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Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

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Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous...
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Nucleophilic substitution in aromatic compounds is feasible in substrates bearing strong electron-withdrawing substituents positioned ortho or para to the leaving group. The reaction proceeds via two steps: the addition of the nucleophile and the elimination of the leaving group.
The reaction begins with an attack of the nucleophile on the carbon that holds the leaving group. This results in the delocalization of the π electrons over the ring carbons. The resonance interaction between...
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Aromatic Hydrocarbon Cations: Structural Overview01:18

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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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Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

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Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?  
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The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.
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π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

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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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Adaptive σ-Aromaticity in an Unsaturated Three-Membered Ring.

Yuanyuan Huang1, Chenshu Dai1, Jun Zhu1

  • 1State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China.

Chemistry, an Asian Journal
|August 29, 2020
PubMed
Summary

Adaptive aromaticity, where molecules are aromatic in both singlet (S₀) and triplet (T₁) states, is rare. This study reveals adaptive σ-aromaticity in unsaturated three-membered rings, expanding aromaticity concepts.

Keywords:
DFT calculationsadaptive aromaticityexcited-state aromaticitythree-membered ringσ-aromaticity

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Area of Science:

  • * Organic Chemistry
  • * Quantum Chemistry
  • * Theoretical Chemistry

Background:

  • * Aromaticity is typically confined to either the singlet (S₀) or triplet (T₁) electronic state, as per Hückel's and Baird's rules.
  • * Adaptive aromaticity, exhibiting aromaticity in both S₀ and T₁ states, is exceptionally rare in chemical species.
  • * σ-aromaticity in the T₁ state remains underexplored, with adaptive σ-aromaticity being even less understood.

Purpose of the Study:

  • * To investigate and demonstrate the phenomenon of adaptive σ-aromaticity.
  • * To explore adaptive σ-aromaticity within the context of unsaturated three-membered ring systems.
  • * To elucidate the electronic origins of adaptive σ-aromaticity in these systems.

Main Methods:

  • * Employed various established aromaticity indices, including Nuclear Independent Chemical Shift (NICS), Aromaticity by Charge Delocalization (ACID), and Electron-Delocalization Degree (EDDB).
  • * Analyzed the electronic structure and properties of unsaturated three-membered rings in both singlet (S₀) and triplet (T₁) states.
  • * Investigated the excitation modes leading to the T₁ state, specifically focusing on transitions from out-of-plane π molecular orbitals to π* orbitals.

Main Results:

  • * Demonstrated the presence of adaptive σ-aromaticity in an unsaturated three-membered ring system.
  • * Identified the excitation mechanism involving out-of-plane π molecular orbitals to π* orbitals as the origin of adaptive σ-aromaticity in the T₁ state.
  • * Showcased that the σ-aromaticity observed in the S₀ state is preserved in the T₁ state.

Conclusions:

  • * Extended the concept of adaptive σ-aromaticity to unsaturated three-membered rings, a traditionally π-aromaticity-dominated area.
  • * Provided a mechanistic understanding for adaptive σ-aromaticity originating from specific electronic excitations.
  • * The findings offer a foundation for further advancements in the understanding of both σ-aromaticity and adaptive aromaticity.