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 a 2 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 1 A 1 ground state and the first excited state 1 A 2 , the twisting instability occurred in planar N 2 C 4 H 2 O 2 Z 2 series. The ( 1 A 1 + 1 A 2 ) ⊗ a 2 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.