Electronic States of the NiO Molecule

Generalized valence bond and configuration interaction wave functions have been obtained as a function of R for numerous electronic states of NiO. All the lower states are found to involve the (4s)¹(3d)⁹Ni atom configuration and O in the (2s)²(2p)⁴configuration. There are two groups of states. The lower group of states involves pairing singly occupied Ni(4s) and O(2pσ) orbitals into a (somewhat ionic) σ bond pair with various pairings of the Ni(3d)⁹and 0(2pπ)³configurations. This leads to a number of states, including the ground state, which we find to be X³Σ-(The electronic structure is analogous to that of O₂.) The calculated D₀and R_efor the X³Σ- state of NiO are 89.9 kcal/mol and 1.60 Å, respectively. The bond energy is in good agreement with the experimental value of 86.5 ± 5 kcal/mol, while the R_evalue is not known experimentally. The higher group of states involve a doubly occupied 0(2pσ) orbital. The Ni(4s) orbital in this case is nonbonding and builds in 4_pcharacter to move away from the oxygen orbitals. The bonding mainly involves stabilization of the oxygen orbitals by the Ni(3d)⁹core (somewhat analogously to the bonding in NiCO). Numerous allowed transitions between these states and the states of the lower group are calculated to be in the range 1.0-3.0 eV, where numerous bands are seen in emission.