Beschreibung:
<jats:p>Nonempirical LCAO MO SCF studies of the low lying excited states of BeF2 are reported. These results provide new insight into the differences between the valence molecular orbitals for covalent and ionic AB2-type molecules. The lowest energy states in BeF2 are predicted to result from transitions involving the highest occupied πg (b2) and πg (a2) molecular orbitals of the ground state to the lowest unoccupied σ̄g (a1) molecular orbital. This is in contrast to the case of CO2 where experiments have confirmed Walsh's prediction that the lowest energy transitions are from the πg (b2) and πg (a2) molecular orbitals to the π̄u (a1) molecular orbital. This predicted change in the ordering of the π̄u (a1) and σ̄g (a1) orbitals for BeF2 and CO2 suggests that the lowest energy transition in BeF2 should be quite weak in absorption since a g to g transition is electric dipole forbidden for the linear molecule. A modification of the Walsh type diagram for highly ionic species of the form X−δ−M+2δ−X−δ is discussed.</jats:p>