Potential energy surface for the reaction Sm⁺ + CO₂ → SmO⁺ + CO: guided ion beam and theoretical studies

Abstract

The potential energy surface (PES) for the oxidation of samarium cations by carbon dioxide is explored both experimentally and theoretically. Using guided ion beam tandem mass spectrometry, several reactions are examined as a function of kinetic energy. These include the title reaction as well as its reverse along with the collision-induced dissociation of Sm(CO) and OSm(CO) with Xe. Analysis of the kinetic energy dependent cross sections yields barriers for the forward and reverse oxidation reaction of 1.77 ± 0.11 and 2.04 ± 0.13 eV, respectively, and Sm-OCO and OSm-CO bond dissociation energies (BDEs) of 0.42 ± 0.03 and 0.97 ± 0.07 eV, respectively. BDEs for Sm(CO) for x = 2 and 3 are also determined as 0.40 ± 0.13 and 0.48 ± 0.12 eV, respectively. The PESs for the title reaction along the sextet and octet spin surfaces are also examined theoretically at the MP2 and CCSD(T) levels using both effective core potential and all-electron basis sets. Reasonable agreement between theory and experiment is obtained for the experimentally characterized intermediates, although all-electron basis sets and spin-orbit effects are needed for quantitative agreement. The observed barrier for oxidation is shown to likely correspond to the energy of the crossing between surfaces corresponding to the ground state electronic configuration of Sm (F,4f6s) and an excited surface having two electrons in the valence space (excluding 4f), which are needed to form the strong SmO bond.

Periodic trends in gas‐phase oxidation and hydrogenation reactions of lanthanides and 5d transition metal cations

“…In all cases but Al + , endothermic reactions forming MCO + + O are also observed and, in most cases, formation of MO 2 + + C (not Al + , Cu + , or Y + ) is also observed. These types of behaviors are also illustrated in more recent GIBMS work on reactions of CO 2 with Pr + (Ghiassee et al, 2021), Nd + (Ghiassee et al, 2019), Sm + (Armentrout & Cox, 2017), Gd + (Demireva & Armentrout, 2018c), and Pt + (Zhang & Armentrout, 2003a). Results from these processes are shown in Figure 5, which does not include results for the MO 2 + product.…”

“…Further, a correlation using a slope of negative unity (dashed line) provides almost as good a reproduction of the E p (5d 2 ) data. Calculations on LnO + clearly show that these molecules have a triple bond (1σ 2 1π 4 ) formed by combinations of the 2p orbitals (with four electrons) on oxygen with 5d orbitals (and two electrons) on the lanthanide cation (Armentrout & Cox, 2017; Demireva et al, 2016; Ghiassee et al, 2019; Ghiassee et al, 2021). There is little 6s character in the sigma bond, consistent with the correlation to the 5d 2 electronic configuration.…”

“…Cross sections for reactions of CO 2 to form MO + (solid symbols) and MCO + (open symbols, x10 except for PtCO + ) with Pr + (blue squares) (Ghiassee et al, 2021), Nd + (pink diamonds) (Ghiassee et al, 2019), Sm + (purple triangles) (Armentrout & Cox, 2017), Gd + (green inverted triangles) (Demireva & Armentrout, 2018c), and Pt + (red inverted triangles) (Zhang & Armentrout, 2003a), as measured using GIBMS. The arrow indicates the BDE of CO 2 at 5.45 eV.…”

“…In the case of Sm + , the SmO + + CO → Sm + + CO 2 reaction was also observed to be endothermic, with a threshold of 2.04 ± 0.13 eV. The difference between the thresholds for the forward (1.77 eV) and reverse (2.04 eV) reactions is equivalent to the exothermicity of reaction 4 for Sm + , 0.27 ± 0.07 eV (Armentrout & Cox, 2017). Here, theoretical calculations indicate that the barrier observed for reaction 4 with Sm + corresponds to the crossing point between the octet spin surface evolving from reactants to the sextet spin surface leading to products, calculated as 1.5 eV.…”

“…Because the exothermic spinforbidden processes are inhibited, spin-allowed reactions, which can be more efficient because no curve crossings are required, can then be observed at higher energies. For example, calculations at multiple levels of theory indicate that the lowest energy MO + states having the same spin as the M + reactant should have thresholds of 0.5-1.5 eV for Pr + (Ghiassee et al, 2021), 0.6-1.8 eV for Nd + (Ghiassee et al, 2019), and 0.6-1.1 eV for Gd + F I G U R E 5 Cross sections for reactions of CO 2 to form MO + (solid symbols) and MCO + (open symbols, x10 except for PtCO + ) with Pr + (blue squares) (Ghiassee et al, 2021), Nd + (pink diamonds) (Ghiassee et al, 2019), Sm + (purple triangles) (Armentrout & Cox, 2017), Gd + (green inverted triangles) (Demireva & Armentrout, 2018c), and Pt + (red inverted triangles) (Zhang & Armentrout, 2003a), as measured using GIBMS. The arrow indicates the BDE of CO 2 at 5.45 eV.…”