Search of cyclic compounds that could serve as analogues of graphene in electronics, is currently the trend in modern chemistry. The mechanical, electric (conductivity), and magnetic properties of such systems are directly related to their geometry in the equilibrium configuration, notably their planarity. For this reason the mechanism of distortion of the planar configurations of cyclic compounds is of special interest. In this work the pseudo Jahn-Teller effect (PJTE) is employed to explain the instability of planar C2v nuclear configurations of the series of 3,6-pyridazinedione derivatives N2C4H2Y2Z2 (Y= O, S, Se, Z= H, F, Cl, Br) with respect to the a2 out-of-plane distortions leading to the puckered structures of C2 symmetry (Fig. 1).figureFig. 1. Instability of planar nuclear configuration of C2v symmetry occurs in all the N2C4H2O2E2 (E= H, F, Cl, Br) compounds due to the PJTE (1A1+1A2) a2 problem.The geometry optimization and the vibrational frequency analysis in planar and equilibrium (puckered) configurations of all the considered compounds with a N2C4 six-member ring were carried out by DFT B3LYP method and Dunning’s correlation consistent basis set cc-pVTZ with the Molpro 2010 package. The cross sections of the adiabatic potentials (APES) along the twisting normal coordinates (Qa2) were calculated with the state-average complete active space self-consistent field CAS(8,8) wavefunctions. Parameters of the PJTE (primary force constants K0, vibronic coupling constants V, and resulting force constants K= K0-V 2/Δ) were estimated by fitting the ab initio data for the APES of the systems to the general formulas. It is shown that the instability of planar C2v nuclear configurations and the out-of-plane distortions of N2C4H2O2Z2 compounds are due to the PJT interaction of the ground 1A1 and the lowest excited 1A2 states. The magnitude of the distortion decreases from N2C4H2O2F2 to N2C4H2O2Cl2, N2C4H2O2Br2 and N2C4O2H4, whereas the molecules N2C4S2H4 and N2C4Se2H4 are stable in the planar nuclear configuration.