Activation of CO₂ by Gadolinium Cation (Gd⁺): Energetics and Mechanism from Experiment and Theory

Abstract

The exothermic and barrierless activation of CO₂ by the lanthanide gadolinium cation (Gd⁺) to form GdO⁺ and CO is investigated in detail using guided ion beam tandem mass spectrometry (GIBMS) and theory. Kinetic energy dependent product ion cross sections from collision-induced dissociation (CID) experiments of GdCO₂ ⁺ are measured to determine the energetics of OGd⁺(CO) and Gd⁺(OCO) intermediates. Modeling these cross sections yields bond dissociation energies (BDEs) for OGd⁺–CO and Gd⁺–OCO of 0.57 ± 0.05 and 0.38 ± 0.05 eV, respectively. The OGd⁺–CO BDE is similar to that previously measured for Gd⁺–CO, which can be attributed to the comparable electrostatic interaction with CO in both complexes. The Gd⁺(OCO) adduct is identified from calculations to correspond to an electronically excited state. The thermochemistry here and the recently measured GdO⁺ BDE allows for the potential energy surface (PES) of the Gd⁺ reaction with CO₂ to be deduced from experiment in some detail. Theoretical calculations are performed for comparison with the experimental thermochemistry and for insight into the electronic states of the GdCO₂ ⁺ intermediates, transition states, and the reaction mechanism. Although the reaction between ground state Gd⁺ (¹⁰D) and CO₂ (¹Σ_g ⁺) reactants to form ground state GdO⁺ (⁸Σ⁻) and CO (¹Σ⁺) products is formally spin-forbidden, calculations indicate that there are octet and dectet surfaces having a small energy gap in the entrance channel, such that they can readily mix. Thereby, the reaction can efficiently proceed along the lowest energy octet surface to yield ground state products, consistent with the experimental observations of an efficient, barrierless process. At high collision energies, the measured GdO⁺ cross section from the Gd⁺ reaction with CO₂ exhibits a distinct feature, attributed to formation of electronically excited GdO⁺ products along a single dectet PES in a diabatic and spin-allowed process. Modeling this high-energy feature gives an excitation energy of 3.25 ± 0.16 eV relative to the GdO⁺ (⁸Σ⁻) ground state, in good agreement with calculated excitation energies for GdO⁺ (¹⁰Π, ¹⁰Σ⁻) electronic states. The reactivity of Gd⁺ with CO₂ is compared with the group 3 transition metal cations and other lanthanide cations and periodic trends are discussed.