Gas-Phase Oxidation Reactions of Neptunium and Plutonium Ions Investigated via Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

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 N 2 O, C 2 H 4 O (ethylene oxide), H 2 O, O 2, CO 2, NO, and CH 2 O 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 CH 2 O, 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 N 2 O and C 2 H 4 O oxidized all AnO + to AnO 2 +; CO 2 and NO oxidized only UO +; H 2 O and O 2 oxidized UO +, NpO +, and PuO +; and CH 2 O was unreactive with all the AnO + ions. The observed formation of PuO 2 + from the oxidants weaker than N 2 O 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 PuO 2 were performed, from which a value of 7.03 ± 0.12 eV was obtained. This IE(PuO 2 ) is 2−3 eV lower than the literature values, but in accord with our observations for the oxidation thermodynamics.