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