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

Spectroscopic and Theoretical Insights into H2 Activation on Uranium Monoxide: Homolytic H2 Cleavage Mediated by Intermediate OU(η2-H2)

Elucidating molecular-level interactions between dihydrogen (H2) and uranium oxides reveals fundamental insights into the intrinsic H2 activation mechanisms underlying processes such as heterogeneous catalysis over uranium oxides and corrosion of uranium induced by H2. Herein, the reactions of H2 with uranium monoxide (UO) molecules have been investigated via a combination of matrix-isolation infrared spectroscopy and quantum chemical calculations. A side-on bonded H2 complex, OU(η2-H2), is identified at 3733.7 and 800.3 cm-1. This species is regarded as a crucial intermediate along H2 activation pathways. Bonding analysis reveals cooperative U(π5f/6d) → H2(σ*) π// backdonation and U ← H2(σ) σ donation in OU(η2-H2) that facilitate the activation of the H2 moiety. Upon λ > 550 nm photoirradiation, OU(η2-H2) isomerizes into H2UO, indicating the homolytic H2 cleavage on UO. Mechanistic details of H2 adsorption and dissociation on UO molecules have been further elucidated.

The bond energy of UN+: Guided ion beam studies of the reactions of U+ with N2 and NO

A guided ion beam tandem mass spectrometer was used to study the reactions of U+ with N2 and NO. Reaction cross sections were measured over a wide range of energy for both systems. In each reaction, UN+ is formed by an endothermic process, thereby enabling the direct measurement of the threshold energy and determination of the UN+ bond dissociation energy. For the reaction of U+ + N2, a threshold energy (E0) of 4.02 ± 0.11 eV was measured, leading to D0 (UN+) = 5.73 ± 0.11 eV. The reaction of U+ + NO yields UO+ through an exothermic, barrierless process that proceeds with 94 ± 23% efficiency at the lowest energy. Analysis of the endothermic UN+ cross section in this reaction provides E0 = 0.72 ± 0.11 eV and, therefore, D0 (UN+) = 5.78 ± 0.11 eV. Averaging the values obtained from both reactions, we report D0 (UN+) = 5.76 ± 0.13 eV as our best value (uncertainty of two standard deviations). Combined with precise literature values for the ionization energies of U and UN, we also derive D0 (UN) = 5.86 ± 0.13 eV. Both bond dissociation energies agree well with high-level theoretical treatments in the literature. The formation of UN+ in reaction of U+ with NO also exhibits a considerable increase in reaction probability above ∼3 eV. Theory suggests that this may be consistent with the formation of UN+ in excited quintet spin states, which we hypothesize are dynamically favored because the number of 5f electrons in reactants and products is conserved.

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.

Conversion of a UO22+ Precursor to UH+ and U+ Using Tandem Mass Spectrometry to Remove Both "yl" Oxo Ligands

Multiple-stage collision-induced dissociation (CID) of a uranyl propiolate cation, [UO2(O2C-C≡CH)]+, can be used to prepare the U-methylidyne species [O═U≡CH]+ [J. Am. Soc. Mass Spectrom. 2019, 30, 796-805]. Here, we report that CID of [O═U≡CH]+ causes elimination of CO to create [UH]+, followed by a loss of H• to generate U+. A feasible, multiple-step pathway for the generation of [UH]+ was identified using DFT calculations. These results provide the first demonstration that multiple-stage CID can be used to prepare both U+ and UH+ directly from a UO22+ precursor for the subsequent investigation of ion-molecule reactivity.

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.

Protactinium and Actinium Monohydrides: A Theoretical Study on Their Spectroscopic and Thermodynamic Properties

Spectroscopic and thermodynamics properties including bond dissociation energies (BDEs), adiabatic electron affinities (AEAs), and ionization energies (IEs) have been predicted for AcH and PaH using the Feller-Peterson-Dixon composite approach. Comparisons with previous studies on ThH and UH were performed to identify possible trends in the actinide series. Multireference CASPT2 calculations were used to predict the spin-orbit effects and obtain potential energy curves for the low-lying Ω states around the equilibrium distance as well as the vertical detachment energies (VDEs) from AcH^- and PaH^- to excited states of the neutral species. The calculated AEA for AnH (An = Ac, Th, Pa, U) showed that the AEA increases from AcH (0.425 eV) to ThH (0.820 eV) and decreases to PaH (0.781 eV) and to UH (0.457 eV), whereas the IE values are 5.887 eV (AcH), 6.181 eV (ThH), 6.204 eV (PaH), and 6.182 eV (UH). The ground state of AcH, AcH^-, PaH, and PaH^- are predicted to be¹Σ⁺₀,²Π_3/2, ³H₄, and ⁴I_9/2, respectively. The BDEs for AcH and PaH are 276.4 and 237.2 kJ/mol, and those for AcH^- and PaH^- are 242.8 and 239.8 kJ/mol, respectively. The natural bond analysis shows a significant ionic character, An⁺H^-, in the bonding of the neutral hydrides.

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.

Experimental and computational investigation of the bond energy of thorium dicarbonyl cation and theoretical elucidation of its isomerization mechanism to the thermodynamically most stable isomer, thorium oxide ketenylidene cation, OTh+CCO

Collision-induced dissociation (CID) of [Th,2C,2O]⁺ with Xe is performed using a guided ion beam tandem mass spectrometer (GIBMS). The only products observed are ThCO⁺ and Th⁺ by sequential loss of CO ligands. The experimental findings and theoretical calculations support that the structure of [Th,2C,2O]⁺ is the bent homoleptic thorium dicarbonyl cation, Th⁺(CO)₂, having quartet spin, which is both thermodynamically and kinetically stable enough in the gas phase to be observed in our GIBMS instrument. Analysis of the kinetic energy-dependent cross sections for this CID reaction yields the first experimental determination of the bond dissociation energy (BDE) of (CO)Th⁺-CO at 0 K as 1.05 ± 0.09 eV. A theoretical BDE calculated at the CCSD(T) level with cc-pVXZ (X = T and Q) basis sets and a complete basis set (CBS) extrapolation is in very good agreement with the experimental result. Although the doublet spin bent thorium oxide ketenylidene cation, OTh⁺CCO, is calculated to be the most thermodynamically stable structure, it is not observed in our experiment where [Th,2C,2O]⁺ is formed by association of Th⁺ and CO in a direct current discharge flow tube (DC/FT) ion source. Potential energy profiles of both quartet and doublet spin are constructed to elucidate the isomerization mechanism of Th⁺(CO)₂ to OTh⁺CCO. The failure to observe OTh⁺CCO is attributed to a barrier associated with C-C bond formation, which makes OTh⁺CCO kinetically inaccessible under our experimental conditions. Chemical bonding patterns in low-lying states of linear and bent Th⁺(CO)₂ and OTh⁺CCO isomers are also investigated.