THEORETICAL-STUDY OF THE ALKALINE-EARTH METAL SUPEROXIDES BEO2 THROUGH SRO2

Three competing bonding mechanisms have been identified for the alkaline-earth metal superoxides: these result in a change in the optimal structure and ground state as the alkaline-earth metal becomes heavier. For example, BeO2 has a linear 3SIGMA(g)-ground-state structure, whereas both CaO2 and SrO2 have C2v 1A1 structures. For MgO2, the theoretical calculations are less definitive, as the 3A2 C2v structure is computed to lie only about 3 kcal/mol above the 3SIGMA(g)- linear structure. The bond dissociation energies for the alkaline-earth metal superoxides have been computed using extensive Gaussian basis sets and treating electron correlation at the modified coupled-pair functional or coupled-cluster singles and doubles level with a perturbational estimate of the triple excitations. Our best estimates for the M-O2 dissociation energies are 88 +/- 4, 25 +/- 4, 54 +/- 4, and 54 +/- 6 kcal/mol for BeO2, MgO2, CaO2, and SrO2, respectively.