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How aromatic are large (4n + 2)pi annulenes?

Chaitanya S Wannere1, Paul von Ragué Schleyer

  • 1Computational Chemistry Annex, Department of Chemistry, University of Georgia, Athens, Georgia 30602-2525, USA.

Organic Letters
|March 14, 2003
PubMed
Summary

Aromatic stabilization energies per pi-electron decrease in larger annulenes, despite total energy convergence. Magnetic properties indicate diminishing aromaticity in these larger systems.

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Area of Science:

  • Organic Chemistry
  • Theoretical Chemistry
  • Computational Chemistry

Background:

  • Annulenes are cyclic hydrocarbons with alternating single and double bonds.
  • Aromaticity is a key concept in understanding the stability and reactivity of cyclic conjugated systems.
  • Previous studies have explored the aromaticity of smaller annulenes, but larger systems present unique challenges.

Purpose of the Study:

  • To investigate the trend of aromatic stabilization energy (ASE) per pi-electron in [n]annulenes from C(6)H(6) to C(66)H(66).
  • To evaluate the impact of bond length alternation on both stabilization energies and magnetic properties.
  • To assess the aromaticity of larger annulenes using various magnetic criteria.

Main Methods:

  • Quantum chemical calculations were employed to determine the structures and energies of [n]annulenes.
  • Aromatic stabilization energies (ASE) were calculated.
  • Bond length alternation was analyzed.
  • Magnetic properties, including Nuclear-Independent Chemical Shifts (NICS), proton chemical shifts, and magnetic susceptibilities, were computed.

Main Results:

  • Total ASE for [n]annulenes up to C(66)H(66) converge around 22 kcal/mol.
  • ASE per pi-electron shows a significant decrease with increasing annulene size.
  • Bond length alternation has a moderate effect on stabilization but a considerable influence on magnetic properties.
  • Magnetic criteria, when applied to optimized structures, consistently show decreasing aromaticity in larger annulenes.

Conclusions:

  • The aromaticity of larger annulenes diminishes significantly as the ring size increases.
  • While total stabilization energy remains relatively constant, the energetic contribution per pi-electron decreases.
  • Magnetic properties serve as reliable indicators of aromaticity, revealing the loss of aromatic character in extended annulene systems.

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