The bond energy of ReO+: Guided ion-beam and theoretical studies of the reaction of Re+(7S) with O2

Direct spectroscopic evidence for the high-spin state of dioxidomanganese(V)

The spin state of metal centers in many catalytic reactions has been demonstrated to be a rate limiting factor when high-valent metal centers such as manganese are involved. Although numerous manganese(V) complexes, including a few manganese(V) oxo complexes, have been identified, thus far only one of these, [MnVH3 buea(O)], has been directly confirmed to exist in a high spin state. Such a high-spin manganese(V) center may play a crucial role in the dioxygen formation process in the elusive S4 state of the Kok cycle in photosystem II. In this study, we provide direct experimental evidence, using X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS), of gas phase [OMnO]+ as the second known high-spin manganese(V) oxo complex. We conclusively assign the ground state as 3B1 (C2v). Additionally, we provide fingerprint spectra not only for [OMnV O]+, but also for the high-spin hydroxidooxidomanganese(IV) ion [OMnIV OH]+ in its 4A'' (Cs) ground state that is expected to exhibit similar XAS and XMCD spectral signatures to neutral dioxidomanganese(IV).

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.

Guided Ion Beam Studies of the Dy + O → DyO+ + e- Chemi-ionization Reaction Thermochemistry and Dysprosium Oxide, Carbide, Sulfide, Dioxide, and Sulfoxide 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 dysprosium cation (Dy⁺) with O₂, SO₂, and CO and reactions of DyO⁺ with CO, O₂, and Xe. DyO⁺ is formed through an exothermic process when Dy⁺ reacts with O₂, whereas all other processes observed are found to be endothermic. The kinetic energy dependences of these cross sections were analyzed to yield 0 K bond dissociation energies (BDEs) for DyO⁺, DyC⁺, DyS⁺, DyO₂⁺, and DySO⁺. The 0 K BDE for DyO⁺ is determined to be 5.60 ± 0.02 eV from the weighted average of six independent thresholds, which are dominated by the slightly endothermic reaction of Dy⁺ with SO₂. Combined with the well-established Dy ionization energy (IE), this value indicates that the chemi-ionization reaction, Dy + O → DyO⁺ + e^-, is endothermic by 0.33 ± 0.02 eV. Theoretical BDEs for Dy⁺-O, Dy⁺-C, Dy⁺-S, ODy⁺-O, and Dy⁺-SO were calculated at several levels of theory and basis sets for comparison with experiment with reasonable agreement achieved.

Gas-Phase Reactivity of Ozone with Lanthanide Ions (Sm+, Nd+) and Their Higher Oxides

The kinetics of SmO_n⁺ (n = 0-2) and NdO_n⁺ (n = 0-2) with O₃ are measured using a selected-ion flow tube. Reaction of Nd⁺ to yield NdO⁺ + O₂ occurs rapidly, with a rate constant near the capture-controlled limit of ∼8 × 10^-10 cm³ s^-1. NdO⁺ reacts at ∼40% of the capture limit to yield NdO₂⁺ with little temperature dependence from 200 to 400 K. NdO₂⁺ likely reacts very slowly (k ∼ 10^-13 cm³ s^-1) to yield NdO⁺ + 2O₂, does not react to yield NdO₃⁺, and associates slowly (k ∼ 10^-12 cm³ s^-1) to yield NdO₂⁺(O₃)_1-3. Reaction of Sm⁺ also yields SmO⁺ at near the capture limit at all temperatures, but a significant fraction (∼50%) of the SmO⁺ is produced in excited states that are long-lived compared to the millisecond time scale of the experiment. These states are evidently resistant to both radiative and collisional relaxation. The excited-state production is likely due to a spin-conservation constraint on the reaction, despite the large spin-orbit coupling typical for lanthanide-containing species. Ground-state SmO⁺ reacts inefficiently (k = 2 × 10^-11 (T/300)^-2.5 cm³ s^-1) to yield SmO₂⁺ + O₂, while the excited-state SmO^+* reacts at the capture limit, with branching to yield Sm⁺ + 2O₂ (ΔH_r,0K = 148.7 ± 0.4 kJ mol^-1 for ground-state SmO⁺) approximately 60% of the time, the remainder forming SmO₂⁺, which further reacts with O₃ to yield SmO⁺ at about 1% of the collisional value.

Thermochemistry and mechanisms of the Pt+ + SO2 reaction from guided ion beam tandem mass spectrometry and theory

The kinetic energy dependences of the reactions of Pt⁺ (²D_5/2) with SO₂ were studied using a guided ion beam tandem mass spectrometer and theory. The observed cationic products are PtO⁺ and PtSO⁺, with small amounts of PtS⁺, all formed in endothermic reactions. Modeling the kinetic energy dependent product cross sections allows determination of the product bond dissociation energies (BDEs): D₀(Pt⁺-O) = 3.14 ± 0.11 eV, D₀(Pt⁺-S) = 3.68 ± 0.31 eV, and D₀(Pt⁺-SO) = 3.03 ± 0.12 eV. The oxide BDE agrees well with more precise literature values, whereas the latter two results are the first such measurements. Quantum mechanical calculations were performed for PtO⁺, PtS⁺, PtO₂ ⁺, and PtSO⁺ at the B3LYP and coupled-cluster with single, double, and perturbative triple [CCSD(T)] levels of theory using the def2-XZVPPD (X = T, Q) and aug-cc-pVXZ (X = T, Q, 5) basis sets and complete basis set extrapolations. These theoretical BDEs agree well with the experimental values. After including empirical spin-orbit corrections, the product ground states are determined as PtO⁺ (⁴Σ_3/2), PtS⁺ (⁴Σ_3/2), PtO₂ ⁺ (²Σ_g ⁺), and PtSO⁺ (²A'). Potential energy profiles including intermediates and transition states for each reaction were also calculated at the B3LYP/def2-TZVPPD level. Periodic trends in the thermochemistry of the group 9 metal chalcogenide cations are compared, and the formation of PtO⁺ from the Pt⁺ + SO₂ reaction is compared with those from the Pt⁺ + O₂, CO₂, CO, and NO reactions.

Effect of Intersystem Crossings on the Kinetics of Thermal Ion-Molecule Reactions: Ti+ + O2, CO2, and N2O

A selected-ion flow tube apparatus has been used to measure rate constants and product branching fractions of ²Ti⁺ reacting with O₂, CO₂, and N₂O over the range of 200-600 K. Ti⁺ + O₂ proceeds at near the Langevin capture rate constant of 6-7 × 10^-10 cm³ s^-1 at all temperatures to yield ⁴TiO⁺ + O. Reactions initiated on doublet or quartet surfaces are formally spin-allowed; however, the 50% of reactions initiated on sextet surfaces must undergo an intersystem crossing (ISC). Statistical theory is used to calculate the energy and angular momentum dependences of the specific rate constants for the competing isomerization and dissociation channels. This acts as an internal clock on the lifetime to ISC, setting an upper limit on the order of τ_ISC < 1e^-11 s. ²Ti⁺ + CO₂ produces ⁴TiO⁺ + CO less efficiently, with a rate constant fit as 5.5 ± 1.3 × 10^-11 (T/300)^{-1.1 ± 0.2} cm³ s^-1. The reaction is formally spin-prohibited, and statistical modeling shows that ISC, not a submerged transition state, is rate-limiting, occurring with a lifetime on the order of 10^-7 s. Ti⁺ + N₂O proceeds at near the capture rate constant. In this case, both Ti⁺ON₂ and Ti⁺N₂O entrance channel complexes are formed and can interconvert over a barrier. The main product is >90% TiO⁺ + N₂, and the remainder is TiN⁺ + NO. Both channels need to undergo ISC to form ground-state products but TiO⁺ can be formed in an excited state exothermically. Therefore, kinetic information is obtained only for the TiN⁺ channel, where ISC occurs with a lifetime on the order of 10^-9 s. Statistical modeling indicates that the dipole-preferred Ti⁺ON₂ complex is formed in ∼80% of collisions, and this value is reproduced using a capture model based on the generic ion-dipole + quadrupole long-range potential.

Activation of D2 by Neodymium Cation (Nd+): Bond Energy of NdH+ and Mechanistic Insights through Experimental and Theoretical Studies

The kinetic-energy-dependent cross section for the reaction of Nd⁺ with D₂ was studied by using a guided ion beam tandem mass spectrometer. The formation of NdD⁺ is endothermic, and analysis of the reaction cross section gave an NdH⁺ 0 K bond dissociation energy (BDE) of 1.99 ± 0.06 eV. Theoretical calculations for the NdH⁺ BDE were performed for comparison with the experimental thermochemistry and generally gave accurate results. Additionally, relaxed potential energy surfaces for NdH₂⁺ were performed, and no strongly bound dihydride intermediate was located. The Nd⁺ + D₂ reactivity and BDE of NdH⁺ are compared with analogous results for the lanthanide cations La⁺, Ce⁺, Pr⁺, Sm⁺, Gd⁺, and Lu⁺ to establish periodic trends and insight into the role of the electronic configurations on this reactivity and the lanthanide hydride cation bond strengths.

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.

Guided Ion Beam Tandem Mass Spectrometry and Theoretical Study of SO2 Activated by Os. J Phys Chem A 2020;124:6629-6644. [PMID: 32702982 DOI: 10.1021/acs.jpca.0c05757]

The kinetic-energy dependence of SO₂ activated by Os⁺ was studied by guided ion beam tandem mass spectrometry. Species observed in endothermic reactions were OsO⁺, OsO₂⁺ or OsS⁺, and OOsS⁺. The kinetic energy-dependent cross sections were modeled to yield 0 K bond dissociation energies (BDEs) of 5.01 ± 0.06 eV (Os⁺-O), 5.15 ± 0.07 eV (Os⁺-O₂), 4.50 ± 0.17 eV (Os⁺-S), and 4.22 ± 0.11 eV (Os⁺-SO). Among these BDE values, the values for OsO⁺ and OsO₂⁺ agree with literature values and those for OsS⁺ and OOsS⁺ are novel measurements. Theoretical calculations were performed at a B3LYP/def2-TZVPPD level for all products, and additional calculations were performed for OsS⁺, OsO₂⁺, and OsSO⁺ using the CCSD(T) level of theory, extrapolated to the complete basis set (CBS) limit, and def2-QZVPPD and aug-cc-pVxZ (x = T, Q, and 5) basis sets. These calculations indicate that the ground states of the products are ⁴Π_5/2 (OsO⁺), ²B₁ (OsO₂⁺), ⁴Π_5/2 (OsS⁺), and ²A″ (OOsS⁺) after including empirical spin-orbit corrections. The potential energy surfaces (PESs) for OsSO₂⁺ intermediates, transition states, and all products were also investigated at the B3LYP/def2-TZVPPD level. The PESs show that none of the reactions have barriers in excess of the product endothermicities. Cross sections for OsO⁺ formation are compared to those from previous guided ion beam studies of related systems (Os⁺ + O₂ and CO and Re⁺ + SO₂) to evaluate their relative behaviors.

Cerium Cation (Ce+) Reactions with H2, D2, and HD: CeH+ Bond Energy and Mechanistic Insights from Guided Ion Beam and Theoretical Studies

Reactions of the atomic lanthanide cerium cation (Ce⁺) with H₂, D₂, and HD were studied by using guided ion beam tandem mass spectrometry. Analysis of the kinetic-energy-dependent endothermic reactions to form CeH⁺ (CeD⁺) led to a 0 K bond dissociation energy (BDE) for CeH⁺ of 2.19 ± 0.09 eV. Theoretical calculations for CeH⁺ were performed at the B3LYP, BHLYP, and PBE0 levels of theory and overestimate the experimental BDE. In contrast, extrapolation to the complete basis set limit using coupled-cluster with single, double, and perturbative triple excitations, CCSD(T), gave a value (2.33 eV) in reasonable agreement with the experimental BDE. The branching ratio of the CeH⁺ and CeD⁺ products in the HD reaction suggests that the reaction occurs via a statistical mechanism involving a long-lived intermediate. Relaxed potential energy surfaces for CeH₂⁺ were computed and are consistent with the availability of such an intermediate, but the crossing point between quartet and doublet surfaces helps explain the inefficiency of the association reaction observed in the literature. The reactivity and CeH⁺ BDE are compared with previous results for group 4 transition metal cations (Ti⁺, Zr⁺, and Hf⁺), other lanthanides (La⁺, Sm⁺, Gd⁺, and Lu⁺), and the isovalent actinide Th⁺. Periodic trends and insight into the role of the electronic configuration on metal-hydride bond strength are discussed.

Guided Ion Beam and Quantum Chemical Investigation of the Thermochemistry of Thorium Dioxide Cations: Thermodynamic Evidence for Participation of f Orbitals in Bonding

Kinetic energy dependent reactions of ThO⁺ with O₂ are studied using a guided ion beam tandem mass spectrometer. The formation of ThO₂⁺ in the reaction of ThO⁺ with O₂ is observed to be slightly endothermic and also exhibits two obvious features in the cross section. These kinetic energy dependent cross sections were modeled to determine a 0 K bond dissociation energy of D₀(OTh⁺-O) = 4.94 ± 0.06 eV. This value is slightly larger but within experimental uncertainty of less precise previously reported experimental values. The higher energy feature in the ThO₂⁺ cross section was also analyzed and suggests formation of an excited state of the product ion lying 3.1 ± 0.2 eV above the ground state. Additionally, the thermochemistry of ThO₂⁺ was explored by quantum chemical calculations, including a full Feller-Peterson-Dixon (FPD) composite approach with correlation contributions up to CCSDT(Q) and four-component spin-orbit corrections, as well as more approximate CCSD(T) calculations including semiempirical estimates of spin-orbit energy contributions. The FPD approach predicts D₀(OTh⁺-O) = 4.87 ± 0.04 eV, in good agreement with the experimental value. Analogous FPD results for ThO⁺, ThO, and ThO₂ are also presented, including ionization energies for both ThO and ThO₂. The ThO₂⁺ bond energy is larger than those of its transition metal congeners, TiO₂⁺ and ZrO₂⁺, which can be attributed partially to an actinide contraction, but also to contributions from the participation of f orbitals on thorium that are unavailable to the transition metal systems.

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.

Activation of H2 by Gadolinium Cation (Gd+): Bond Energy of GdH+ and Mechanistic Insights from Guided Ion Beam and Theoretical Studies

The energy-dependent reactions of the lanthanide gadolinium cation (Gd⁺) with H₂, D₂, and HD are investigated using guided ion beam tandem mass spectrometry. From analysis of the resulting endothermic product ion cross sections, the 0 K bond dissociation energy for GdH⁺ is measured to be 2.18 ± 0.07 eV. Theoretical calculations on GdH⁺ are performed for comparison with the experimental thermochemistry and generally appear to overestimate the experimental GdH⁺ bond dissociation energy. The branching ratio of the products in the HD reaction suggests that Gd⁺ reacts primarily via a statistical insertion mechanism to form the hydride product ion with contributions from direct mechanisms. Relaxed potential energy surfaces for GdH₂⁺ are computed and are consistent with the availability of both statistical and direct reaction pathways. The reactivity and hydride bond energy for Gd⁺ is compared with previous results for the group three metal cations, Sc⁺ and Y⁺, and the lanthanides, La⁺ and Lu⁺, and periodic trends are discussed.

Guided ion beam and theoretical studies of the bond energy of SmS. J Chem Phys 2017;147:214307. [PMID: 29221388 DOI: 10.1063/1.5009916]

Previous work has shown that atomic samarium cations react with carbonyl sulfide to form SmS⁺ + CO in an exothermic and barrierless process. To characterize this reaction further, the bond energy of SmS⁺ is determined in the present study using guided ion beam tandem mass spectrometry. Reactions of SmS⁺ with Xe, CO, and O₂ are examined. Results for collision-induced dissociation processes with all three molecules along with the endothermicity of the SmS⁺ + CO → Sm⁺ + COS exchange reaction are combined to yield D₀(Sm⁺-S) = 3.37 ± 0.20 eV. The CO and O₂ reactions also yield a SmSO⁺ product, with measured endothermicities that indicate D₀(SSm⁺-O) = 3.73 ± 0.16 eV and D₀(OSm⁺-S) = 1.38 ± 0.27 eV. The SmS⁺ bond energy is compared with theoretical values characterized at several levels of theory, including CCSD(T) complete basis set extrapolations using all-electron basis sets. Multireference configuration interaction calculations with explicit spin-orbit calculations along with composite thermochemistry using the Feller-Peterson-Dixon method and all-electron basis sets were also explored for SmS⁺, and for comparison, SmO, SmO⁺, and EuO.

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

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⁺(CO₂) 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⁺(CO₂)_x 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⁺ (⁸F,4f⁶6s¹) and an excited surface having two electrons in the valence space (excluding 4f), which are needed to form the strong SmO⁺ bond.

Gadolinium cation (Gd+) reaction with O2: Potential energy surface mapped experimentally and with theory

Guided ion beam tandem mass spectrometry is used to measure the kinetic energy dependent cross sections for reactions of the lanthanide metal gadolinium cation (Gd⁺) and GdO⁺ with O₂ and for collision-induced dissociation (CID) of GdO₂⁺ with Xe. Gd⁺ reacts with O₂ in an exothermic and barrierless reaction to form GdO⁺ and O. GdO₂⁺ is also formed in this reaction, but this product ion is formed in a sequential reaction, as verified by pressure dependent measurements and comparison with the results for the reaction of GdO⁺ with O₂. The CID experiments of GdO₂⁺ indicate the presence of two GdO₂⁺ precursor ion populations, assigned to a weakly bound oxygen molecule adduct (Gd⁺-O₂) and an inserted cyclic Gd⁺ dioxide species (O-Gd⁺-O). Analysis of the resulting product ion cross sections yields bond dissociation energies (BDEs, D₀) for Gd⁺-O₂ and OGd⁺-O, where the latter BDE is also independently measured in an exchange reaction between GdO⁺ and O₂. The CID experiments also provide the energy of the barrier for the rearrangement of the Gd⁺-O₂ adduct to the inserted O-Gd⁺-O structure (as identified by loss of a single oxygen atom). The thermochemistry measured here yields D₀(OGd⁺-O) = 2.86 ± 0.08 eV, D₀(Gd⁺-O₂) = 0.75 ± 0.11 eV, and a barrier height relative to Gd⁺-O₂ of 0.31 ± 0.07 eV. These data are sufficient to characterize in some detail the potential energy surface of the Gd⁺ reaction with O₂ entirely from experiment. Theoretical calculations are performed for comparison with the experimental energetics and for further insight into the reaction mechanisms.

Guided ion beam and theoretical studies of the reactions of Re+, Os+, and Ir+ with CO

The kinetic-energy dependences of the reactions M⁺ + CO where M⁺ = Re⁺, Os⁺, and Ir⁺ are studied using guided ion-beam tandem mass spectrometry. Formation of both MO⁺ and MC⁺ was observed in endothermic processes for all three metals. Modeling of the data provides thresholds that yield 0 K bond dissociation energies (BDEs, in eV) of 4.67 ± 0.09 (Re⁺-O), 4.82 ± 0.14 (Os⁺-O), 4.25 ± 0.11 (Ir⁺-O), 5.13 ± 0.12 (Re⁺-C), 6.14 ± 0.14 (Os⁺-C), and 6.58 ± 0.12 (Ir⁺-C). These BDEs agree well with literature values within experimental uncertainties demonstrating that ground state products are formed for all cases even though some of the reactions are formally spin forbidden. Quantum mechanical calculations at several levels of theory and using several basis sets were performed for MC⁺ and MO⁺ (with comparable results taken from the literature in some cases). B3LYP and CCSD(T) calculated ground state BDEs agree reasonably well with experimental values. The ground states in B3LYP and CCSD(T)/CBS calculations are Σ-3 (ReC⁺), Δ2 (OsC⁺), and Σ+1 or Δ3 (IrC⁺) after including spin-orbit considerations. Relaxed potential energy surfaces (PESs) for the M⁺ + CO reactions show crossings between surfaces of different spin states such that products can be formed with no barriers in excess of the substantial endothermicities. Unlike results for these metal cations reacting with O₂, the kinetic energy dependent cross sections for the formation of MO⁺ in the M⁺ + CO reactions exhibit only one feature. Reasons for this differential behavior are discussed in detail.

Gadolinium (Gd) Oxide, Carbide, and Carbonyl Cation Bond Energies and Evaluation of the Gd + O → GdO+ + e- Chemi-Ionization Reaction Enthalpy

Guided ion beam mass spectrometry (GIBMS) is used to measure the kinetic energy dependent product ion cross sections for reactions of the lanthanide metal gadolinium cation (Gd⁺) with O₂, CO₂, and CO and for reactions of GdO⁺ with CO, O₂, and Xe. GdO⁺ is formed through barrierless and exothermic processes in the reactions of Gd⁺ with O₂ and CO₂. All other reactions observed are endothermic, and analyses of their kinetic energy dependent cross sections yield 0 K bond dissociation energies (BDEs) for GdO⁺, GdC⁺, and GdCO⁺. The 0 K BDE for GdO⁺ is determined from five different reactions to be 7.69 ± 0.10 eV, and this value is combined with literature data to derive the ionization energy (IE) of GdO as 5.82 ± 0.16 eV. Additionally, GdC⁺ and GdCO⁺ BDEs of 3.18 ± 0.18 eV and 0.65 ± 0.06 eV are obtained from analysis of the Gd⁺ reactions with CO and CO₂, respectively. Theoretical GdO⁺, GdC⁺, and GdCO⁺ BDEs are calculated for comparison with experiment using various Gd basis sets with an effective core potential and several levels of theory. For calculations that correctly predict a ¹⁰D ground state for Gd⁺, good agreement between theoretical and measured GdC⁺ and GdCO⁺ BDEs is obtained, whereas the GdO⁺ BDE is underestimated in these calculations by about 0.8 eV. Additional BDEs for GdO⁺ and GdC⁺ are calculated using triple- and quadruple-ζ correlation consistent all-electron basis sets for Gd. Calculations with these basis sets provide better agreement with experiment for GdO⁺ but not for GdC⁺. The measured Gd⁺ oxide, carbide, and carbonyl BDEs are similar to those for the group 3 metal ions, Sc⁺ and Y⁺. This is attributed to similarities in the ground state electronic configurations of these metal ions leading to similar interaction strengths. The experimental GdO⁺ BDE measured here combined with the known IE of Gd is used to determine an exothermicity of 1.54 ± 0.10 eV for the Gd chemi-ionization reaction with atomic oxygen. This value is consistent with but more precise than previous literature values.