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 Å) 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.