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Local aromaticities in large polyacene molecules.

Jun-ichi Aihara1, Hideaki Kanno

  • 1Department of Chemistry, Faculty of Science, Shizuoka University, Oya, Shizuoka 422-8529, Japan.

The Journal of Physical Chemistry. A
|July 15, 2006
PubMed
Summary

Controversial aromaticity in polyacenes is resolved: inner rings are not more aromatic than outer ones. Bond resonance energies show outer rings possess higher aromaticity than inner rings in large polyacene molecules.

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Area of Science:

  • Organic Chemistry
  • Theoretical Chemistry
  • Computational Chemistry

Background:

  • The relative aromaticity of individual rings within large polyacene molecules remains a debated topic.
  • Nucleus-independent chemical shift (NICS) values have suggested that inner rings might exhibit higher aromaticity than outer rings, potentially exceeding that of benzene.

Purpose of the Study:

  • To investigate and clarify the relative aromaticity of rings in linear polyacene molecules.
  • To determine whether inner rings of large polyacenes are indeed more aromatic than outer rings.

Main Methods:

  • Evaluation of bond resonance energies (BREs) for a series of linear polyacenes.
  • Calculation of hypothetical geometry-independent pi-electron currents.
  • Correlation analysis between global HOMA (harmonic oscillator model of aromaticity) values and percentage topological resonance energies (% TREs).

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  • Assessment of the relationship between magnetic properties (NICS, ring-current intensity) and molecular geometry.

    • Main Results:

    • For large polyacene molecules, inner rings were consistently found to be less aromatic than outer rings.
    • Global HOMA values showed a strong correlation with percentage topological resonance energies (% TREs).
    • Average NICS values did not correlate well with global HOMA or % TREs.
    • Magnetic properties like NICS and ring-current intensity are significantly influenced by molecular geometry.

    Conclusions:

    • The study concludes that inner rings of large polyacenes are not more aromatic than outer rings.
    • Aromaticity assessment should carefully consider molecular geometry, as magnetic properties can be misleading.
    • Bond resonance energies provide a more reliable measure of aromaticity in these systems compared to NICS values.