Enthalpy of the Cerium (Ce) Chemi-Ionization Reaction and CeO+, CeC+, and CeCO+ Bond Energies Determined by Energy-Resolved Guided Ion Beam Mass Spectrometry Experiments.

Abstract

Cerium (Ce) oxide, carbide, and carbonyl cation bond energies and the exothermicity of the Ce chemi-ionization (CI) reaction with atomic oxygen were investigated using guided-ion beam tandem mass spectrometry (GIBMS). The kinetic energy dependent product cross sections for reactions of Ce⁺ with O₂, CO, and CO₂ and CeO⁺ with O₂, CO, Xe, and Ar were measured using GIBMS. For the reactions of Ce⁺ with O₂ and CO₂, CeO⁺ is formed through an exothermic reaction, whereas CeO⁺ formation is endothermic in the reaction with CO. This reaction also forms CeC⁺ and CeCO⁺ is formed in the reaction of Ce⁺ with CO₂, both in endothermic processes. Reactions of CeO⁺ with all four gases led to endothermic collision-induced dissociation as well as exchange reactions to form CeO₂⁺ for the O₂ and CO reactants. Bond dissociation energies (BDEs) at 0 K were determined through analyses of the kinetic energy dependent cross sections for all endothermic reactions. We determined that the BDEs of CeO⁺, CeC⁺, and CeCO⁺ are 8.46 ± 0.15, 3.93 ± 0.16, and ≥0.25 ± 0.07 eV, respectively, where the CeO⁺ BDE is a weighted average of 6 independent 0 K threshold measurements. The CeO⁺ BDE is combined with the ionization energy (IE) of Ce to determine an exothermicity of 2.91 ± 0.15 eV for the Ce + O → CeO⁺ + e^- chemi-ionization reaction. Combined with neutral BDEs from the literature, the thermochemistry determined here also provides IE(CeO) = 5.03 ± 0.12 eV and IE(CeC) = 6.50 ± 0.16 eV. Theoretical calculations for ground and some excited states were performed for CeO⁺, CeC⁺, and CeCO⁺ to provide insight into the bonding and a comparison with the experimentally determined BDEs for each molecule.