The double Renner effect: A theoretical study of the MgNC/MgCN isomerization in the A˜ 2Πelectronic state

This paper reports the first theoretical treatment of the MgNC↔MgCN isomerization in the A˜ 2Π electronic state. In this state, the electronic energy is doubly degenerate at Mg-N-C and Mg-C-N linear geometries, while at bent geometries it splits to produce two non-degenerate electronic states 1 2A″ and 2 2A′. The rovibronic states associated with these two electronic states are coupled by the Renner effect. Since two different linear geometries are involved, and the electronic energy is doubly degenerate at both, we speak about the 'double Renner effect.' Based on ab initio data, global potential energy surfaces for the 1 2A″ and 2 2A′ states have been constructed; they encompass the MgNC and MgCN minima (which both correspond to linear geometries). These surfaces have been used, in conjunction with the newly developed computer program dr, for calculating 24MgNC/24MgCN rovibronic energies while accounting correctly for the double Renner effect. The lowest rovibronic state, in which there is significant tunneling between the MgNC and MgCN linear-geometry minima, lies 2835 cm−1 above the vibronic ground state of 24MgCN. In the lowest tunneling state, the molecule tunnels between being 1 2A″ MgCN and 2 2A′ MgNC, and so one can say that the isomerization is driven by the Renner interaction.