Theoretical characterization of the potential energy curve for hydrogen atom addition to molecular oxygen

Large scale polarization configuration interaction (POL-CI) calculations are reported on the HO2 molecule. For the structure of the stable complex the calculations predict (with experimental values in parentheses) ROH = 0.99 Å(0.971 Å), ROO = 1.37 Å (1.335 Å), and qHOO = 103°(104.1°). The calculated harmonic frequencies (3655, 1416, and 1181 cm−1) are also in good agreement with those derived from experiment. On the other hand, the H–O2 bond energy obtained in the calculations (42 kcal/mole) is too small by ∼10 kcal/mole (an inherent defect in the POL-CI method). Although both the GVB and GVB-CI calculations predict barriers of ∼3 kcal/mole to the addition of a hydrogen atom to molecular oxygen, the POL-CI calculations reduce the barrier to less than 0.4 kcal/mole. Taking the computational limitations into account, there may well be no barrier at all to the addition reaction.