impurezas iónicas en cristales líquidos (CL). Un nuevo modelo explica cómo los iones afectan la anisotropía dieléctrica y la viscosidad rotacional, validado por experimentos con CL dopados con grafeno.

Palabras clave:
cristales líquidosimpurezas iónicasanisotropía dieléctricaviscosidad rotacionalcribado iónicofísica de materia blandaciencia de materialesquímica física

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Área de la Ciencia:

  • Física de materia blanda
  • Ciencia de materiales
  • Química física

Sus antecedentes:

  • Las impurezas iónicas impactan las propiedades de los cristales líquidos (CL) nemáticos, pero falta una comprensión cuantitativa.
  • Los modelos existentes no capturan completamente la compleja interacción entre los iones y los parámetros del material de los CL.

Objetivo del estudio:

  • Desarrollar y validar un marco teórico predictivo para los efectos iónicos en CL nemáticos.
  • Vincular cuantitativamente el cribado iónico con el comportamiento reo-óptico macroscópico.

Principales métodos:

  • Se integró la autoenergía culómbica iónica en el formalismo de Landau-de Gennes.
  • Se desarrolló un modelo para el efecto del arrastre iónico electrostático en la viscosidad rotacional.
  • Se verificaron experimentalmente los modelos utilizando CL nemáticos de doble frecuencia dopados con grafeno.
  • Se realizaron mediciones dinámicas de conmutación óptica.

Principales resultados:

  • El marco teórico predice con precisión la supresión no lineal de la anisotropía dieléctrica por iones libres.
  • El modelo de arrastre iónico explica con éxito el aumento de la viscosidad rotacional.
  • Los resultados experimentales muestran una fuerte concordancia con las predicciones teóricas en varios regímenes dieléctricos.
  • Las mediciones de conmutación óptica confirman los hallazgos sobre la viscosidad rotacional.

Conclusiones:

  • Se estableció un vínculo autoconsistente entre el cribado iónico microscópico y el comportamiento reo-óptico macroscópico en cristales líquidos.
  • Se avanzó en la comprensión fundamental de la electrostática relacionada con los iones en fluidos complejos.
  • Se proporcionó una herramienta predictiva para diseñar materiales de CL con propiedades a medida.