MRCI investigation of Ni2O2 and Ni2O2+

Abstract

The geometrical and electronic structures of rhombic Ni₂O₂ and Ni₂O₂⁺ are investigated by multireference configuration interaction (MRCI) calculations using natural atomic orbital basis sets. The ground term of Ni₂O₂ is a ¹A_g term with a Ni–O distance of 178.9 pm and a Ni–O–Ni angle of 79.6°. At energies below 1 eV there are at least 21 further terms of singlet, triplet, and quintet multiplicity. The lowest-lying term of Ni₂O₂⁺ is a ²B₁ term with a distorted C_2v symmetric structure with two non-equivalent Ni centres and Ni–O distances of 170.6 and 182.6 pm. A ⁴B₂ term is found at an energy higher by only 0.02 eV. At energies below 0.6 eV there are at least 6 further terms of doublet and quartet multiplicity. The lowest-lying sextet terms appear at about 0.6 eV. The calculated Ni–O–Ni angle of Ni₂O₂ is in very close agreement with the value derived from IR spectra. The fact of a doublet ground state of Ni₂O₂⁺ does not support the assumption that spin restrictions are the reason for the absence of any observable reaction of Ni₂O₂⁺ with H₂ and CH₄.