Electron-Spin Magnetic Moments (g Factors) of the Oxides LiO, NaO, and the Superoxides LiO2, NaO2. An ab Initio Study

The electron-spin magnetic moments of the superoxides LiO 2 and NaO 2, as parametrized by the g factors, are studied at the uncorrelated (ROHF) and correlated (MRCI) ab initio levels. The present method, which uses a perturbative approach complete to second order, is based on the Breit−Pauli Hamiltonian. In the standard C 2 v notation for the MO 2 radicals, Δ g yy > Δ g zz ≫ |Δ g xx |, where Δ g aa = g aa − g e . The perpendicular component Δ g xx, which is small and negative, is dictated by first-order terms (ground-state expectation values). The in-plane components Δ g yy and Δ g zz are large and positive. Δ g yy is governed by the second-order magnetic coupling between X 2 A 2 and 1 2 B 2 (electron excitation from the highest b 2 MO into the a 2 (π*) SOMO), and Δ g zz by the coupling with two 2 A 1 states (excitations from the two highest a 1 MOs into π*). The calculated data reproduce the experimental trends reasonable well. For the ground state X 2 A 2, the MRCI results for Δ g xx, Δ g yy, Δ g zz (in ppm, with ppm = 10 -6 ) are −373 (−350), 56 800 (56 250), 7273 (6600) for LiO 2; and −393 (150), 110 492 (108 600), 6868 (4600) for NaO 2, with the experimental results given in parentheses (average from Ar and Kr matrices). For comparison purposes, the g shifts of the low-lying excited state 1 2 B 2 of LiO 2 and NaO 2, as well as the magnetic coupling parameters for O 2 -, LiO, and NaO, are also reported.