Oxidation studies of dipositive actinide ions, An2+ (An = Th, U, Np, Pu, Am) in the gas phase:: Synthesis and characterization of the isolated uranyl, neptunyl, and plutonyl ions UO22+(g), NpO22+(g), and PuO22+(g)

Reactions of atomic and ligated dipositive actinide ions, An(2+), AnO(2+), AnOH(2+), and AnO(2)(2+) (An = Th, U, Np, Pu, Am) were systematically studied by Fourier transform ion cyclotron resonance mass spectrometry. Kinetics were measured for reactions with the oxidants, N2O, C2H4O (ethylene oxide), H2O, O-2, CO2, NO, and CH2O. Each of the five An(2+) ions reacted with one or more of these oxidants to produce AnO(2+), and reacted with H2O to produce AnOH(2+). The measured pseudo-first- order reaction rate constants, k, revealed disparate reaction efficiencies, k/k(COL): Th2+ was generally the most reactive and Am 21 the least. Whereas each oxidant reacted with Th2+ to give ThO2+, only C2H4O oxidized AM(2+) to AMO(2+). The other An(2+) exhibited intermediate reactivities. Based on the oxidation reactions, bond energies and formation enthalpies were derived for the AnO(2+), as were second ionization energies for the monoxides, IE[AnO(+)]. The bare dipositive actinyl ions, UO22+, NpO22+, and PuO22+, were produced from the oxidation of the corresponding Ano(2+) by N2O, and by O-2 in the cases of UO2+ and NpO2+. Thermodynamic properties were derived for these three actinyls, including enthalpies of formation and electron affinities. It is concluded that bare UO22+, NpO22+, and PuO22+ are thermodynamically stable toward Coulomb dissociation to {AnO(+) + O+} or {An(+) + O-2(+)}. It is predicted that bare AMO(2)(2+) is thermodynamically stable. In accord with the expected instability of Th(VI), ThO2+ was not oxidized to ThO22+ by any of the seven oxidants. The gas-phase results are compared with the aqueous thermochemistry. Hydration enthalpies were derived here for uranyl and plutonyl; our Delta H-hyd[UO22+] is substantially more negative than the previously reported value, but is essentially the same as our Delta H-hyd[PuO22+].