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Thermal Sigmatropic Reactions: Overview01:16

Thermal Sigmatropic Reactions: Overview

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Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in...
2.0K
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.0K
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
2.0K
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

1.8K
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
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Atomic Nuclei: Nuclear Relaxation Processes01:23

Atomic Nuclei: Nuclear Relaxation Processes

598
In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
598
[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement01:21

[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement

2.6K
The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
2.6K
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

795
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...
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Video Experimental Relacionado

Updated: May 23, 2025

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets
06:26

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

Published on: May 15, 2017

7.1K

La aromatización de estado excitado impulsa la dinámica de planarización sin equilibrio

Yusuke Yoneda1,2, Tomoaki Konishi3, Kensuke Suga3,4

  • 1Research Center of Integrative Molecular Systems (CIMoS), Institute for Molecular Science, National Institutes of Natural Sciences, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan.

Journal of the American Chemical Society
|March 10, 2025
PubMed
Resumen

La aromaticidad en estado excitado impulsa cambios conformacionales en sistemas cíclicos. Este estudio revela que los cambios en la estructura electrónica preceden a la planarización estructural en los derivados de ciclooctatetraeno, lo que aclara la dinámica fotoinducida.

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Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex
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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals
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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals
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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

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Área de la Ciencia:

  • La fotoquímica
  • Química Física
  • Espectroscopia

Sus antecedentes:

  • La aromaticidad en estado excitado predice cambios conformacionales fotoinducidos en sistemas cíclicos π conjugados.
  • La relación precisa entre las dinámicas electrónicas y estructurales durante estos cambios sigue siendo poco conocida.

Objetivo del estudio:

  • Para investigar la dinámica de planarización de no equilibrio de un derivado del ciclooctatetraeno (COT).
  • Para aclarar la secuencia temporal de los cambios en la estructura electrónica y los cambios conformacionales tras la fotoexcitación.

Principales métodos:

  • Se utilizó la espectroscopia de absorción transitorio de femtosegundos.
  • Se empleó la espectroscopia Raman de dominio de tiempo con resolución de tiempo con etiquetado 13C.

Principales resultados:

  • Se observó un cambio continuo de pico en los datos de Raman resueltos en el tiempo, lo que indica una transición estructural de curvado a plano.
  • Demostró que la modificación de la estructura electrónica precede a la planarización observada.
  • Identificó sin ambigüedad el cambio de banda marcador con etiquetado 13C.

Conclusiones:

  • La fotoexcitación induce aromaticidad en el derivado COT, seguido de un cambio conformacional a la planarización.
  • Proporciona una perspectiva dinámica sobre la aromaticidad en estado excitado y la evolución estructural fotoinducida.