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Related Concept Videos

Aryldiazonium Salts to Azo Dyes: Diazo Coupling01:11

Aryldiazonium Salts to Azo Dyes: Diazo Coupling

The reaction of weakly electrophilic aryldiazonium (also called arenediazonium) salts with highly activated aromatic compounds leads to the formation of products with an —N=N— link, called an azo linkage. This reaction, presented in Figure 1, is known as diazo coupling and occurs without the loss of the nitrogen atoms of the aryldiazonium salt. Highly activated aromatic compounds such as phenols or arylamines favor the diazo coupling reaction. The coupling generally occurs at the para position.
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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
Structure of Benzene: Molecular Orbital Model01:18

Structure of Benzene: Molecular Orbital Model

According to the molecular orbital (MO) model, benzene has a planar structure with a regular hexagon of six sp2 hybridized carbons. As shown in Figure 1, each carbon is bonded to three other atoms with C–C–C and H–C–C bond angles of 120°. The C–H bond length is 109 pm, and the C–C bond length is 139 pm which is midway between the single bond length of sp3 hybridized carbons (154 pm) and sp2 hybridized carbons (133 pm).
Nucleophilic Aromatic Substitution: Elimination–Addition01:11

Nucleophilic Aromatic Substitution: Elimination–Addition

Simple aryl halides do not react with nucleophiles. However, nucleophilic aromatic substitutions can be forced under certain conditions, such as high temperatures or strong bases. The mechanism of substitution under such conditions involves the highly unstable and reactive benzyne intermediate. Benzyne contains equivalent carbon centers at both ends of the triple bond, each of which is equally susceptible to nucleophilic attack. This 50–50 distribution of products is confirmed through isotopic...
Reactions at the Benzylic Position: Oxidation and Reduction00:59

Reactions at the Benzylic Position: Oxidation and Reduction

The benzylic position describes the position of a carbon atom attached directly to a benzene ring. Benzene by itself does not undergo oxidation. In contrast, the benzylic carbon is quite reactive in the presence of strong oxidizing agents such as KMnO4 or H2CrO4. Therefore, alkylbenzenes are readily oxidized to benzoic acid, irrespective of the type of alkyl groups.
Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene01:17

Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene

The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule.

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Related Experiment Video

Updated: Jun 20, 2026

Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles
11:45

Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles

Published on: August 22, 2018

Electron attachment to trans-azobenzene.

Alberto Modelli1, Paul D Burrow

  • 1Dipartimento di Chimica G. Ciamician, Università di Bologna, via Selmi 2, 40126 Bologna, Italy.

Physical Chemistry Chemical Physics : PCCP
|September 24, 2009
PubMed
Summary

Electron transmission spectroscopy reveals temporary anion states in trans-azobenzene. These states, formed by electron capture into pi* orbitals, lead to long-lived molecular anions, ruling out direct core-excited anion formation.

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Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions
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Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions

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Area of Science:

  • Physical Chemistry
  • Molecular Spectroscopy
  • Quantum Chemistry

Background:

  • Trans-azobenzene is a molecule with potential applications in materials science and photochemistry.
  • Understanding its electronic structure and anion states is crucial for predicting its behavior.

Purpose of the Study:

  • To characterize the temporary anion states of gas-phase trans-azobenzene.
  • To investigate the mechanisms of molecular anion formation.

Main Methods:

  • Electron transmission spectroscopy (ETS) was used to probe anion states in the 0-6 eV range.
  • High-level computational methods (HF, MP2, B3LYP, DFT) were employed to calculate electronic structures and energies.
  • Anion currents were measured as a function of electron impact energy.

Main Results:

  • Vertical electron attachment energies were measured and compared with calculated pi* virtual orbital energies.
  • A positive vertical electron affinity of 0.83 eV was predicted.
  • Long-lived parent molecular anions were detected at 0 eV and near 1 eV.
  • The results suggest shape resonances are the initial step in anion formation.

Conclusions:

  • The study provides quantitative agreement between experimental spectral features and theoretical calculations.
  • Shape resonances, not direct core-excited anion states, are the primary pathway for forming long-lived trans-azobenzene anions.
  • DFT calculations suggest a possible conversion of shape resonances to sigma-pi* core-excited states.