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Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

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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

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

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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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Aromatic Hydrocarbon Anions: Structural Overview01:18

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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

Aromatic Hydrocarbon Cations: Structural Overview

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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...
2.8K
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

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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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Quest for Double Möbius Aromaticity.

Kangkan Sarmah1, Ankur K Guha1

  • 1Advanced Computational Chemistry Centre, Department of Chemistry, Cotton University, Panbazar, Guwahati, Assam, INDIA-, 781001.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|March 7, 2024
PubMed
Summary
This summary is machine-generated.

Researchers identified double Möbius aromaticity in a synthetic Rh2B2 cluster, overcoming challenges of rare and toxic elements. A related RhB2- cluster also exhibits Möbius aromaticity, representing the smallest known ring with this property.

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Area of Science:

  • Quantum Chemistry
  • Aromaticity Theory
  • Computational Materials Science

Background:

  • Neutral four-membered rings with 4n electrons are typically Hückel antiaromatic.
  • Möbius aromaticity in 4n electron systems is theoretically possible but experimentally elusive.
  • Previous proposals for double Möbius aromaticity involved challenging elements like Protactinium (Pa).

Purpose of the Study:

  • To computationally investigate a synthetically viable four-membered cluster exhibiting double Möbius aromaticity.
  • To explore the potential of Rhodium (Rh) and Boron (B) for creating such systems.
  • To identify the smallest possible ring system displaying Möbius aromaticity.

Main Methods:

  • Density Functional Theory (DFT) calculations were employed.
  • Analysis of electronic structure and aromaticity indices (e.g., NICS values) was performed.
  • Geometric optimization and electronic state characterization of proposed clusters.

Main Results:

  • A stable, four-membered Rh2B2 cluster was computationally designed, exhibiting double Möbius aromaticity.
  • This Rh2B2 cluster presents a synthetically accessible alternative to previous proposals.
  • A three-membered RhB2- cluster was found to possess Möbius aromaticity, the smallest reported to date.

Conclusions:

  • The Rh2B2 cluster offers a promising platform for studying double Möbius aromaticity with accessible elements.
  • The RhB2- cluster represents a significant advancement in understanding Möbius aromaticity in small ring systems.
  • These findings open new avenues for the design of novel aromatic materials.