Influence of pseudo-Jahn-Teller activity on the singlet-triplet gap of azaphenalenes.

We analyze the possibility of symmetry-lowering induced by pseudo-Jahn-Teller interactions in six previously studied azaphenalenes that are known to have their first excited singlet state (S) lower in energy than the triplet state (T). The primary aim of this study is to explore whether Hund's rule violation is observed in these molecules when their structures are distorted from or point group symmetries by vibronic coupling. Along two interatomic distances connecting these point groups to their subgroups or , we relaxed the other internal degrees of freedom and calculated two-dimensional potential energy subsurfaces.

Resilience of Hund's rule in the chemical space of small organic molecules.

We embark on a quest to identify small molecules in the chemical space that can potentially violate Hund's rule. Utilizing twelve TDDFT approximations and the ADC(2) many-body method, we report the energies of S and T excited states of 12 880 closed-shell organic molecules within the bigQM7ω dataset with up to 7 CONF atoms. In this comprehensive dataset, none of the molecules, in their minimum energy geometry, exhibit a negative S-T energy gap at the ADC(2) level while several molecules display values <0.