A Study of the X²A₂ State of KO₂ Using Ab Initio and Density Functional Theory:  The Equilibrium Geometry and Vibrational Frequencies

Geometry optimization and harmonic vibrational frequency calculations at the CASSCF, CISD, UMP2, QCISD, CCSD(T), and B3LYP levels of theory were performed on the ground X̃ 2 A 2 state of KO 2 . Various augmented effective core potential and all-electron basis sets were employed, with the largest being the LANL2DZ effective core potential augmented by [6s6p3d1f], the all-electron basis set, [11s10p5d3f] for potassium, and aug-cc-pVTZ for oxygen. These calculations lead to the conclusion that the C 2 v X̃ 2 A 2 state of KO 2 has an equilibrium bond angle of 32.3 ± 0.5°, a K−O bond length of 2.410 ± 0.005 Å, and an O−O bond length of 1.341 ± 0.001 Å. The B3LYP density functional method is also employed for NaO 2 and LiO 2, and it appears that this method does not suffer from symmetry breaking; a similar conclusion is reached for the QCISD method. Isotopic shifts for all three molecules are also reported and compared to available experimental values.