A new yardstick for benzenoid polycyclic aromatic hydrocarbons

Area of Science:

• Chemical Graph Theory
• Computational Chemistry
• Organic Chemistry

Background:

• Benzenoids are polycyclic aromatic hydrocarbons with unique pi-electron systems.
• Characterizing and ordering benzenoid isomers is crucial for understanding their properties.

Purpose of the Study:

• Introduce a novel benzenoid signature (S1-S6) for pi-electron partitioning.
• Develop a new ordering criterion for benzenoid isomers based on this signature.

Main Methods:

• Defined a signature of six real numbers (S1-S6) representing pi-electron distribution in benzenoid rings.
• Investigated linear correlations between sums of signature terms (e.g., S6+S5, S4+S3) for isomeric benzenoids.
• Explored other partial sums (S6+S5+S3, S6+S2+S1) for complexity and ordering.

Main Results:

• An almost perfect linear correlation exists between S6+S5 and S4+S3 for isomeric cata- or peri-fused benzenoids.
• The proposed ordering based on signature sums aligns well with ordering by Clar sextets and Kekule structures.
• Branched cata-condensed benzenoids exhibit higher S6+S5 sums than nonbranched isomers.
• For nonisomeric peri-condensed benzenoids, sums increase with ring count and decrease with internal carbon atoms.
• The S6+S2+S1 sum shows general applicability for benzenoid complexity and isomer ordering.

Conclusions:

• The benzenoid signature offers a versatile tool for characterizing pi-electron distribution.
• Signature-based sums provide an effective and generalizable method for ordering benzenoid isomers.
• This approach enhances the understanding of structure-property relationships in polycyclic aromatic hydrocarbons.