Oxygen molecular ionization energies

The highest occupied molecular orbital (HOMO) in formaldehyde and heteroaldehydes, H2C=E, is the lone pair at E (nE), and the second highest MO (SOMO) is the C=E 77-bonding orbital. The LUMO is the 77 CE orbital composed of the antibonding combination of pz(C) and pz(E). The ionization energy of the HOMO in formaldehyde is 10.88 eV and of the SOMO 14.5 eV, as determined by photoelectron spectroscopy.33 The ionization energy of the HOMO and the SOMO both decrease considerably when the oxygen atom in formaldehyde is replaced by sulfur or selenium (see Fig. 1, data are compiled from Refs. 33-37). 

He(I) photoelectron spectra of lanthanum tris-yS -diketonate complexes have been studied and the spectra interpreted on the basis of quantum chemical calculations of ground and ionized states by X -DV and ab initio methods [54]. The comparative analysis of the photoelectron spectra of Sc, and Lu tris-acetylacetonates showed that substitution of one oxygen atom by NH group shifts the two pairs of n and n orbitals to a different extent. The IE (ionization energy) values for (e) and 7T3 (ai) orbitals decreased by 0.8-1.0 eV. The n and n+ molecular orbital two pair bands showed a weak shift of 0.1 eV or less. 

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