Influence of Pseudo-Jahn-Teller Activity on the Singlet-Triplet Gap of Azaphenalenes

We probe the sensitivity of the singlet-triplet energy gap of selected azaphenalenes to symmetry lowering induced by Jahn-Teller interactions. While cyclazine in its characteristic D₃ₕ structure defies Hund's rule, CCSD(T)-level modeling suggests its structure corresponds to two equivalent minima of C₃ₕ symmetry undergoing rapid automerization. The combined effect of symmetry reduction and high-level corrections indicates a negligible singlet-triplet gap in cyclazine. Notably, pentazine and heptazine prefer symmetric structures exhibiting negative gaps in accord with experiments. Azaphenalenes containing nitrogen atoms at electron-deficient sites exhibit stronger in-plane structural distortion; in their low-symmetry energy minima, they adhere to Hund's rule.