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

This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, redistribution , reselling , loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. 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. 1. Introduction Triazine is the simplest member of the azobenzenes family, and its isomers have attracted considerable experimental and theoretical attention [1–12]. Triazines are isoelectronic with benzene, the simplest aromatic molecule, however, the presence of heteoroatoms introduces an interesting twist in their electronic structure and allows one to investigate how chemical and physical properties of benzene (e.g. aromaticity) are modulated in homologous series. One important difference between azobenzenes and benzene is their very dense electronic spectra [1]. In …