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Related Concept Videos

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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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.
Removing one hydrogen from the intervening CH2 group...
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Frost Circles for Different Conjugated Systems01:18

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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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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?  
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as...
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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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Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

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Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom,...
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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Tuning (Anti)Aromaticity: Variations on the [8]-Circulene Framework.

Patrick W Fowler1, Cate S Anstöter2

  • 1Department of Chemistry, University of Sheffield, Sheffield, United Kingdom.

Chemphyschem : a European Journal of Chemical Physics and Physical Chemistry
|January 27, 2024
PubMed
Summary
This summary is machine-generated.

This study models π-electron delocalisability in [8]-circulenes using the ab initio ipsocentric method. Modifications to the framework reveal a full range of electronic current patterns, aiding in tuning aromaticity and antiaromaticity.

Keywords:
ab initio calculationsantiaromaticityaromaticitycurrent-density mapsipsocentric

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Area of Science:

  • * Quantum chemistry
  • * Organic electronics
  • * Materials science

Background:

  • * Optoelectronic properties of organic molecules depend on π-electron delocalisation.
  • * π-electron delocalisability is sensitive to changes in electron count via heteroatom substitution or redox chemistry.
  • * Induced magnetic field currents diagnose π-electron delocalisability and aromaticity.

Purpose of the Study:

  • * To model ring-current patterns in [8]-circulenes based on tetracyclopenta[def,jkl,pqr,vwx]tetraphenylene (TCPTP).
  • * To investigate the effects of charge state, heteroatom substitution, and carbon-boron-nitrogen (BN) pair substitution on π-delocalisability.
  • * To interpret current density patterns using the ipsocentric approach and develop design strategies for macrocyclic frameworks.

Main Methods:

  • * Employed the ab initio ipsocentric method for modeling.
  • * Utilized CHF/CTOCD-DZ2/6-31G** level calculations.
  • * Analyzed induced current density partitioned into orbital contributions based on symmetry, energy, and nodal character.

Main Results:

  • * Modeled diverse ring-current patterns in modified TCPTP frameworks.
  • * Demonstrated access to a full spectrum of current possibilities, from global circulation to localization.
  • * Identified selection rules for interpreting current density and guiding molecular design.

Conclusions:

  • * The ipsocentric method effectively models π-electron delocalisability in complex organic systems.
  • * Modifications to the TCPTP framework allow fine-tuning of aromatic and antiaromatic properties.
  • * Insights gained can inform the design of novel macrocyclic materials with tailored optoelectronic properties.