Gas-phase reactions of U⁺ and U²⁺ with O₂ and H₂O in a quadrupole ion trap

Reaction pathways and rate constants of gas-phase uranium and uranium oxide ions with O₂ and H₂O have been investigated using a quadrupole ion trap mass spectrometer (QIT-MS). A new reaction pathway is identified for the reaction between U²⁺ and H₂O, which leads to the formation of UO⁺ via the intermediate UOH²⁺. Reaction rate constants are determined for several reactions by measuring the reaction rate at different partial pressures of the reagent gas and are found to be in reasonable agreement with the literature. These rate constants include the first known measurement for the reaction of U²⁺ with H₂O (∼0.4 k_ADO). New limits on thermochemical values are also provided for certain species. These include ΔH_f (UO²⁺) ≤ 1742 kJ mol^-1 and 1614 ≤ ΔH_f (UOH²⁺) ≤ 1818 kJ mol^-1 and are based on the assumption that only exothermic or thermoneutral reactions are possible under the conditions used. This assumption is supported by simulations of the root-mean-square (RMS) ion kinetic energy of stored uranium ions in the QIT. Only a slight increase in the RMS ion kinetic energies, from 0.1 to 0.2 eV, is predicted over the range of trapping conditions studied (0.05 ≤ q_z ≤ 0.75) corresponding to a theoretical reaction temperature of ∼384 K. The simulations also compare helium and neon as bath gases and show that the RMS kinetic energies are found to be very similar at long trapping times (> 20 ms), although neon establishes steady state conditions in approximately half the time.