Quantitative Resonance Theory Based on the Clar Sextet Model.

Area of Science:

• Theoretical Chemistry
• Organic Chemistry
• Computational Chemistry

Background:

• The Clar sextet rule is a qualitative model for polycyclic aromatic hydrocarbon (PAH) chemistry.
• This rule has limitations and exceptions in certain cases.
• A quantitative approach is needed to fully understand chemical resonance in PAHs.

Purpose of the Study:

• To develop a quantitative theory of chemical resonance based on Clar-type structures.
• To create a computational tool for analyzing resonance in PAHs.
• To propose extended Clar rules and a unified quantitative model for PAH aromaticity.

Main Methods:

• Development of a quantitative theory using semilocalized Clar resonators.
• Wave function expansion based on DFT or Hartree-Fock calculations.
• Implementation in the open-source Python code EzReson for large-scale analysis.

Main Results:

• Quantitative measures (weights and energies) for Clar resonators were established.
• Resonator weight increases exponentially with sextet number; energy decreases linearly.
• Over a thousand PAH molecules were analyzed, confirming the general validity of the Clar rule.

Conclusions:

• The study proposes three extended Clar rules and a unified quantitative model.
• Minor Clar resonators play a crucial, previously overlooked role in resonance stabilization and ring aromaticity.
• The developed theory provides a robust framework for understanding PAH chemistry.