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Related Concept Videos

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 1,5-hexadiene, referred...
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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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Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

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Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei...
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Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

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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...
2.3K
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
1.6K
UV–Vis Spectroscopy: Molecular Electronic Transitions01:16

UV–Vis Spectroscopy: Molecular Electronic Transitions

3.1K
In Ultraviolet–Visible (UV–Vis) spectroscopy, the absorption of electromagnetic radiation is used to probe the electronic structure of molecules. This technique provides insights into molecular electronic transitions, particularly the movement of electrons between different molecular orbitals. Radiation is absorbed if the energy of the electromagnetic radiation passing through the molecule is precisely equal to the energy difference between the excited and ground states. During this...
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Related Experiment Video

Updated: Feb 24, 2026

Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
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Sigmatropic proton shifts: a quantum chemical study.

Yi Wang1, Zhi-Xiang Yu

  • 1Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China. yuzx@pku.edu.cn.

Organic & Biomolecular Chemistry
|August 24, 2017
PubMed
Summary

Woodward-Hoffmann rules govern [1,j] sigmatropic proton shifts in polyenyl anions. Only the [1,6] proton shift is facile intramolecularly; others require proton shuttles for rearrangement.

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Area of Science:

  • Organic Chemistry
  • Quantum Chemistry
  • Reaction Mechanisms

Background:

  • Sigmatropic rearrangements are fundamental organic reactions.
  • Understanding proton shifts in conjugated systems is crucial for reaction design.

Purpose of the Study:

  • To investigate [1,j] sigmatropic proton shifts in polyenyl anions using quantum chemical methods.
  • To elucidate the stereochemical outcomes and energetic barriers of these reactions.

Main Methods:

  • Quantum chemical calculations were employed.
  • Analysis of transition structures and activation energies was performed.

Main Results:

  • The Woodward-Hoffmann rules accurately predict the stereochemistry of [1,j] proton shifts.
  • Activation barriers vary significantly with 'j', with [1,6] being the lowest.
  • Intramolecular [1,6] proton shifts are feasible under mild conditions.

Conclusions:

  • Polyenyl anion [1,j] proton shifts follow predictable stereochemical pathways based on 'j'.
  • Proton shuttles are necessary for most [1,j] shifts due to high activation barriers.
  • The [1,6] shift represents a unique, facile intramolecular rearrangement.