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

Abstract

The HO(3) and HO(4) polyoxide radicals have attracted some attention due to their potential role in ozone chemistry. Experimentally, the geometrical structure of HO(3) is known whereas that of HO(4) 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 HO(2), with O(2). 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 HO(2)) and O(2) have been computed amounting to -3.21 kcal mol(-1) for HO(3) (trans conformation) and 11.33 kcal mol(-1) for HO(4) (cis conformation). The corresponding formation enthalpies are 6.12 and 11.83 kcal mol(-1). The trans conformation of HO(4) is less stable than the cis one by 6.17 kcal mol(-1). Transition states for HO(4) dissociation and for cis/trans conversion are also described.

The Gas‐Phase Reaction between O3 and HO Radical: A Theoretical Study

“…[12] This large O2ÀO3 bond length is similar to that occurring in the HO 3 radical. [53] A more detailed discussion of the features of this tetraoxide radical will be reported elsewhere, [54] however, the HO 4 radical can be seen as an adduct of the HO 2 radical with O 2 , in the same way as HO 3 can be seen as and adduct between HO radical and O 2 . [53] Energetically, single-point CASPT2 calculations at the MRCI optimised geometry predict the HO 4 [12] For the HO 4 decomposition, we also carried out an MRCI numerical optimisation, and we found a transition state with a O2ÀO3 bond distance of 1.997 (TS3, see Figure 2), with an energy barrier of about 5.5 kcal mol À1 relative to the HO 4 radical (DE according to single-point CASPT2 energy calculation at the MRCI optimised geometry).…”

On the Dissociation of Ground State trans-HOOO Radical: A Theoretical Study

“…Therefore, there is a clear discrepancy between the experimental findings of Le Picard et al 52 and the theoretical prediction of Varner et al 47 concerning the existence of an exit barrier for the unimolecular dissociation of trans-HOOO • (X 2 A′′) to HO • (X 2 Π) + O 2 (X 3 Σ g -). Since it has been argued 10,11,24,41,42,44 that multireferencebased methods are needed to correctly describe the electronic structure of HOOO • , the results of Varner et al 47 motivated us to reinvestigate the minimum energy reaction path of trans-HOOO • (X 2 A′′) dissociation to HO • (X 2 Π) + O 2 (X 3 Σ g -) using high level multireference-based methodologies. With this aim, herein, we report the results of CASSCF and CASPT2 electronic structure calculations on the equilibrium geometry of ground-state trans-HOOO • and its unimolecular dissociation.…”

Structure, stability, and dynamics of hydrogen polyoxides

“…The structure of HO 3 has been determined in gas phase recently5. That of HO 4 is not experimentally known but it has been described with the help of accurate ab initio calculations6.…”