Exchange Rules for Diradical π-Conjugated Hydrocarbons

Area of Science:

• Quantum Chemistry
• Materials Science
• Organic Chemistry

Background:

• Planar π-conjugated hydrocarbons like triangulene possess degenerate molecular orbitals.
• Hund's rules often predict ground state spin (S=1), but this is not universally accurate.

Purpose of the Study:

• To develop a comprehensive set of rules for predicting zero mode states.
• To accurately determine spin multiplicity and open-/closed-shell nature for ground and excited states in these systems.

Main Methods:

• Analytical arguments combined with configuration interaction calculations.
• Utilized a Hubbard model supported by quantum chemistry methods and large Gaussian basis sets.

Main Results:

• Established novel rules that correctly predict zero mode states.
• Accurately determined spin multiplicity and shell nature for both ground and excited states, surpassing existing theorems.

Conclusions:

• The developed rules provide a more accurate framework for understanding electronic states in planar π-conjugated hydrocarbons.
• This work extends beyond Lieb's theorem and Ovchinnikov's rule, offering deeper insights into molecular electronic structures.