Experimental and Theoretical Study of the Formation of Noble Gas Monoxide Cations in the Gas Phase

Reactions of Ng⁺ ions (Ng = Kr, Xe) with strong oxidants O₃ and N₂O were examined by FTICR/MS, employing electron ionization in the ICR cell under very low pressure. For Kr⁺, only electron transfer was observed, showing that, due to the high ionization energy of Kr, this highly exoenergetic reaction channel dominated. The lower ionization energy of Xe compared to those of O₃ and N₂O allowed for the formation of XeO⁺ in the reaction of Xe⁺ with O₃; however, Xe⁺ was nonreactive with N₂O. The experimental O atom dissociation energies of O₃ and N₂O at 298.15 K, 25.5 and 39.9 kcal mol^-1, respectively, indicate that, if a previous experimental and theoretical estimate of the dissociation energy D(Xe⁺-O) = 51 kcal mol^-1 is right, formation of XeO⁺ in the reaction of Xe⁺ with N₂O should be observed under our experimental conditions. Therefore, we assessed the reaction profiles of Xe⁺ with O₃ and N₂O using state-specific CASPT2 and MP2, respectively, to try to comprehend the different outcomes of these oxygen-bond cleavage reactions. Calculations showed that, although exoenergetic, the N₂O reaction has a higher activation energy than that of O₃, which agrees with the experiments where N₂O was ineffective in the formation of XeO⁺.