Lanthanides as Catalysts: Guided Ion Beam and Theoretical Studies of Sm+ + COS

A guided ion beam investigation of UO2+ thermodynamics and f orbital participation: Reactions of U+ + CO2, UO+ + O2, and UO+ + CO

A guided ion beam tandem mass spectrometer was employed to study the reactions of U+ + CO2, UO+ + O2, and the reverse of the former, UO+ + CO. Reaction cross sections as a function of kinetic energy over about a three order of magnitude range were studied for all systems. The reaction of U+ + CO2 proceeds to form UO+ + CO with an efficiency of 118% ± 24% as well as generating UO2+ + C and UCO+ + O. The reaction of UO+ + O2 forms UO2+ in an exothermic, barrierless process and also results in the collision-induced dissociation of UO+ to yield U+. In the UO+ + CO reaction, the formation of UO2+ in an endothermic process is the dominant reaction, but minor products of UCO+ + O and U+ + (O + CO) are also observed. Analysis of the kinetic energy dependences observed provides the bond energies, D0(U+-O) = 7.98 ± 0.22 and 8.05 ± 0.14 eV, D0(U+-CO) = 0.73 ± 0.13 eV, and D0(OU+-O) = 7.56 ± 0.12 eV. The values obtained for D0(U+-O) and D0(OU+-O) agree well with the previously reported literature values. To our knowledge, this is the first experimental measurement of D0(U+-CO). An analysis of the oxide bond energies shows that participation of 5f orbitals leads to a substantial increase in the thermodynamic stability of UO2+ relative to ThO2+ and especially transition metal dioxide cations.

Quantitative Aspects of Gas-Phase Metal Ion Chemistry: Conservation of Spin, Participation of f Orbitals, and C-H Activation and C-C Coupling

In this Featured Article, I reflect on over 40 years of guided ion beam tandem mass spectrometry (GIBMS) studies involving atomic metal cations and their clusters throughout the periodic table. Studies that have considered the role of spin conservation (or lack thereof) are a primary focus with a quantitative assessment of the effects examined. A need for state-specific studies of heavier elements is noted, as is a more quantitative assessment of spin-orbit interactions in reactivity. Because GIBMS experiments explicitly evaluate the kinetic energy dependence of reactions over a wide range, several interesting and unusual observations are highlighted. More detailed studies of such unusual reaction events would be welcome. Activation of C-H bonds and ensuing C-C coupling events are reviewed, with future work encouraged. Finally, studies of lanthanides and actinides are examined with an eye on understanding the role of f orbitals in the chemistry, both as participants (or not) in the bonding and as sources/sinks of electron density. This area seems to be ripe for more quantitative experiments.

Electronic Spectroscopy of Jet-Cooled NdO

Chemi-ionization reactions of the type M + O → MO⁺ + e^- (M = Nd or Sm) are currently being investigated as a method to artificially increase the electron density in the ionosphere for control of micro- and radio wave propagation. Experiments involving the release of atomic Nd into the upper atmosphere have resulted in the production of a cloud that, on excitation by solar radiation, emits green light. It has been assumed that NdO was the carrier of this emission, but the existing spectroscopic data needed for this attribution is lacking. While the electronic spectrum of NdO has been well-characterized at wavelengths greater than 590 nm, relatively little spectroscopic data exist for emission wavelengths in the blue-green spectral range. In this study, spectra for jet-cooled NdO were recorded over the range 15,500-21,000 cm^-1. Rotationally resolved laser induced fluorescence and vibronically resolved dispersed laser-induced fluorescence spectra were recorded, and nine new electronically excited states were identified. The data indicate that the electronic spectrum of NdO has relatively few allowed transitions in the green spectral range, casting doubt on the assignment of the Nd high-altitude release cloud green emission to NdO.

Electronic Spectroscopy of SmO in the 645 to 670 nm Range

The associative ionization reaction Sm + O → SmO⁺ + e^- is being investigated as an electron source that could transiently modify high-altitude electron densities via Sm vapor release. Electronic spectra have been obtained from tests where sounding rockets released Sm vapor, but the interpretation of these results has been hampered by the limited laboratory spectral data available for both SmO and SmO⁺. The present study extends the spectroscopic characterization of SmO in the 645-670 nm range, where the field data show the most prominent molecular emission features. Rotationally resolved excitation spectra, dispersed laser-induced fluorescence spectra, and fluorescence decay lifetimes are reported. The results are consistent with the assignment of a subset of the red-region bands to configurational transitions of the form Sm²⁺(4f⁵6s)O^2- ↔ Sm²⁺(4f⁵5d)O^2-. Analysis of the excited state hyperfine structure supports this configurational description.

Thermochemistry of the Ir+ + SO2 reaction using guided ion beam tandem mass spectrometry and theory

Kinetic energy dependences of the reactions of Ir⁺ (⁵F₅) with SO₂ were studied using a guided ion beam tandem mass spectrometer and theory. The observed cationic products are IrO⁺, IrS⁺, and IrSO⁺, formed in endothermic reactions. Bond dissociation energies (BDEs) of the products are determined by modeling the kinetic energy dependent product cross sections: D₀(Ir⁺-O) = 4.27 ± 0.11 eV, D₀(Ir⁺-S) = 4.03 ± 0.06 eV, and D₀(Ir⁺-SO) ≥ 2.95 ± 0.06 eV. The oxide BDE agrees well with literature values, whereas the two latter results are novel measurements. Quantum mechanical calculations are performed at the B3LYP level of theory using the def2-TZVPPD basis set for all product BDEs with additional calculations for IrS⁺, IrO₂ ⁺, and IrSO⁺ at the coupled cluster with single, double, and perturbative triple excitation levels with def2-QZVPPD and aug-cc-pVXZ (X = T and Q and for IrS⁺, also X = 5) basis sets and complete basis set extrapolations. These theoretical BDEs agree reasonably well with the experimental values. ¹A₁ (IrO₂ ⁺), ⁵Δ₄ (IrS⁺), and ³A″/¹A' (IrSO⁺) are found to be the ground states after including empirical spin-orbit corrections. The potential energy surfaces including intermediates and transition states for each reaction are also calculated at the B3LYP/def2-TZVPPD level. The formation of MO⁺ (M = Re, Os, and Ir) from M⁺ + SO₂ reactions is compared with those from the M⁺ + O₂ and M⁺ + CO reactions, where interesting trends in cross sections are observed. Overall, these studies suggest that the M⁺ + O₂ reactions had restrictions associated with reactions along A' and A″ surfaces.

Evaluation of the Pr + O → PrO+ + e- chemi-ionization reaction enthalpy and praseodymium oxide, carbide, dioxide, and carbonyl cation bond energies

Guided ion beam tandem mass spectrometry (GIBMS) was used to measure the kinetic energy dependent product ion cross sections for reactions of the lanthanide metal praseodymium cation (Pr+) with O2, CO2, and CO and reactions of PrO+ with CO, O2, and Xe. PrO+ is formed through barrierless exothermic processes when the atomic metal cation reacts with O2 and CO2, whereas all other reactions are observed to be endothermic. Analyses of the kinetic energy dependences of these cross sections yield 0 K bond dissociation energies (BDEs) for PrO+, PrC+, PrCO+, and PrO2+. The 0 K BDE for PrO+ is determined to be 7.62 ± 0.09 eV from the weighted average of five independent thresholds. This value is combined with the well-established ionization energy (IE) of Pr to indicate an exothermicity of the chemi-ionization reaction, Pr + O → PrO+ + e-, of 2.15 ± 0.09 eV. Additionally, BDEs of Pr+-C, OPr+-O, and Pr+-CO are determined to be 2.97 ± 0.10. 2.47 ± 0.11, and 0.31 ± 0.07 eV. Theoretical Pr+-O, Pr+-C, OPr+-O, and Pr+-CO BDEs are calculated for comparison with experimental values. The Pr+-O BDE is underestimated at the B3LYP and PBE0 level of theory but better agreement is obtained using the coupled-cluster with single, double, and perturbative triple excitations, CCSD(T), level. Density functional theory approaches yield better agreement for the BDEs of Pr+-C, OPr+-O, and Pr+-CO.

Thermochemical studies of reactions of Re+ with SO2 using guided ion beam experiments and theory

The kinetic energy dependent reactions of Re⁺ with SO₂ were studied with guided ion beam tandem mass spectrometry. ReO⁺, ReO₂⁺, and OReS⁺ species were observed as products, all in endothermic reactions. Modeling of the kinetic energy dependent cross sections yields 0 K bond dissociation energies (BDEs, in eV) of 4.78 ± 0.06 (Re⁺-O), 5.75 ± 0.02 (Re⁺-O₂), and 4.35 ± 0.14 (Re⁺-SO). The latter two values can be combined with other information to derive the additional values 6.05 ± 0.05 (ORe⁺-O) and 4.89 ± 0.19 (ORe⁺-S). BDEs of ReO⁺ and ReO₂⁺ agree with literature values whereas the values for OReS⁺ are the first measurements. The former result is obtained even though formation of ground state ReO⁺ is spin-forbidden. Quantum mechanical calculations at the B3LYP level of theory with a def2-TZVPPD basis set yield results that agree reasonably well with experimental values. Additional calculations at the BP86 and CCSD(T) levels of theory using def2-QZVPPD and aug-cc-pVxZ (x = T, Q, and 5) basis sets were performed to compare thermochemistry with experiment to determine that ReO₂⁺ rather than the isobaric ReS⁺ is formed. Product ground states are ³Δ₃(ReO⁺), ³B₁(OReO⁺), ⁵Π_-1(ReS⁺), and ³A''(OReS⁺) after including empirical spin-orbit corrections, which means that formation of ground state products is spin-forbidden for all three product channels. The potential energy surfaces for the ReSO₂⁺ system were also explored at the B3LYP/def2-TZVPPD level and exhibited no barriers in excess of the endothermicities for all products. BDEs for rhenium oxide and sulfide diatomics and triatomics are compared and discussed. The present result for formation of ReO⁺ is compared to that for formation of ReO⁺ in the reactions of Re⁺ + O₂ and CO, where the former system exhibited interesting dual cross section features. Results are consistent with the hypothesis that the distinction of in-plane and out-of-plane C_S symmetry in the triatomic systems might be the explanation for the two endothermic features observed in the Re⁺ + O₂ reaction.

Evaluation of the exothermicity of the chemi-ionization reaction Nd + O → NdO+ + e- and neodymium oxide, carbide, dioxide, and carbonyl cation bond energies

The exothermicity of the chemi-ionization reaction, Nd + O → NdO⁺ + e^-, has been indirectly determined by measuring the thermochemistry for reactions of the lanthanide metal neodymium cation (Nd⁺) with O₂, CO₂, and CO and reactions of NdO⁺ with CO, O₂, and Xe. Guided ion beam tandem mass spectrometry was used to measure the kinetic energy dependent product ion cross sections for these reactions. NdO⁺ is formed through a barrierless exothermic process when the atomic metal cation reacts with O₂ and CO₂. All other reactions are observed to be endothermic. Analyses of the kinetic energy dependences of these cross sections yield 0 K bond dissociation energies (BDEs) for several species. The 0 K BDE for Nd⁺-O is determined to be 7.28 ± 0.10 eV from the average of four independent thresholds. This value is combined with the well-established Nd ionization energy to indicate an exothermicity of the title reaction of 1.76 ± 0.10 eV, which is lower and more precise than literature values. In addition, the Nd⁺-C, ONd⁺-O, and Nd⁺-CO BDEs are determined to be 2.61 ± 0.30, 2.12 ± 0.30, and 0.30 ± 0.21 eV. Additionally, theoretical BDEs of Nd⁺-O, Nd⁺-C, ONd⁺-O, and Nd⁺-CO are calculated at several levels for comparison with the experimental values. B3LYP calculations seriously underestimate the Nd⁺-O BDE, whereas MP2 and coupled-cluster with single, double-and perturbative triple excitations values are in reasonable agreement. Good agreement is generally obtained for Nd⁺-C, ONd⁺-O, and Nd⁺-CO BDEs as well.