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Related Concept Videos

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

2.0K
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...
2.0K
Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

4.3K
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...
4.3K
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

4.2K
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.
4.2K
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

4.3K
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...
4.3K
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

14.8K
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 Hückel’s rule or the 4n +...
14.8K
NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

6.7K
Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
6.7K

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Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles
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An electronic aromaticity index for large rings.

Eduard Matito1

  • 1Faculty of Chemistry, University of the Basque Country UPV/EHU, and Donostia International Physics Center (DIPC), P.K. 1072, 20080 Donostia, Euskadi, Spain. ematito@gmail.com and IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain and Instituto de Química at the Universidad Autónoma de México (UNAM), Mexico.

Physical Chemistry Chemical Physics : PCCP
|February 16, 2016
PubMed
Summary

We developed a new electronic aromaticity index, AV1245, to measure electron delocalization in molecules. This index accurately assesses aromaticity in large ring systems, offering a reliable and efficient computational tool for chemists.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Organic Chemistry

Background:

  • Aromaticity is a key concept in chemistry, influencing molecular stability and reactivity.
  • Existing aromaticity indices have limitations regarding applicability and precision.
  • Accurate assessment of aromaticity in large and complex ring systems remains challenging.

Purpose of the Study:

  • Introduce a novel electronic aromaticity index, AV1245.
  • Develop a new algorithm for calculating multicenter indices (MCI) in large rings.
  • Validate the AV1245 index for diverse molecular structures, including large rings.

Main Methods:

  • The AV1245 index is based on the average of 4-center multicenter indices (MCI) with a 1,2,4,5 positional relationship.
  • A new algorithm was developed for efficient MCI calculation in large molecular rings.
  • The index was calibrated using computational data and tested on various systems, including porphyrins.

Main Results:

  • AV1245 effectively quantifies electron delocalization and aromaticity.
  • The index demonstrates size-extensivity and low computational cost, scaling linearly with ring size.
  • AV1245 shows no limitations regarding atomic composition, molecular geometry, or calculation level.
  • Successful application to large systems like octaphyrins demonstrates its utility.

Conclusions:

  • AV1245 provides a robust, versatile, and computationally efficient measure of electronic aromaticity.
  • The index is particularly suitable for studying large molecular rings and complex conjugated systems.
  • AV1245 overcomes limitations of previous aromaticity indices, offering improved accuracy and applicability.