Why is the Energy of the Singly Occupied Orbital in Some Radicals below the Highest Occupied Orbital Energy

Organic (mono)radicals where the singly occupied molecular orbital (SOMO) is energetically below the highest occupied molecular orbital (HOMO) level have recently attracted much interest. A clear understanding of the electronic factors that lead to this energetic SOMO/HOMO inversion (SHI) would be desirable to aid the rational design of SHI radicals with high stability and other desired properties. The electronic factors that govern SHI in known SHI radicals are studied computationally. Then, the findings are applied to design potential SHI candidates 'in silico'. The electrostatic repulsion among the frontier orbitals and the repulsion between the alpha and beta spin components of the orbitals in a closed-shell 'parent' compound are key to understanding the occurrence of SHI in a radical.