Singlet–triplet interaction in Group 2 M2O hypermetallic oxides

Abstract

This ab initio study of Group 2 M2O hypermetallic oxides focuses mainly on the two heaviest members, Ba2O and Ra2O. In accordance with previous studies in our group on the Be, Mg, Ca and Sr hypermetallic oxides, we find that the BaOBa and RaORa molecules have a linear X∼1Σg+ ground electronic state and a very low lying first excited ã3Σu+ triplet electronic state. Special attention is placed on calculating and understanding how the singlet–triplet splitting and singlet–triplet interaction strength vary down the series. The calculations reveal that MgOMg shows the largest singlet–triplet splitting and does not fit into the overall trend down the Group 2 series of elements. However, in all cases the extent of the singlet–triplet interaction between vibronic levels of the X∼ and ã states is very small. On the experimental side, there is literature evidence for the formation of electronically excited Ba2O in oxidation reactions of barium dimers, and our calculations of excited singlet and triplet state energies support that assignment.