Page | 739 Received: 01 November 2019 Revised: 12 March 2020 Accepted: 27 March 2020 DOI: 10.33945/SAMI/ECC.2020.7.1 Eurasian Chem. Commun. 2 (2020) 739-749 http:/echemcom.com FULL PAPER Quantum chemical study of the Jahn – Teller effect on the distortions of XO 2 (X = O, S, Se, Te) systems Ali Esmaeili a |Reza Fazaeli b, * |Esmat Mohammadi Nasab a a Department of Chemistry, Arak Branch, Islamic Azad University, Arak, Iran b Department of Chemistry, South Tehran Branch, Islamic Azad University, P.O. BOX 11365-4435, Tehran, Iran *Corresponding Author: Reza Fazaeli Tel.: +98 (21) 77248534 Preliminary researches provided essential information about the optimized configuration of triatomic XO2 (X = O, S, Se, Te) systems, which were bent in the ground state and linear in their first excited state. The Jahn-Teller effects including the Jahn- Teller (JTE), the Renner-Teller effect (RTE), and the pseudo Jahn–Teller effect (PJTE) are parts of the most important reasons for structural distortion in the high symmetry configurations for each molecular system. This study purpose was to investigate the dependence between PJT parameters including the vibronic coupling constant values (F), energy gap between reference states (Δ), and initial force constant (K0). In all above mentioned molecules, stability were increased with the reduction in the symmetry level. This increment was attributed to the PJTE. The vibronic coupling interaction between the ground (Σg), and the first excited states (Πu) through the PJTE problem (PJT (Σg+ Πu) × Πu) was because of the asymmetry and molecules bending phenomenon. The hardness difference parameter Δ[η (C2V) -η (D∞h)] decreases from O to Te (30.42, 22.66, 22.65, 22.58 Kcal/mol). These changes could explain the trend, which were observed for the D∞h → C2V conversion process. KEYWORDS The Jahn-Teller effect; vibronic interactions; Ab initio; quantum; symmetry breaking. Introduction Despite of the triatomic systems’ apparent simplicity, the origin of their equilibrium geometry is not still entirely known. Preliminary researches provided essential information on these specific molecular systems’ optimized configuration, but this information cannot explain the geometric origin and the difference in configurations directly; specifically, the electron structure calculations by its own cannot explain why many of the triatomic systems (such as + + + + 2 2 2 SiH ,H O ,CH ,HNO ,HNF,HBF ,…) are bent in the ground state, and also are linear in the first excited state [1]. So many models have been used in many studies like Walsh diagrams and molecular orbital theory, the valence shell electron pair repulsion model (VSEPR), polarized ion models as well as the softness hardness–based models, but they cannot totally explain about this issue, [2]. The triatomic molecules and free radicals extensive application has been of interest in combustion, photochemical, atmospheric chemistry, interstellar chemistry processes, and frequently considered as intermediate in chemical reactions [3-7]. The vibronic coupling theory gives us information about the molecules shape. It is popular that the Jahn-Teller effect (JTE) (including the Jahn-Teller, which is suitable for the systems in the electronically degenerate states, the Renner-Teller effect (RTE) which is designed for the linear molecules, and the pseudo Jahn–Teller effect