Theoretical study of the vertical electronic spectra of O2- and the superoxides LiO2 and NaO2

The vertical electronic spectra of the ion O-2(-) and superoxides LiO2 and NaO2 are studied at the multireference configuration interaction (MRCI) level. For isolated O-2(-) with a ground state X(2)Pi(g)(2 sigma(g)(2)1 pi(u)(4)1 pi(g)(3)), the present calculations place 1(2)Pi(u)(1 pi(u)-->1 pi(g)) at 5.20eV, 1(2)Sigma(g)(+)(2 sigma(g)-->1 pi(g)) at 6.32 eV and 2(2)Sigma(u)(+)(1 sigma(u)-->1 pi(g)) at 15.62 eV, confirming the assignments made on the basis of electron scattering experiments on O-2(X(3)Sigma(g)(-)). An absorption band at 6.41 eV observed for O-2(-)-doped KCl crystals is reassigned to 1 pi g-->2 sigma(u) states of O-2(-), instead of a charge-transfer excitation O-2(-)-->K+. A strong absorption band near 5.5 eV for NaO2 crystals is confirmed to correspond to a pi-->pi* (B-2(1)<--X(2)A(2)) excitation localized on the O-2(-) moiety (f value approximate to 0.078 and vertical Delta E = 5.28 eV). Another band at 3.8 eV for NaO2 crystals, previously assigned to 1(2)Delta(u)<--X(2)Pi(g) of O-2(-), is reassigned to charge-transfer states 1 pi(g)(O-2(-))-->3s, 3p(x), 3p(y)(Na+) with vertical Delta E's from 3.95 to 4.45 eV and S values from 0.008 to 0.023. No experimental data are available for LiO2. Our calculations assign to this radical a strong pi-->pi* absorption, with Delta E = 4.71 eV and f approximate to 0.1. Next in intensity one finds the charge-transfer state occupying 2p(y)(Li+), with Delta E = 5.18 eV and f approximate to 0.031. For LiO2 and NaO2, the transition 1(2)B(2)<--X(2)A(2) is of very low intensity (f less than or equal to 10(-3)) as it involves the components of the 1 pi(g) MO of O-2(-) (i.e. both the transition moment and Delta E are small).