Inversion isomerism induced by the Pseudo Jahn-Teller Effect in X₂CE molecules (X = H, F; E = O, S, Se) in their reduced and excited states

The pseudo-Jahn-Teller effect (PJTE) is employed to reveal the origin of molecular geometry changes induced by reduction and excitation, using a series of X 2 CE molecules (X = H, F, Cl; E = C, S, Se) as examples. Their nuclear configuration is planar with C 2v symmetry in the ground electronic state. However, when reduced, they acquire a nonplanar pyramidal structure with an elongated C-E bond in their ground and lowest excited electronic states 1 A 2 and 3 A 2 . It is shown that these distortions are due to the PJT mixing of the reference electronic states with appropriate excited states via the b 1 vibrational mode. The PJTE is shown to reasonably parametrize the adiabatic potential energy curve ε ( Q b 1 ) along the instability coordinate Q b 1 .