THE FRATERNAL-TWINS OF QUARTET O-4+

Eleven stationary geometries of quartet O4+ have been studied by ab in itio methods. The geometries were optimized at the complete active spa ce self-consistent field (CASSCF) level of theory and the energies wer e calculated by the multiconfigurational second order pertubation meth od (CASPT2), using double-zeta plus polarization (DZP), triple-zeta pl us double polarization (TZ2P), average atomic natural orbital (ANO) [5 s4p2d] and average ANO [6s5p3d2f] basis sets. The rectangular and tran s-planar structures are found to be the most stable, with an energy ba rrier to conversion between the two at the threshold of dissociation. Both have a delocalized hole and are stable relative to separated O2 a nd O2+ by 11.0 and 11. 5 kcal/mol for the rectangular and the trans-pl anar structure, respectively, compared with the experimentally deduced energy in the range of 9.2 to 10.8 kcal/mol. The adiabatic ionization potentials of O4 and O2 are computed to be 11.67 and 12.21 eV, while experimental values are 11.66 and 12.07 eV, respectively. The vibratio nal frequencies have been computed for all degrees of freedom at the C ASSCF level of theory. Symmetry breaking is found to be a particular p roblem in the computation of the antisymmetric stretch frequency for t he delocalized structures at the CASSCF level of theory. Attempts to r ectify these problems using the restricted active space self-consisten t field (RASSCF) method leads to additional difficulties, but further analysis yields insight into the symmetry breaking and problems with e arlier calculations. Finally, a nonorthogonal configuration interactio n (CI) calculation based on the interaction of localized CASSCF wave f unctions using the complete active space state interation (CASSI) meth od leads to a balanced treatment of the antisymmetric stretch which is free from symmetry breaking. The study explains the four most promine nt absorption frequencies observed in the partially unassigned IR spec trum of 04+ isolated in solid neon as the antisymmetric OO stretch, an d the combination band of the symmetric and antisymmetric OO stretch o f both the rectangular and trans-planar structures.