Pseudo Jahn–Teller origin of twisting in 3,6-pyridazinedione derivatives; N2C4H2Y2Z2 (Y = O, S, Se, Z = H, F, Cl, Br) compounds

Abstract

3,6-pyridazinedione and two of its derivatives where oxygen atoms of the molecule are substituted by two sulfur or selenium ( N 2 C 4 Y 2 H 4) were studied with the goal of answering the following question: "Which N 2 C 4 Y 2 H 4 compounds are unstable in their planar configuration?" Additionally, the origin of the twisting instability of 3,6-pyridazinedione planar configuration and three of its 1,2-dihalo derivatives ( N 2 C 4 H 2 O 2 Z 2) were rationalized by employing the pseudo Jahn–Teller effect (PJTE) to explain the difference between N 2 C 4 H 2 O 2 Z 2 structures in series. Therefore, the structures of six 3,6-pyridazinediones ( N 2 C 4 H 2 Y 2 Z 2) were optimized in both equilibrium and planar configurations, and their vibrational frequencies were calculated. Then the adiabatic potential energy surface (APES) profiles along the a2 distortion coordinates were calculated. Based on the calculation results, N 2 C 4 S 2 H 4 and N 2 C 4 Se 2 H 4 compounds were stable in the planar structure; but, due to the vibronic coupling interaction between the 1A1 ground state and the first excited state 1A2, the twisting instability occurred in planar N 2 C 4 H 2 O 2 Z 2 series. The (1 A 1 + 1 A 2) ⊗ a2 problem was found to be the reason of the breaking symmetry phenomena in all the four N 2 C 4 H 2 O 2 Z 2 in series from unstable planar configuration (highest-symmetry C 2v ) to the stable twisted geometry with C 2 symmetry. Finally, the vibronic coupling constants of the PJTE of the compounds in series were estimated by fitting the secular equation roots along the normal coordinates of distortion.