MRCI investigation of Ni2O2 and Ni2O2+

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 degrees. 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 C2v 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.