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

Criteria for Aromaticity and the Hückel 4n + 2 Rule

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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...
10.4K
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

2.8K
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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ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3

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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
5.9K
NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

4.6K
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.
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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Excited-state aromaticity reversals in norcorrole.

Peter B Karadakov1, Edward Cummings1

  • 1Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK. peter.karadakov@york.ac.uk.

Physical Chemistry Chemical Physics : PCCP
|September 25, 2024
PubMed
Summary

Aromaticity reversals occur in Ni(II) norcorrole and norcorrole upon electronic excitation. Antiaromatic ground states become aromatic in excited states, forming 24 π-electron systems.

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Organic Chemistry

Background:

  • Norcorrole molecules, including Ni(II) norcorrole (NiNc) and norcorrole (H2Nc), exhibit antiaromatic characteristics in their ground electronic state (S0).
  • Understanding the electronic behavior of these macrocycles upon excitation is crucial for their potential applications.

Purpose of the Study:

  • To investigate aromaticity reversals between the ground (S0) and excited (T1 and S1) states of NiNc and H2Nc.
  • To analyze the changes in aromaticity metrics and molecular geometry upon electronic excitation.

Main Methods:

  • Calculated harmonic oscillator model of aromaticity (HOMA) values at optimized S0, T1, and S1 geometries.
  • Analyzed nucleus-independent chemical shift (NICS) values and isotropic magnetic shielding distributions between S0 and T1 states.

Main Results:

  • Aromaticity reversals were observed: antiaromatic S0 states transform into aromatic T1 and S1 states.
  • Excited states exhibit Baird aromaticity with 24 π electrons.
  • The geometries of the aromatic T1 and S1 states are more non-planar (larger bowl depths) than the antiaromatic S0 states.

Conclusions:

  • NiNc and H2Nc undergo significant aromaticity changes upon electronic excitation.
  • The study reveals a novel pathway to Baird aromaticity in these systems.
  • The interplay between electronic state and molecular geometry influences aromaticity in norcorroles.