Jahn-Teller distortions in the electronically excited states of sym-triazine

Electronic structure of the low-lying excited states of sym-triazine is analysed. Excitation energies are computed at the equilibrium ground state geometry and at the geometry of the cation. Full geometry optimisations of the selected excited states were performed, and the magnitude of the Jahn-Teller distortions in these states was quantified. Analysis of the electronic structure of these states offers a simple recipe for predicting the symmetry of the equilibrium geometries by a single point calculation of the vertical excitations. Whereas the states derived from excitations between a degenerate pair and a non-degenerate orbital form a familiar two-state conical intersection and are always distorted by virtue of the Jahn-Teller theorem, the states derived from the excitations between two pairs of degenerate orbitals form a more complicated glancing-like manifold characterised by negligible Jahn-Teller linear terms. Our analysis shows that regardless of the degeneracy pattern in these four-state manifolds, the top two states are always nearly-symmetric. The particular shape of the potential energy surfaces in a glancing intersection depends on the relative ordering of the states within the manifold, and different topologies of both types of intersections around D-3h geometry are discussed. Predicted symmetry of the excited states is compared to the optimised structures of the selected electronic states.