Reactions of Th+ + H2, D2, and HD Studied by Guided Ion Beam Tandem Mass Spectrometry and Quantum Chemical Calculations

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.

Sequential Bond Dissociation Energies of Th+(CO)x, x = 3-6: Guided Ion Beam Collision-Induced Dissociation and Quantum Computational Studies

Collision-induced dissociation (CID) of [Th,xC,xO]⁺, x = 3-6, with Xe is performed using a guided ion beam tandem mass spectrometer (GIBMS). Products are formed exclusively by the loss of CO ligands. Analyses of the kinetic energy-dependent CID product cross sections yield bond dissociation energies (BDEs) of (CO)_x-1Th⁺-CO at 0 K as 1.09 ± 0.05, 0.82 ± 0.07, 0.63 ± 0.05, and 0.70 ± 0.05 eV, respectively. Different structures of [Th,xC,xO]⁺ were explored using various electronic structure methods, and BDEs for CO ligand loss from precursor [Th,xC,xO]⁺ complexes were computed. Both experimental and theoretical results corroborate that the structures of [Th,xC,xO]⁺, x = 3-6, formed experimentally are homoleptic thorium cation carbonyl complexes, Th⁺(CO)_x. The nonmonotonic trend in experimental BDEs is reproduced theoretically, although ambiguities in the spin states of the x = 4-6 complexes (doublet or quartet) remain. BDEs calculated at the coupled cluster with single, double, and perturbative triple excitations (CCSD(T))/cc-pVXZ//B3LYP/cc-PVXZ (X = T and Q) level and a complete basis set (CBS) extrapolation agree reasonably well with the experimental values for all complexes. Thorium oxide ketenylidene carbonyl cations, OTh⁺CCO(CO)_y, y = 1-4, were calculated to be the most stable structures of [Th,xC,xO]⁺, x = 3-6, respectively; however, these are not observed in our experiment. Potential energy profiles (PEPs) having either quartet or doublet spin calculated at the B3LYP/cc-pVQZ level suggest that the failure to observe OTh⁺CCO(CO)_y, y = 1-4, is the result of a barrier corresponding to the C-C bond formation, making the formation of OTh⁺CCO(CO)_y inaccessible kinetically under the present experimental conditions.

Experimental and Computational Description of the Interaction of H and H- with U

The results of ab initio correlated molecular orbital theory electronic structure calculations for low-lying electronic states are presented for UH and UH^- and compared to photoelectron spectroscopy measurements. The calculations were performed at the CCSD(T)/CBS and multireference CASPT2 including spin-orbit effects by the state interacting approach levels. The ground states of UH and UH^- are predicted to be ⁴Ι_9/2 and ⁵Λ₆, respectively. The spectroscopic parameters T_e, r_e, ω_e, ω_ex_e, and B_e were obtained, and potential energy curves were calculated for the low energy Ω states of UH. The calculated adiabatic electron affinity is 0.468 eV in excellent agreement with an experimental value of 0.462 ± 0.013 eV. The lowest vertical detachment energy was predicted to be 0.506 eV for the ground state, and the adiabatic ionization energy (IE) is predicted to be 6.116 eV. The bond dissociation energy (BDE) and heat of formation values of UH were obtained using the IE calculated at the Feller-Peterson-Dixon level. For UH, UH^-, and UH⁺, the BDEs were predicted to be 225.5, 197.9, and 235.5 kJ/mol, respectively. The BDE for UH is predicted to be ∼20% lower in energy than that for ThH. The analysis of the natural bond orbitals shows a significant U⁺H^- ionic component in the bond of UH.

Reactions of Atomic Thorium and Uranium Cations with SF6 Studied by Guided Ion Beam Tandem Mass Spectrometry

The fundamental chemistry of the thorium and uranium fluorides continues to be an area of interest because of the use of thorium and uranium fluoride compounds in nuclear fuel systems. Here, we study the reaction of thorium cations with sulfur hexafluoride for the first time and revisit the reaction of uranium cations with sulfur hexafluoride. By using guided ion beam tandem mass spectrometry, we explore the reaction pathways that become accessible well above thermal energies (E ∼ 0.04 eV). Overall, we find that both Th⁺ and U⁺ react very efficiently with SF₆, approaching the collision limit at both thermal and elevated energies. The primary products observed at low energies include Th_1-3⁺, UF_1-4⁺, and SF_1-4⁺, all of which are formed in barrierless, exothermic processes. SF₅⁺ was also observed, although the pressure dependence of this channel reveals that SF₅⁺ forms exothermically through secondary reactions, which the energy dependences suggest result from reactions between ThF₂⁺ and UF₃⁺ with SF₆. At higher energies, both AnF₃⁺ products are observed to decay to AnF⁺ + F₂, and both SF₄⁺ and SF₂⁺ exhibit cross sections with endothermic features. For both systems, the rise in SF₄⁺ can be attributed to a secondary collision between AnF⁺ with SF₆ on the basis of the pressure dependence of the SF₄⁺ channel at higher energies, and the rise in SF₂⁺ appears to result from the decomposition of SF₃⁺ to SF₂⁺ + F.

Activation of CO2 by Actinide Cations (Th+, U+, Pu+, and Am+) as Studied by Guided Ion Beam and Triple Quadrupole Mass Spectrometry

Reactions of CO₂ with Th⁺ have been studied using guided ion beam tandem mass spectrometry (GIBMS) and with An⁺ (An⁺ = Th⁺, U⁺, Pu⁺, and Am⁺) using triple quadrupole inductively coupled plasma mass spectrometry (QQQ-ICP-MS). Additionally, the reactions ThO⁺ + CO and ThO⁺ + CO₂ were examined using GIBMS. Modeling the kinetic energy-dependent GIBMS data allowed the determination of bond dissociation energies (BDEs) for D₀(Th⁺-O) and D₀(OTh⁺-O) that are in reasonable agreement with previous GIBMS measurements. The QQQ-ICP-MS reactions were studied at higher pressures where multiple collisions between An⁺ and the neutral CO₂ occur. As a consequence, both AnO⁺ and AnO₂⁺ products were observed for all An⁺ except Am⁺, where only AmO⁺ was observed. The relative abundances of the observed monoxides compared to the dioxides are consistent with previous reports of the AnO_n⁺ (n = 1, 2) BDEs. A comparison of the periodic trends of the group 4 transition metal, lanthanide (Ln), and actinide atomic cations in reactions with CO₂ (a formally spin-forbidden reaction for most M⁺ ground states) and O₂ (a spin-unrestricted reaction) indicates that spin conservation plays a minor role, if any, for the heavier Ln⁺ and An⁺ metals. Further correlation of Ln⁺ and An⁺ + CO₂ reaction efficiencies with the promotion energy (E_p) to the first electronic state with two valence d-electrons (E_p(5d²) for Ln⁺ and E_p(6d²) for An⁺) indicates that the primary limitation in the activation of CO₂ is the energetic cost to promote from the electronic ground state of the atomic metal ion to a reactive state.

Interaction of Th with H0/-/+: Combined Experimental and Theoretical Thermodynamic Properties

High-level electronic structure calculations of the low-lying energy electronic states for ThH, ThH^-, and ThH⁺ are reported and compared to experimental measurements. The inclusion of spin-orbit coupling is critical to predict the ground-state ordering as inclusion of spin-orbit switches the coupled-cluster CCSD(T) ordering of the two lowest energy states for ThH and ThH⁺. At the multireference spin-orbit SO-CASPT2 level, the ground states of ThH, ThH^-, and ThH⁺ are predicted to be the ²Δ_3/2, ³Φ₂, and ³Δ₁ states, respectively. The adiabatic electron affinity is calculated to be 0.820 eV, and the vertical detachment energy is calculated to be 0.832 eV in comparison to an experimental value of 0.87 ± 0.02 eV. The observed ThH^- photoelectron spectrum has many transitions, which approximately correlate with excitations of Th⁺ and/or Th. The adiabatic ionization energy of ThH including spin-orbit corrections is calculated to be 6.181 eV. The natural bond orbital results are consistent with a significant contribution of the Th⁺H^- ionic configuration to the bonding in ThH. The bond dissociation energies for ThH, ThH^-, and ThH⁺ using the Feller-Peterson-Dixon approach were calculated to be similar for all three molecules and lie between 259 and 280 kJ/mol.

Quantum chemical study of reaction mechanism between plutonium and nitrogen

The study of the reaction between plutonium and nitrogen is helpful in further understanding the interaction between plutonium and air molecules. Currently, there is no research on the microscopic reaction mechanism of plutonium nitridation reactions. Therefore, the microscopic mechanism of the Pu with N₂ gas phase reaction is explored in this study, based on density functional theory (DFT) using different basis functions. In this paper, the geometry of stationary points on the potential energy surface is optimized. In addition, the transition states are verified by frequency analysis and intrinsic reaction coordination (IRC). Finally, we obtained the reaction potential energy curve and micro reaction pathways. Analysis of the reaction mechanism shows that the reaction of Pu with N₂ has two pathways. Pathway 1 (Pu + N₂ → R1 → TS1 → PuN₂) has a T-shaped transition state and pathway 2 (Pu + N₂ → R2 → TS2 → PuN + N) has an L-shaped transition state. Both transition states have only one imaginary frequency. According to the comparison of the energy at each stagnation point along the two pathways, and the heat energy emitted by the two reaction paths, we found that pathway 1 is the main reaction pathway. The nature of Pu-N bonding evolution along the pathways was studied by atoms in molecules (AIM) and electron localization function (ELF) topological approaches. In order to analyze the role of the plutonium atom 5f orbital in the reaction, the variation in density state along the pathways was measured. Results show that the 5f orbital mainly contributes to the formation of Pu-N bonds, and the influence of temperature on the reaction rate is revealed by calculating the rate constants of the two reaction pathways.

Reactions of U+ with H2, D2, and HD Studied by Guided Ion Beam Tandem Mass Spectrometry and Theory

The kinetic energy-dependent reactions of the atomic actinide uranium cation (U⁺) with H₂, D₂, and HD were examined by guided ion beam tandem mass spectrometry. An average 0 K bond dissociation energy of D₀(U⁺ - H) = 2.48 ± 0.06 eV is obtained by analysis of the endothermic product ion cross sections. Quantum chemistry calculations were performed for comparison with experimental thermochemistry, including high-level CASSCF-CASPT2-RASSI calculations of the spin-orbit corrections. CCSD(T) and the CASSCF levels show excellent agreement with experiment, whereas B3LYP and PBE0 slightly overestimate and the M06 approach badly underestimates the bond energy for UH⁺. Theory was also used to investigate the electronic structures of the reaction intermediates and potential energy surfaces. The experimental product branching ratio for the reaction of U⁺ with HD indicates that these reactions occur primarily via a direct reaction mechanism, despite the presence of a deep-well for UH₂⁺ formation according to theory. The reactivity and hydride bond energy for U⁺ are compared with those for transition metal, lanthanide, and actinide cations, and periodic trends are discussed. These comparisons suggest that the 5f electrons on uranium are largely core and uninvolved in the reactive chemistry.

Guided Ion Beam Studies of the Thorium Monocarbonyl Cation Bond Dissociation Energy and Theoretical Unveiling of Different Isomers of [Th,O,C]+ and Their Rearrangement Mechanism

Threshold collision-induced dissociation (TCID) of the thorium monocarbonyl cation, ThCO⁺, with xenon is performed using a guided ion beam tandem mass spectrometer. The only product observed is Th⁺ resulting from loss of the CO ligand. Analysis of the kinetic energy-dependent cross sections for this CID reaction yields the first experimental determination of the bond dissociation energy (BDE) of Th⁺-CO at 0 K as 0.94 ± 0.06 eV. Calculated BDEs at the CCSD(T) level of theory with cc-pVXZ (X = T and Q) basis sets and a complete basis set (CBS) extrapolation are in good agreement with the experimental result. The Feller-Peterson-Dixon composite coupled-cluster methodology was also applied on both ThCO⁺ and ThCO, with contributions up to CCSDT(Q) and a four-component treatment of spin-orbit coupling effects. The final 0 K Th⁺-CO BDE of 0.94 ± 0.04 eV is in excellent agreement with the current experimental result. The ionization energy of ThCO, as well as the atomization energies and heats of formation for both ThCO and ThCO⁺, is reported at this same level of theory. Complete potential energy profiles of both quartet and doublet spin are also constructed to elucidate the mechanism for the formation and interconversion of different isomers of [Th,O,C]⁺. Chemical bonding patterns in low-lying states of ThCO⁺ and potential energy curves for ThCO⁺ dissociation are also investigated.

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.

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.

Activation of Water by Thorium Cation: A Guided Ion Beam and Quantum Chemical Study

The reaction of atomic thorium cations with deuterated water as a function of kinetic energy from thermal to 10 eV was studied using guided ion beam tandem mass spectrometry. At thermal energies, both ThO⁺ + D₂ and DThO⁺ + D are formed in barrierless exothermic processes and reproduce results in the literature obtained using ion cyclotron resonance mass spectrometry. As the energy is increased, the branching ratio between these two channels changes such that the dominant product changes from ThO⁺ to DThO⁺ and back to ThO⁺, until ThD⁺ + OD is energetically available and is the dominant product channel. To help understand these experimental results, a variety of theoretical approaches were tried and used to establish a potential energy surface, which compares well with previous theoretical studies. Utilizing the theoretical results, the kinetic energy dependent branching ratio between the ThO⁺ + D₂ and DThO⁺ + D channels was calculated using both RRKM and phase space theory (PST). The results indicate that consideration of angular momentum conservation (as in PST) and spin-orbit corrected energies are needed to reproduce experimental results quantitatively. The PST modeling also provides relative energies for the two competing transition states that lead to the primary products, for which theory provides reasonable agreement. Graphical Abstract Note: This data is.

Samarium cation (Sm+) reactions with H2, D2, and HD: SmH+ bond energy and mechanistic insights from guided ion beam and theoretical studies

Guided ion beam tandem mass spectrometry is used to study the reaction of the lanthanide samarium cation (Sm⁺) with H₂ and its isotopologues (HD and D₂) as a function of collision energy. Modeling the resulting energy dependent product ion cross sections from these endothermic reactions yields 2.03 ± 0.06 eV (two standard deviations) for the 0 K bond dissociation energy of SmH⁺. Quantum chemical calculations are performed to determine stabilities of the ground and low-energy states of SmH⁺ for comparison with the experimentally measured thermochemistry. The calculations generally overestimate the SmH⁺ bond energy, but a better agreement between experiment and theory is achieved after correcting for spin-orbit energy contributions, with coupled-cluster with single, double and perturbative triple excitations/complete basis set [CCSD(T)/CBS] results reproducing the experiment well. In the HD reaction, the SmH⁺ product is observed to be favored over the SmD⁺ by about a factor of three, indicating that the reaction proceeds via a direct mechanism with short-lived intermediates. This is consistent with quantum chemical calculations of relaxed potential energy surface scans of SmH₂ ⁺, which show that there is no strongly bound dihydride intermediate. The reactivity and hydride bond energy of Sm⁺, which has a valence electron configuration typical of most lanthanides, are compared with previous results for the lanthanide cations La⁺, Gd⁺, and Lu⁺, which exhibit configurations more closely related to the group 3 metal cations, Sc⁺ and Y⁺. Periodic trends across the lanthanide series and insights into the role of the electronic configurations on hydride bond strength and reactivity with H₂ are discussed.

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.