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Videos de Conceptos Relacionados

Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

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In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
678
NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

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The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
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Molecular and Ionic Solids02:54

Molecular and Ionic Solids

17.3K
Crystalline solids are divided into four types: molecular, ionic, metallic, and covalent network based on the type of constituent units and their interparticle interactions.
Molecular Solids
Molecular crystalline solids, such as ice, sucrose (table sugar), and iodine, are solids that are composed of neutral molecules as their constituent units. These molecules are held together by weak intermolecular forces such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds, which...
17.3K
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

1.2K
In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...
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π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

1.1K
An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0,...
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Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

1.2K
The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...
1.2K

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Author Spotlight: Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks
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Mapas de interacción dependientes del desplazamiento químico en sólidos moleculares

Manuel Cordova1,2, Lyndon Emsley1,2

  • 1Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.

Journal of the American Chemical Society
|July 13, 2023
PubMed
Resumen
Este resumen es generado por máquina.

Este estudio presenta mapas de interacción 3D derivados de la estructura molecular y los cambios químicos experimentales. Estos mapas aceleran la predicción de la estructura cristalina (CSP) y evalúan las estructuras candidatas sin cálculos DFT intensivos desde el punto de vista computacional.

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Área de la Ciencia:

  • Química del estado sólido
  • La cristalografía
  • Química computacional

Sus antecedentes:

  • La cristalografía RMN es clave para determinar las estructuras sólidas moleculares.
  • Los protocolos de predicción de la estructura cristalina (CSP) son computacionalmente intensivos, lo que dificulta la exploración polimórfica.
  • La generación de estructuras cristalinas candidatas es un cuello de botella en CSP.

Objetivo del estudio:

  • Desarrollar un método para acelerar los protocolos de predicción de la estructura cristalina (CSP).
  • Evaluar la probabilidad de estructuras cristalinas candidatas de manera eficiente.
  • Para reducir el costo computacional asociado con CSP.

Principales métodos:

  • Construyendo mapas tridimensionales de interacción de una base de datos de estructuras cristalinas y cambios químicos experimentales.
  • Derivar mapas directamente de la estructura molecular y los cambios químicos experimentales asociados.
  • Utilizando estos mapas para identificar las limitaciones estructurales para la CSP.

Principales resultados:

  • Se ha demostrado la capacidad de identificar las limitaciones estructurales para acelerar los protocolos CSP.
  • Mostró un método para evaluar las estructuras cristalinas candidatas sin cálculos DFT.
  • Desarrolló mapas de interacción 3D como una herramienta novedosa para la determinación de la estructura en estado sólido.

Conclusiones:

  • Los mapas de interacción 3D ofrecen un enfoque computacionalmente eficiente para acelerar el CSP.
  • El método desarrollado reduce el cuello de botella en la generación de estructuras cristalinas candidatas.
  • Esta técnica proporciona una alternativa viable para evaluar los candidatos a la estructura cristalina.