Abstract
The pseudo Jahn–Teller effect (PJTE) is employed to explain the origin of the puckered structures of tetra-heterocyclic 1,2-diazetes, C2N2E4, E=H, F, Cl, Br, and to reveal the conditions of restoration of their planar configuration. The high-symmetry C2v planar configuration of all these compounds is unstable with respect to puckering, a2-type distortions produced by the PJT coupling between their ground 1A1 and excited 1A2 electronic states. The PJTE coupling constants are estimated by fitting ab initio calculated energy profiles to the formulas of the vibronic coupling problem (A1+A2)⊗a2. The conditions for the restoration of the planar configurations of the C2N2E4 cycles were revealed: the PJTE can be quenched by removing the outer two electrons, e.g. by producing a “triple-decker sandwich” structure with two electron acceptors on both sides of the neutral cycle.
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