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This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
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Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of conjugated systems can be understood from their π molecular orbitals.
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The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the radicals. Examples of such electronically-stabilized radicals are triphenylmethyl, tetramethylpiperidine‐N‐oxide, and 2,2‐diphenyl‐1‐picrylhydrazyl. These radicals are remarkably stable and are known as persistent radicals. Some of the persistent radicals can even be isolated and purified.
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Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
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A Stable [4,3]Peri-acene Diradicaloid: Synthesis, Structure, and Electronic Properties.

Jun-Jian Shen1, Yi Han1, Shaoqiang Dong1

  • 1Department of Chemistry, National University of Singapore, 3 Science drive 3, 117543, Singapore, Singapore.

Angewandte Chemie (International Ed. in English)
|November 9, 2020
PubMed
Summary

Researchers synthesized a stable derivative of [4,3]peri-acene, a molecule with high diradical character. This breakthrough in synthesizing complex peri-acenes offers insights into their electronic properties and stability.

Keywords:
aromaticitydiradicaloidsnanographenesperi-aceneszigzag edges

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Area of Science:

  • Organic Chemistry
  • Materials Science
  • Physical Chemistry

Background:

  • Peri-fused acenes (peri-acenes) are challenging to synthesize due to their inherent open-shell diradical character.
  • Large peri-acenes, like [4,3]peri-acene, exhibit significant diradical character (y₀ = 94.8%) due to aromatic stabilization.

Purpose of the Study:

  • To report the isolation and characterization of a stable derivative of [4,3]peri-acene.
  • To investigate the electronic and structural properties of this novel peri-acene derivative.

Main Methods:

  • Synthesis and isolation of a [4,3]peri-acene derivative (4) in crystalline form.
  • X-ray crystallographic analysis to determine molecular structure.
  • SQUID measurements and electrochemical analysis to study electronic properties.

Main Results:

  • A stable crystalline derivative (4) of [4,3]peri-acene was successfully isolated.
  • Structural analysis revealed eight localized Clar's sextets, indicating aromaticity.
  • The derivative exhibited an open-shell singlet ground state with a narrow electrochemical energy gap (1.13 eV) and a small singlet-triplet energy gap (-0.57 kcal/mol).
  • The compound showed reasonable stability with a half-life of approximately 157 hours under ambient conditions.

Conclusions:

  • The kinetic blocking strategy enabled the isolation of a stable peri-acene derivative.
  • The study provides valuable insights into the electronic structure and stability of complex peri-acenes.
  • This work paves the way for further exploration of peri-acene chemistry and applications.