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Videos de Conceptos Relacionados

¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Two-Dimensional (2D) NMR: Overview01:12

Two-Dimensional (2D) NMR: Overview

The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse.
¹H NMR Signal Multiplicity: Splitting Patterns01:13

¹H NMR Signal Multiplicity: Splitting Patterns

When protons A and X are coupled, their nuclear spin energy levels are slightly modified. This is because the energy required to excite proton A to a spin state parallel to proton X is slightly different from the energy required for it to become anti-parallel to spin X. Consequently, there are two possible excitation frequencies for A (A1 and A2), depending on the spin state of X, and vice versa. The mutual nature of coupling implies that the difference between frequencies A1 and A2, indicated...
Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule01:10

Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule

In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal (1:1) intensity. When proton A is coupled to two equivalent protons (AX2 spin system), the spin states of each X can be aligned with or against the external field, creating three possible scenarios. This results in a 1:2:1  triplet signal, where the central peak corresponds to the chemical shift of A and is twice as large or intense as the others.
¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
¹³C NMR: ¹H–¹³C Decoupling01:04

¹³C NMR: ¹H–¹³C Decoupling

The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...

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Nuclear Magnetic Resonance Spectroscopy for the Identification of Multiple Phosphorylations of Intrinsically Disordered Proteins
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Nuclear Magnetic Resonance Spectroscopy for the Identification of Multiple Phosphorylations of Intrinsically Disordered Proteins

Published on: December 27, 2016

Espectroscopia de RMN de campo local separado en núcleos cuádrupolares de medio entero.

Julia Grinshtein1, Christopher V Grant, Lucio Frydman

  • 1Department of Chemical Physics, Weizmann Institute of Sciences, 76100 Rehovot, Israel.

Journal of the American Chemical Society
|November 7, 2002
PubMed
Resumen

Nuevos métodos de RMN de estado sólido que utilizan espectroscopia de campo local separado caracterizan núcleos cuádrupolares. Estas técnicas correlacionan patrones espectrales para asignar resonancias y estudiar la dinámica de los sólidos.

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Área de la Ciencia:

  • Espectroscopia de Resonancia Magnética Nuclear (RMN) de estado sólido Espectroscopia de Resonancia Magnética Nuclear (RMN) de estado sólido Espectroscopia de estado sólido
  • Ciencia de los materiales Ciencia de los materiales.
  • Química Física es la química física.

Sus antecedentes:

  • La caracterización de las resonancias en la espectroscopia de RMN de estado sólido de núcleos cuádrupolares medio enteros presenta desafíos.
  • Es posible que los métodos existentes no resuelvan completamente los sitios o dinámicas no equivalentes en materiales sólidos complejos.

Objetivo del estudio:

  • Explorar nuevos enfoques para la caracterización de las resonancias en RMN de estado sólido de núcleos cuádrupolares medio enteros.
  • Desarrollar técnicas avanzadas de RMN para asignar resonancias a entornos estructurales específicos e investigar la dinámica molecular.

Principales métodos:

  • Adquisición de espectros de campos locales separados heteronucleares en sólidos giratorios.
  • Desarrollo de experimentos bidimensionales (2D) que correlacionan patrones de polvo MAS cuadrupolar de segundo orden con patrones de banda lateral MAS dipolar.
  • Extensión a secuencias de RMN tridimensionales (3D) para separar sitios químicos no equivalentes a lo largo de una dimensión isotrópica.

Principales resultados:

  • Demostró la correlación de patrones de polvo MAS cuadrupolar con patrones de banda lateral MAS dipolar para sitios químicos específicos.
  • Separó con éxito resonancias no equivalentes utilizando secuencias de RMN 3D basadas en espectros de correlación anisotrópica.
  • Se ilustró la aplicación de estos métodos con experimentos de recopilación de 1H-23Na en mononucleótidos.

Conclusiones:

  • Los enfoques de RMN de campo local separado extendido se acercan a los núcleos cuádrupolares, lo que facilita la asignación de resonancia a entornos estructurales específicos.
  • Proporcionó nuevas herramientas para la investigación de la dinámica de los materiales en estado sólido.
  • Validar la utilidad de estos experimentos de RMN 2D y 3D a través de aplicaciones prácticas.