A Computational Study on the Instability of Linear Structures in TeX₂ (X = F, Cl, Br):The Role of the Pseudo-Jahn–Teller Effect

The instability of linear molecular structures is a fundamental phenomenon closely associated with symmetry breaking and vibronic interactions. In this study, we systematically investigate the structural stability of linear TeX₂ molecules (X = F, Cl, Br) using quantum chemical calculations at the B3LYP/def2-TZVPP, HF/def2-TZVP, and M06-2X/def2-TZVPP levels of theory. The results demonstrate that the linear D∞h configurations of all three molecules are intrinsically unstable with respect to bending distortions, leading to energetically favored non-linear equilibrium structures. Analysis of the adiabatic potential energy surfaces reveals that these distortions originate from the pseudo-Jahn–Teller effect (PJTE), resulting from vibronic coupling between the ground (Σg) and low-lying excited (Πu) electronic states. The curvature of the ground-state potential energy surface along the Πu bending coordinate is negative, confirming the presence of PJTE-induced instability. Natural Bond Orbital (NBO) analysis further indicates that bending distortions enhance donor–acceptor interactions and electronic delocalization, contributing to the stabilization of the bent structures. Moreover, the energy difference between the linear and bent configurations decreases systematically from TeF₂ to TeBr₂, reflecting a progressive softening of the molecular framework with increasing halogen atomic size. This trend is accompanied by a reduction in the electronic energy gap and an enhancement of vibronic coupling strength, resulting in increased PJTE stabilization energies. These findings provide a consistent theoretical explanation for the instability of linear TeX₂ molecules and highlight the decisive role of the pseudo-Jahn–Teller effect in governing their structural behavior.