On the nature of the unusually long OO bond in HO3 and HO4 radicals

Abstract

The HO3 and HO4 polyoxide radicals have attracted some attention due to their potential role in ozone chemistry. Experimentally, the geometrical structure of HO3 is known whereas that of HO4 is not. Moreover, the existence of the latter radical has been questioned. Theoretical calculations on the two species have been reported before, showing important structural differences depending on the computational level. Both radicals present an unusually long OO bond (around 1.7–1.8 A) that can be associated with an intricate interaction between HO, or HO2, with O2. The nature of such interaction is investigated in detail using large scale ab initio methods (CASSCF, CASPT2, MRCI, QCISD, CCSD(T)) and density functional techniques (B3LYP) in connection with extended basis sets. Stabilization enthalpies at 298 K with respect to HO (or HO2) and O2 have been computed amounting to -3.21 kcal mol-1 for HO3 (trans conformation) and 11.33 kcal mol-1 for HO4 (cis conformation). The corresponding formation enthalpies are 6.12 and 11.83 kcal mol-1. The trans conformation of HO4 is less stable than the cis one by 6.17 kcal mol-1. Transition states for HO4 dissociation and for cis/trans conversion are also described.