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, Ba20 and Ra20. In accordance with previous studies in our group on the Be, Mg, Ca and Sr hypermetallic oxides, we find that the Ba0Ba and Ra0Ra molecules have a linear X1Eg+ ground electronic state and a very low lying first excited 5/3Eu+ 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 singlettriplet interaction between vibronic levels of the (X) over tilde and (alpha) over tilde 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.