The electronic structure of X₃, X = O, S, Se and Te and their anions

The electronic and geometrical structural properties of the group VIA trimers, O₃, S₃, Se₃ and Te₃ in their ground and lowest-energy anion states have been calculated using an ab initio complete active space (CAS) MCSCF method with a double-zeta+polarization Gaussian basis set. Compact effective potentials were used to replace the atomic core electrons derived from relativistic atomic orbitals for the selenium and tellurium atoms. UMPn (n=2-4) calculations were also carried out at the CAS MCSCF equilibrium geometries. The bending energy curves for the trimer ground states are characterized by an increasing stability of the ring relative to the open geometry and decreasing barrier to D3h⇌C_2v interconversion with increasing atomic number. Calculated electron affinities of the trimers are in better agreement with experiment for the UMP4(SDTQ) method than with the CAS MCSCF approach. In each case, the lowest energy state of the anion has a bond angle close to the neutral ground-state value, explaining the absence of a bending mode progression in the photoelectron spectra of O₃^- and S₃^-.