The electronic structure of diatomic nickel oxide†

The nature of the Ni–O bond is relevant to catalytic and environmental applications. The vibrational frequency and electronic structure of NiO were calculated using CASSCF, icMRCI+Q, CCSD(T), and DFT. CASSCF predicted a quintet state (⁵Σ⁻) ground state for the equilibrium bond distance with a state crossing at 1.65 Å, where the triplet (³Σ⁻) state becomes of lower energy. These states arise from the 3d⁸(³F)4s² (³F) and 3d⁹(²D)4s¹ (³D) configurations of Ni. The icMRCI+Q method predicts a triplet (³Σ⁻) ground state and does not predict a state crossing with the quintet. This state has significant ionic character with the 2p_z of O bonding with the 4s/3d_z² of the Ni to form a σ bond. The NiO frequency at the icMRCI+Q level of 835.0 cm⁻¹ is in excellent agreement with experiment; the value of r_e is 1.5992 Å at this computational level. CCSD(T) predicts ω_e = 888.80 cm⁻¹ when extrapolated to the complete basis set limit. Frequencies predicted using CCSD(T) deviate from experiment consistent with the calculations showing large multireference character. A wide array of density functionals were benchmarked. Of the 43 functionals tested, the ones that gave the best prediction of the frequency are ωB97XD, CAM-B3LYP, and τ-HCTH with respective values of 831.8, 838.3, and 837.4 cm⁻¹ respectively. The bond dissociation energy (BDE) of NiO is predicted to be 352.4 kJ mol⁻¹ at the Feller–Peterson–Dixon (FPD) level in good agreement with one of the experimental values. The calculated BDEs at the DFT level are sensitive to the choice of functional and atomic asymptote. Sixteen functionals predicted the BDE within 20 kJ mol⁻¹ of the FPD value.