Non-planarity in four-membered homo-cyclic compounds A4 (A = O, S, Se, Te, Po) and restoring their planarity: a study of the pseudo-Jahn–Teller effect

Abstract

The bent D2d structures of the four-membered homo-cyclic compounds A4 (A = O, S, Se, Te, Po) were examined computationally to understand the pseudo-Jahn–Teller effect (PJTE). To do this, ab initio geometry optimizations and corresponding frequency calculations (at the MP2/cc-pVQZ-(PP) level of theory) show that all A4 compounds under-consideration are unstable in their planar (D4h) configuration. The ground state and six low-lying non-degenerate and degenerate electronic excited states were computed at the CASSCF (6,7)/cc-pVQZ-(PP) along the bending normal coordinate connecting the D4h and D2d geometries; these represent the adiabatic potential energy surfaces (APESs). Based on the APESs, the coupling between the ground state (1A1g) and the 1B2u excited state is demonstrated to be the reason for the planar structure bends from the high-symmetry D4h geometry into the lower-symmetry D2d stable equilibrium configuration. The solution to the PJTE (1A1g + 1B2u) ⊗ b2u problem is useful to answer the question of “how instability rises in A4 planar configuration?”. Although all A4 compounds in the series are non-planar with D4h symmetry, but geometrical optimizations and frequency calculations show that coordination of two noble gas cations (NG+ = He+, Ne+ and Ar+) above and below the σh plane of the A4 (A = O, S, Se) ring could restore ring planarity in (A4 NG)2+ complexes. The PJTE is also quenched in the A42+ (A = O, S, Se, Te, Po) cation and dication series, and planarity of the rings is also restored, i.e., the high-symmetry D4h structure becomes the equilibrium configuration.