The first gas-phase ion chemistry studies of the transuranium actinides Np and Pu by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS) have been performed. Reactions of An+ and AnO+ (An = Th, U, Np, and Pu) with N2O, C2H4O (ethylene oxide), H2O, O2, CO2, NO, and CH2O have been studied, with a focus on the oxidation of transuranium ions. All the An+ ions formed AnO+ with all the oxidants studied, with the exception of Pu+ with CH2O, in accord with the known bond dissociation energies, BDE(An+−O). The reaction efficiencies appear to correlate with the magnitude of the promotion energies from the ground states to reactive-state configurations of the An+ ions. Only N2O and C2H4O oxidized all AnO+ to AnO2 +; CO2 and NO oxidized only UO+; H2O and O2 oxidized UO+, NpO+, and PuO+; and CH2O was unreactive with all the AnO+ ions. The observed formation of PuO2 + from the oxidants weaker than N2O is in conflict with the literature values for BDE(OPu+−O), which we conclude are significantly too low. Charge-transfer “bracketing” experiments to determine the ionization energy of PuO2 were performed, from which a value of 7.03 ± 0.12 eV was obtained. This IE(PuO2) is 2−3 eV lower than the literature values, but in accord with our observations for the oxidation thermodynamics.

Reactions of atomic and ligated dipositive actinide ions, An2+, AnO2+, AnOH2+, and AnO2(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, O2, CO2, NO, and CH2O. Each of the five An2+ ions reacted with one or more of these oxidants to produce AnO2+, and reacted with H2O to produce AnOH2+. The measured pseudo-first-order reaction rate constants, k, revealed disparate reaction efficiencies, k/k(COL): Th2+ was generally the most reactive and Am2+ the least. Whereas each oxidant reacted with Th2+ to give ThO2+, only C2H4O oxidized Am2+ to AmO2+. The other An2+ exhibited intermediate reactivities. Based on the oxidation reactions, bond energies and formation enthalpies were derived for the AnO2+, as were second ionization energies for the monoxides, IE[AnO+]. The bare dipositive actinyl ions, UO2(2+), NpO2(2+), and PuO2(2+), were produced from the oxidation of the corresponding AnO2+ by N2O, and by O2 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 UO2(2+), NpO2(2+), and PuO2(2+) are thermodynamically stable toward Coulomb dissociation to [AnO+ + O+] or [An+ + O2+]. It is predicted that bare AmO2(2+) is thermodynamically stable. In accord with the expected instability of Th(VI), ThO(2+) was not oxidized to ThO2(2+) 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 deltaH(hyd)[UO2(2+)] is substantially more negative than the previously reported value, but is essentially the same as our deltaH(hyd)[PuO2(2+)].

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Activation of small (C1−C4) alkanes and alkenes by bare and oxo-ligated actinide cations (Th+ through Cm+) has been systematically examined using Fourier transform ion cyclotron resonance mass spectrometry. The reduced reactivities of Pu+, Am+, and Cm+ correlate with the energies for electronic excitation to a 5f n- 26d7s configuration, which indicates that their 5f electrons are chemically inert. The reactivity trend identified for the highly reactive early actinide ions, Th+ > Pa+ > U+ > Np+, is interpreted to indicate significant 5f electron participation in organoactinide σ-type bond formation for Pa+. Among the seven studied AnO+ ions, only ThO+, PaO+, and UO+ activated at least one hydrocarbon, with the reactivity of PaO+ being distinctively high. Electronic structure calculations for PaO+ show that its ground state is Pa(5f6d)O+, i.e., with one 5f and one 6d nonbonding electrons available on the metal, and all of its excited states up to 1.8 eV have a 5f orbital occupancy of ≥0.8. The high reactivity and substantial 5f character of PaO+ indicate participation of 5f electrons in hydrocarbon bond activation for oxo-ligated Pa+. The results of this work reveal that 5f electrons play a distinctive role in protactinium chemistry involving σ-type organometallic bonding.