Gas-Phase Oxidation of Cm+ and Cm2+ – Thermodynamics of Neutral and Ionized CmO

Abstract

Fourier transform ion cyclotron resonance mass spectrometry was employed to study the products and kinetics of gas-phase reactions of Cm+ and Cm2+; parallel studies were carried out with La+/2+, Gd+/2+ and Lu+/2+. Reactions with oxygen-donor molecules provided estimates for the bond dissociation energies, D[M+-O] (M ) Cm, Gd, Lu). The first ionization energy, IE[CmO], was obtained from the reactivity of CmO+ with dienes, and the second ionization energies, IE[MO+] (M ) Cm, La, Gd, Lu), from the rates of electrontransfer reactions from neutrals to the MO2+ ions. The following thermodynamic quantities for curium oxide molecules were obtained: IE[CmO] ) 6.4 ( 0.2 eV; IE[CmO+] ) 15.8 ( 0.4 eV; D[Cm-O] ) 710 ( 45 kJ mol-1; D[Cm+-O] ) 670 ( 40 kJ mol-1; and D[Cm2+-O] ) 342 ( 55 kJ mol-1. Estimates for the M2+-O bond energies for M ) Cm, La, Gd, and Lu are all intermediate between D[N2-O] and D[OC-O] - that is, 167 kJ mol-1 < D[M2+-O] < 532 kJ mol-1 - such that the four MO2+ ions fulfill the thermodynamic requirement for catalytic oxygen-atom transport from N2O to CO. It was demonstrated that the kinetics are also favorable and that the CmO2+, LaO2+, GdO2+, and LuO2+ dipositive ions each catalyze the gas-phase oxidation of CO to CO2 by N2O. The CmO2 + ion appeared during the reaction of Cm+ with O2 when the intermediate, CmO+, was not collisionally cooled - although its formation is kinetically and/or thermodynamically unfavorable, CmO2 + is a stable species.