Capturing the elusive aromaticity of bicalicene

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Organic Chemistry

Background:

• Aromaticity is a key concept in chemistry, influencing molecular stability and reactivity.
• Bicalicene, a non-benzenoid hydrocarbon, presents a unique case for studying aromaticity due to its structure.
• Understanding the electronic properties of bicalicene is crucial for its potential applications.

Purpose of the Study:

• To investigate the origin of ring-current aromaticity in the bicalicene molecule.
• To analyze the electronic circulation patterns and their contribution to aromaticity.
• To explain the limitations of traditional models in describing bicalicene's aromaticity.

Main Methods:

• Computation of current-density maps using ipsocentric RHF/6-311G** and DFT/6-311G** levels of theory.
• Analysis of conjugated-circuit models and valence-bond theory.
• Canonical molecular-orbital analysis, focusing on frontier-orbital contributions.

Main Results:

• Strong local diatropic circulations on the pentagonal rings were identified as the source of bicalicene's aromaticity.
• Conjugated-circuit models failed to capture the observed circulation pattern due to significant 'ionic' contributions.
• Cancellation of paratropic and diatropic frontier-orbital contributions was observed, explaining difficulties with Hückel-based models.

Conclusions:

• Bicalicene's aromaticity is primarily driven by local circulations and ionic contributions, deviating from simple conjugated models.
• The study highlights the inadequacy of Hückel-based models for accurately predicting current-density maps in such systems.
• Alternative measures of aromaticity confirm the dominant 'tetraionic' contribution to bicalicene's aromatic character.