Pseudo Jahn–Teller origin of instability of planar configurations of hexa-heterocycles C4N2H4X2 (X = H, F, Cl, Br)

Symmetry breaking in planar molecular-ring systems in nondegenerate states is due to the pseudo Jahn–Teller effect (PJTE), and the knowledge of the detailed mechanism of the latter allows one to manipulate the molecular properties, in particular, by restoring the planar configuration. We explore the origin of structural nonplanarity of hexa-heterocycles C4N2H4X2 with X = H, F, Cl, Br, by means of ab initio electronic structure calculations of ground and excited states, and revealing the PJT interactions that produce the instability of the ground state. The four structures are optimized in both equilibrium and unstable planar ground state geometries, and their electronic configurations and vibrational frequencies are evaluated. In all these compounds the imaginary frequency in the planar configuration has b3u symmetry, thus showing the direction (symmetrized coordinate) of bending (puckering) instability. Then the energy profiles (cross sections of the adiabatic potential energy surfaces) of the ground and two excited states in the planar configuration along the coordinate of instability are calculated, and the excited states producing the instability of the ground state via the PJTE are revealed. Only one of these excited states contributes to the b3u instability via a two-level PJT vibronic coupling problem. The vibronic coupling constants are estimated by fitting the solutions of the secular equations to the corresponding ab initio calculated energy profiles.