Ab initio study of Σ and Π electronic states of carbonyl radical cations

The Σ and Π states of radical cations of formaldehyde (1), acetaldehyde (2), s-trans-acrolein (3), s-cis-acrolein (4) and cyclopentadienone (5) were examined by ab initio calculations with the MIDI-4 basis set. It was found that the Σ state is more stable than the Π state for the cation radicals 1–4, and the breakdown of Koopmans' theorem for the ionization of acrolein was confirmed.