Correlation vs. exchange competition drive the singlet-triplet excited-state inversion in non-alternant hydrocarbons

Abstract

In this work, we focus on the understanding of the driving force behind the S1 - T1 excited-state energy inversion (which would thus violate the Hund’s rule, situating the S1 state lower in energy than the T1 state) of two non-benzenoid non-alternant hydrocarbons, composed of odd-membered rings. The molecules considered here have identical chemical composition but different atomic configuration in space. The delicate interplay between structural and electronic factors that might induce the inversion and its energy extension, only by few meV, are systematically investigated here by state-of-the-art calculations. Qualitative and quantitative accurate predictions are obtained employing post-HF methods, thanks to the balanced and careful inclusion of electron correlation effects. The obtained results might guide and rationalize new searches of molecules violating Hund’s rule, concomitantly advising the importance of key contributions from the theoretical method of choice.