Electronic Structure\nof Diatomic Nickel Sulfide

The nature of the Ni–S bond is investigated due\nto its role\nin the absorption of atmospheric Lewis acid gases such as SO<sub>2</sub> and SO<sub>3</sub> onto Ni surfaces. The vibrational frequency and\nelectronic structure of NiS were predicted using CCSD(T), CASSCF,\nand internally contracted multireference configuration interaction\n(icMRCI) + Q. 43 density functional theory (DFT) functionals were\nbenchmarked. CASSCF predicted the ground state of NiS to be the <sup>5</sup>Δ state arising from the 3d<sup>8</sup>(<sup>3</sup>F)4s<sup>2</sup> (<sup>3</sup>F) and 3d<sup>9</sup>(<sup>2</sup>D)4s\n(<sup>3</sup>D) electronic configurations of Ni. When dynamical correlation\neffects are included at the icMRCI + Q level, the ground state of\nNi–S is predicted to be <sup>3</sup>Σ<sup>–</sup> consistent with the experiment. The vibrational frequency of Ni–S\nis calculated to be 519.1 cm<sup>–1</sup> at the icMRCI + Q\nlevel, in reasonable agreement with the experimental value of 512.68\ncm<sup>–1</sup>. CCSD(T) predicts the frequency of Ni–S\nto be 543.2 cm<sup>–1</sup> when extrapolated to the complete\nbasis set (CBS) limit. The Feller–Peterson–Dixon value\nbased on the CCSD(T)/CBS extrapolation for the bond dissociation energy\nof NiS is 350.6 kJ/mol, within <4 kJ/mol of experiment. Of the\n43 DFT functionals, BP86 and O3LYP predicted the vibrational frequency\nin closest agreement with the experiment. The applicability of DFT\nto such acid gas systems was further demonstrated by calculating the\nenergy for displacement of NiO by SO to yield NiS and O<sub>2</sub>. This displacement energy was calculated to be within experimental\nerror for ∼50% of the DFT functionals, but large differences\nwere also predicted for some functionals.