The Pseudo Jahn-Teller effect and NBO analysis for untangling the symmetry breaking in the planar configurations of M2X4+ (M = Si, Ge and X = Cl, Br, I): effect on electronic structure and chemical properties

Context The Pseudo Jahn- Teller effect is a significant tool for evaluating molecular distortion and symmetry breaking. The PJT effect associated with NBO analysis can be a powerful method for studying the structural properties variations arising from D-2h -> C-2h distortions. The theoretical studies on Si2X4 + and Ge2X4+ radical cations have been rare. The calculations have shown that C-2h non-planar structures are more stable than planar structures with D-2h symmetry. The (B-3u + B-1u) circle times b(2g) PJTE problem of M2X4+ compounds is a result of the coupling between the ground B-3u state and the exited B-1u state in the Q(b2g) direction causes. Also, the difference in M and X atoms can affect the PJT instability of compounds. The findings of this work show that the energy gap between the ground and excited states that have D-2h symmetry decreases from M2Cl4+ to M2I4+ and increases from Si2X4+ to Ge2X4+. In fact, there is a significant relationship between instability of high-symmetry configurations, geometric parameters, electron delocalization, chemical hardness, electronegativity, electrophilicity index, and PJT stabilization energies. These results may serve to evaluate the distortion of similar systems. Methods The structures of Si2X4+ and Ge2X4+ are optimized by LC.BLYP, M06.2X, and B3LYP methods with def2.TZVPP basis set in GAMESS software. The details of the excited states of compounds are studied by the TD-DFT method. NBO analysis for planar and non.planar structures is carried out at B3LYP/def2.TZVPP level by the NBO 5. G program that demonstrates HOMO, LUMO, ED, bonding and antibonding orbital occupancies, bond order, and E-2.