Structure, Stability, and Reactivity of Cationic Hydrogen Trioxides and Thermochemistry of their Neutral Analogs. A Fourier-Transform Ion Cyclotron Resonance Study

The kinetics and the reaction patterns of the HO 3 + and H 2 O 3 + ions toward a variety of inorganic and organic substrates have been investigated by using Fourier-transform ion cyclotron resonance (FT-ICR) spectrometry. The thermochemistry of the HO 3 + and H 2 O 3 + ions is evaluated from correlations between their proton transfer (PT) efficiencies and the proton affinity (PA) of the selected substrates. Similarly, thermochemical data on HO 3 and H 2 O 3 species are inferred from a comparison between the electron transfer (ET) efficiencies of their cationic counterparts and the standard ionization energies (IE) of the substrates. Thus, in striking contrast with most literature theoretical and empirical estimates, an experimental value of −1 ± 5 kcal mol - 1 is obtained for the standard heat of formation of HO 3 . Accordingly, ground-state HO 3 ( 2 A) is thermochemically stable toward dissociation to HO( 2 Π) and O 2 ( 3 Σ g - ), and therefore, its existence as a true intermediate in key ionic reactions occurring in the upper atmosphere cannot be excluded. The standard formation enthalpy of H 2 O 3 + (198 ± 5 kcal mol - 1 ) is evaluated by two independent approaches, while that of the HOOOH neutral molecule is estimated as ≤−26 kcal mol - 1 . The HO 3 + ion displays a variegated chemistry. Depending of the nature of the reactive centers of the neutral substrate, the HO 3 + ion may react as a Brønsted or a Lewis acid, as an oxenium ion or an oxygen-centered free radical. When all these pathways are thermochemically precluded, as with CO, a ligand swiching process takes place in HO 3 + to give the CHO 2 + ion, which may promote a three-step acid-catalyzed cycle for the O 3 oxidation of CO to CO 2 and O 2 . Likewise, the less reactive H 2 O 3 + ion undergoes ligand swiching by water.