Electronic structure of dichalcogenide and dipnictide anions

The Self-Consistent-Field Xα Scattered Wave Cluster MO method is used to calculate molecular orbital energy level diagrams for O₂, O₂^?, O₂^2?, S₂, S₂^2?, Se₂^2?, P₂^4?, As₂^4? and AsS^3?. Calculated energies are in good agreement with those measured by photoemission spectroscopy for O₂, S₂, S₂^2? in ZrS₃, Se₂^2? in ZrSe₃ and P₂^4? in TiP₂. For O₂^? the calculated energy of the π_u → π_g transition agrees fairly well with that of the UV absorption observed in LiO₂. The predicted photoemission spectrum of O₂^2? is consistent with a preliminary study on BaO₂. Calculated valence region widths in peroxides are found to be significantly greater than the calculated oxygen atomic valence _s-p separation while valence widths for the other anions closely match atomic valence s-p splittings. This effect arises from the instability of the peroxide antibonding orbitals which correlates with the absence of dsun peroxides and Superoxides. Calculations on a Cin2v geometry FeO₆su9? cluster show that the difference in energy of O2p nonbonding and Fe3d crystal field type orbitals drops sharply as an O-O edge is contracted, providing a mechanism for Fe³⁺ → Fe²⁺ reduction at high pressure.