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A comprehensive understanding of when and how 5f orbitals participate in complex chemical bonding is important for a variety of applications. The actinides are unique in that they possess 5f orbitals and can access high oxidation states, which make them attractive for use in catalysis. Fundamental studies of actinide-ligand interactions offer a mechanism to examine the activation of the 5f orbitals so that the selectivity of 5f orbitals can be assessed. A previous study examined the reaction of Pu + + CO 2 and determined that the reaction efficiency is restricted by a barrier, namely, promotion from the Pu + ground-state configuration, 5f 6 7s, to a reactive-state configuration, 5f 5 6d 2 . The present study illustrates the benefit of activation of Pu's 5f orbitals when studying the reaction of Pu + + NO. In this reaction, PuO + forms in an exothermic, barrierless process. The 5f orbitals can and do participate in forming a linear intermediate, [N-Pu-O] + , and this drives the exothermic reaction. Understanding the conditions under which 5f orbitals are active in chemical bonding is the key to exploiting the actinides' selective catalytic capabilities.

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., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

In a previous study, electrospray ionization, collision-induced dissociation (CID), and gas-phase ion-molecule reactions were used to create and characterize ions derived from homogeneous precursors composed of a uranyl cation (U VI O 2 2+ ) coordinated by either formate or acetate ligands [E. Perez, C. Hanley, S. Koehler, J. Pestok, N. Polonsky and M. Van Stipdonk, Gas phase reactions of ions derived from anionic uranyl formate and uranyl acetate complexes, J. Am. Soc. Mass Spectrom. , 2016, 27 , 1989-1998]. Here, we describe a follow-up study of anionic complexes that contain a mix of formate and acetate ligands, namely [UO 2 (O 2 C-CH 3 ) 2 (O 2 C-H)] - and [UO 2 (O 2 C-CH 3 )(O 2 C-H) 2 ] - . Initial CID of either anion causes decarboxylation of a formate ligand to create carboxylate-coordinated U-hydride product ions. Subsequent CID of the hydride species causes elimination of acetaldehyde or formaldehyde, consistent with reactions that include intra-complex hydride attack upon bound acetate or formate ligands, respectively. Density functional theory (DFT) calculations reproduce the experimental observations, including the favored elimination of formaldehyde over acetaldehyde by hydride attack during CID of [UO 2 (H)(O 2 C-CH 3 )(O 2 C-H)] - . We also discovered that MS n CID of the acetate-formate complexes leads to generation of the oxyl-methide species, [UO 2 (O)(CH 3 )] - , which reacts with H 2 O to generate [UO 2 (O)(OH)] - . DFT calculations support the observation that formation of [UO 2 (O)(OH)] - by elimination of CH 4 is favored over H 2 O addition and rearrangement to create [UO 2 (OH) 2 (CH 3 )] - .

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., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

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 SF 6 , 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. SF 5 + was also observed, although the pressure dependence of this channel reveals that SF 5 + forms exothermically through secondary reactions, which the energy dependences suggest result from reactions between ThF 2 + and UF 3 + with SF 6 . At higher energies, both AnF 3 + products are observed to decay to AnF + + F 2 , and both SF 4 + and SF 2 + exhibit cross sections with endothermic features. For both systems, the rise in SF 4 + can be attributed to a secondary collision between AnF + with SF 6 on the basis of the pressure dependence of the SF 4 + channel at higher energies, and the rise in SF 2 + appears to result from the decomposition of SF 3 + to SF 2 + + F.

Uranium trioxide, UO 3 , has a T-shaped structure with bent uranyl, UO 2 2+ , coordinated by an equatorial oxo, O 2- . The structure of cation UO 3 + is similar but with an equatorial oxyl, O •- . Neutral and cationic uranium trioxide coordinated by nitrates were characterized by collision induced dissociation (CID), infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory. CID of uranyl nitrate, [UO 2 (NO 3 ) 3 ] - (complex A1 ), eliminates NO 2 to produce nitrate-coordinated UO 3 + , [UO 2 (O • )(NO 3 ) 2 ] - ( B1 ), which ejects NO 3 to yield UO 3 in [UO 2 (O)(NO 3 )] - ( C1 ). Finally, C1 associates with H 2 O to afford uranyl hydroxide in [UO 2 (OH) 2 (NO 3 )] - ( D1 ). IRMPD of B1 , C1 , and D1 confirms uranyl equatorially coordinated by nitrate(s) along with the following ligands: ( B1 ) radical oxyl O •- ; ( C1 ) oxo O 2- ; and ( D1 ) two hydroxyls, OH - . As the nitrates are bidentate, the equatorial coordination is six in A1 , five in B1 , four in D1 , and three in C1 . Ligand congestion in low-coordinate C1 suggests orbital-directed bonding. Hydrolysis of the equatorial oxo in C1 epitomizes the inverse trans influence in UO 3 , which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl ν 3 IR frequencies indicate the following donor ordering: O 2- [best donor] ≫ O •- > OH - > NO 3 - .

While the strong axial U[double bond, length as m-dash]O bonds confer high stability and inertness to UO22+, it has been shown that the axial oxo ligands can be eliminated or replaced in the gas-phase using collision-induced dissociation (CID) reactions. We report here tandem mass spectrometry experiments initiated with a gas-phase complex that includes UO22+ coordinated by a 2,6-difluorobenzoate ligand. After decarboxylation to form a difluorophenide coordinated uranyl ion, [UO2(C6F2H3)]+, CID causes elimination of CO, and then CO and C2H2 in sequential dissociation steps, to leave a reactive uranium fluoride ion, [UF2(C2H)]+. Reaction of [UF2(C2H)]+ with CH3OH creates [UF2(OCH3)]+, [UF(OCH3)2]+ and [UF(OCH3)2(CH3OH)]+. Cleavage of C-O bonds within these species results in the elimination of methyl cation (CH3+). Subsequent CID steps convert [UF(OCH3)2]+ to [UO2(F)]+ and similarly, [U(OCH3)3]+ to [UO2(OCH3)]+. Our experiments show removal of both uranyl oxo ligands in "top-down" CID reactions and replacement in "bottom-up" ion-molecule and dissociation steps.

We recently reported a detailed investigation of the collision-induced dissociation (CID) of [UO 2 (NO 3 ) 3 ] - and [UO 2 (NO 3 ) 2 (O 2 )] - in a linear ion trap mass spectrometer (J. Mass Spectrom. DOI:10.1002/jms.4705). Here, we describe the CID of [UO 2 (NO 3 )(O 2 )] - which is created directly by ESI, or indirectly by simple elimination of O 2 from [UO 2 (NO 3 )(O 2 ) 2 ] - . CID of [UO 2 (NO 3 )(O 2 )] - creates product ions as at m/z 332 and m/z 318. The former may be formed directly by elimination of O 2 , while the latter required decomposition of a nitrate ligand and elimination of NO 2 . DFT calculations identify a pathway by which both product ions can be generated, which involves initial isomerization of [UO 2 (NO 3 )(O 2 )] - to create [UO 2 (O)(NO 2 )(O 2 )] - , from which elimination of NO 2 or O 2 will leave [UO 2 (O)(O 2 )] - or [UO 2 (O)(NO 2 )] - , respectively. For the latter product ion, the composition assignment of [UO 2 (O)(NO 2 )] - rather than [UO 2 (NO 3 )] - is supported by ion-molecule reaction behavior, and in particular, the fact that spontaneous addition of O 2 , which is predicted to be the dominant reaction pathway for [UO 2 (NO 3 )] - is not observed. Instead, the species reacts with H 2 O, which is predicted to be the favored pathway for [UO 2 (O)(NO 2 )] - . This result in particular demonstrates the utility of ion-molecule reactions to assist the determination of ion composition. As in our earlier study, we find that ions such as [UO 2 (O)(NO 2 )] - and [UO 2 (O)(O 2 )] - form H 2 O adducts, and calculations suggest these species spontaneously rearrange to create dihydroxides., (© 2021 John Wiley & Sons, Ltd.)

Electrospray ionization (ESI) can produce a wide range of gas-phase uranyl (UO 2 2+ ) complexes for tandem mass spectrometry studies of intrinsic structure and reactivity. We describe here the formation and collision-induced dissociation (CID) of [UO 2 (NO 3 ) 3 ] - and [UO 2 (NO 3 ) 2 (O 2 )] - . Multiple-stage CID experiments reveal that the complexes dissociate in reactions that involve elimination of O 2 , NO 2 , or NO 3 , and subsequent reactions of interesting uranyl-oxo product ions with (neutral) H 2 O and/or O 2 were investigated. Density functional theory (DFT) calculations reproduce experimental results and show that dissociation of nitrate ligands, with ejection of neutral NO 2 , is favored for both [UO 2 (NO 3 ) 3 ] - and [UO 2 (NO 3 ) 2 (O 2 )] - . DFT calculations also suggest that H 2 O adducts to products such as [UO 2 (O)(NO 3 )] - spontaneously rearrange to create dihydroxides and that addition of O 2 is favored over addition of H 2 O to formally U(V) species., (© 2021 John Wiley & Sons, Ltd.)

Because of the high stability and inertness of the U=O bonds, activation and/or functionalization of UO 2 2+ and UO 2 + remain challenging tasks. We show here that collision-induced dissociation (CID) of the uranyl-propiolate cation, [U VI O 2 (O 2 C-C≡CH)] + , can be used to prepare [U VI O 2 (C≡CH)] + in the gas phase by decarboxylation. Remarkably, CID of [U VI O 2 (C≡CH)] + caused elimination of CO to create [OU VI CH] + , thus providing a new example of a well-defined substitution of an "yl" oxo ligand of U VI O 2 2+ in a unimolecular reaction. Relative energies for candidate structures based on density functional theory calculations suggest that the [OU VI CH] + ion is a uranium-methylidyne product, with a U≡C triple bond composed of one σ-bond with contributions from the U df and C sp hybrid orbitals, and two π-bonds with contributions from the U df and C p orbitals. Upon isolation, without imposed collisional activation, [OU VI CH] + appears to react spontaneously with O 2 to produce [U V O 2 ] + . Graphical Abstract .