Revealing Disparate Chemistries of Protactinium and Uranium. Synthesis of the Molecular Uranium Tetroxide Anion, UO4–

Ion-molecule reactions of mass-selected ions

Gas-phase reactions of mass-selected ions with neutrals covers a very broad area of fundamental and applied mass spectrometry (MS). Oftentimes, ion-molecule reactions (IMR) can serve as a viable alternative to collision-induced dissociation and other ion dissociation techniques when using tandem MS. This review focuses on the literature pertaining applications of IMR since 2013. During the past decade considerable efforts have been made in analytical applications of IMR, including advances in one of the major techniques for characterization of unsaturated fatty acids and lipids, ozone-induced dissociation, and the development of a new technique for sequencing of large ions, hydrogen atom attachment/abstraction dissociation. Many advances have also been made in identifying gas-phase chemistry specific to a functional group in organic and biological compounds, which are useful in structure elucidation of analytes and differentiation of isomers/isobars. With "soft" ionization techniques like electrospray ionization having become mainstream for quite some time now, the efforts in the area of metal ion catalysis have firmly moved into exploring chemistry of ligated metal complexes in their "natural" oxidation states allowing to model individual steps of mechanisms in homogeneous catalysis, especially in combination with high-level DFT calculations. Finally, IMR continue to contribute to the body of knowledge in the area of chemistry of interstellar processes.

Unusual Actinyl Complexes with a Redox-Active N,S-Donor Ligand

Understanding the fundamental chemistry of soft N,S-donor ligands with actinides across the series is critical for separation science toward sustainable nuclear energy. This task is particularly challenging when the ligands are redox active. We herein report a series of actinyl complexes with a N,S-donor redox-active ligand that stabilizes different oxidation states across the actinide series. These complexes are isolated and characterized in the gas phase, along with high-level electronic structure studies. The redox-active N,S-donor ligand in the products, C₅H₄NS, acts as a monoanion in [U^VIO₂(C₅H₄NS^-)]⁺ but as a neutral radical with unpaired electrons localized on the sulfur atom in [Np^VO₂(C₅H₄NS^•)]⁺ and [Pu^VO₂(C₅H₄NS^•)]⁺, resulting in different oxidation states for uranium and transuranic elements. This is rationalized by considering the relative energy levels of actinyl(VI) 5f orbitals and S 3p lone pair orbitals of the C₅H₄NS^- ligand and the cooperativity between An-N and An-S bonds that provides additional stability for the transuranic elements.

Electrochemistry of Uranyl Peroxide Solutions during Electrospray Ionization

The ionization of uranyl triperoxide monomer, [(UO₂)(O₂)₃]^4- (UT), and uranyl peroxide cage cluster, [(UO₂)₂₈(O₂)_42 - x(OH)_2x]^28- (U₂₈), was studied with electrospray ionization mass spectrometry (ESI-MS). Experiments including tandem mass spectrometry with collision-induced dissociation (MS/CID/MS), use of natural water and D₂O as solvent, and use of N₂ and SF₆ as nebulizer gases, provide insight into the mechanisms of ionization. The U₂₈ nanocluster under MS/CID/MS with collision energies ranging from 0 to 25 eV produced the monomeric units UO_x^- (x = 3-8) and UO_xH_y^- (x = 4-8, y = 1, 2). UT under ESI conditions yielded the gas-phase ions UO_x^- (x = 4-6) and UO_xH_y^- (x = 4-8, y = 1-3). Mechanisms that produce the observed anions in the UT and U₂₈ systems are: (a) gas-phase combinations of uranyl monomers in the collision cell upon fragmentation of U₂₈, (b) reduction-oxidation resulting from the electrospray process, and (c) ionization of surrounding analytes, creating reactive oxygen species that then coordinate to uranyl ions. The electronic structures of anions UO_x^- (x = 6-8) were investigated using density functional theory (DFT).

Hydrolysis of Small Oxo/Hydroxo Molecules Containing High Oxidation State Actinides (Th, Pa, U, Np, Pu): A Computational Study

The energetics of hydrolysis reactions for high oxidation states of oxo/hydroxo monomeric actinide species (Th^IVO₂, Pa^IVO₂, U^IVO₂, Pa^VO₂(OH), U^VO₂(OH), U^VIO₃, Np^VIO₃, Np^VIIO₃(OH), and Pu^VIIO₃(OH)) were calculated at the CCSD(T) level. The first step is the formation of a Lewis acid/base adduct with H₂O (hydration), followed by a proton transfer to form a dihydroxide molecule (hydrolysis); this process is repeated until all oxo groups are hydrolyzed. The physisorption (hydration) for each H₂O addition was predicted to be exothermic, ca. -20 kcal/mol. The hydrolysis products are preferred energetically over the hydration products for the +IV and +V oxidation states. The compounds with An^VI are a turning point in terms of favoring hydration over hydrolysis. For An^VIIO₃(OH), hydration products are preferred, and only two waters can bind; the complete hydrolysis process is now endothermic, and the oxidation state for the An in An(OH)₇ is +VI with two OH groups each having one-half an electron. The natural bond order charges and the reaction energies provide insights into the nature of the hydrolysis/hydration processes. The actinide charges and bond ionicity generally decrease across the period. The ionic character decreases as the oxidation state and coordination number increase so that covalency increases moving to the right in the actinide period.

Collision-induced dissociation of [UO2 (NO3 )(O2 )]- and reactions of product ions with H2 O and O2

We recently reported a detailed investigation of the collision-induced dissociation (CID) of [UO₂ (NO₃ )₃ ]^- and [UO₂ (NO₃ )₂ (O₂ )]^- in a linear ion trap mass spectrometer (J. Mass Spectrom. DOI:10.1002/jms.4705). Here, we describe the CID of [UO₂ (NO₃ )(O₂ )]^- which is created directly by ESI, or indirectly by simple elimination of O₂ from [UO₂ (NO₃ )(O₂ )₂ ]^- . CID of [UO₂ (NO₃ )(O₂ )]^- creates product ions as at m/z 332 and m/z 318. The former may be formed directly by elimination of O₂ , while the latter required decomposition of a nitrate ligand and elimination of NO₂ . DFT calculations identify a pathway by which both product ions can be generated, which involves initial isomerization of [UO₂ (NO₃ )(O₂ )]^- to create [UO₂ (O)(NO₂ )(O₂ )]^- , from which elimination of NO₂ or O₂ will leave [UO₂ (O)(O₂ )]^- or [UO₂ (O)(NO₂ )]^- , respectively. For the latter product ion, the composition assignment of [UO₂ (O)(NO₂ )]^- rather than [UO₂ (NO₃ )]^- is supported by ion-molecule reaction behavior, and in particular, the fact that spontaneous addition of O₂ , which is predicted to be the dominant reaction pathway for [UO₂ (NO₃ )]^- is not observed. Instead, the species reacts with H₂ O, which is predicted to be the favored pathway for [UO₂ (O)(NO₂ )]^- . 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 [UO₂ (O)(NO₂ )]^- and [UO₂ (O)(O₂ )]^- form H₂ O adducts, and calculations suggest these species spontaneously rearrange to create dihydroxides.

Molecular oxides of high-valent actinides

The past decade has been very productive in the field of actinide (An) oxides containing high-valent An. Novel gas-phase experimental and an impressive number of theoretical studies have been performed, mostly on pure oxides or oxides extended with other ligands. The review covers the structural properties of molecular An oxides with high (An≥V) oxidation states. The presented compounds include the actinide dioxide cations [AnO2]+ and [AnO2]2+, neutral and ionic AnOx (x = 3–6), oxides with more than one An atom like neutral dimers, trimers and dimers from cation–cation interactions, as well as large U-oxide clusters observed very recently in the gaseous phase.

Collision-induced dissociation of [UVI O2 (ClO4 )]+ revisited: Production of [UVI O2 (Cl)]+ and subsequent hydrolysis to create [UVI O2 (OH)]. RAPID COMMUNICATIONS IN MASS SPECTROMETRY : RCM 2018;32:1085-1091. [PMID: 29645301 DOI: 10.1002/rcm.8135] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

RATIONALE

In a previous study [Rapid Commun Mass Spectrom. 2004;18:3028-3034], collision-induced dissociation (CID) of [U^VI O₂ (ClO₄ )]⁺ appeared to be influenced by the high levels of background H₂ O in a quadrupole ion trap. The CID of the same species was re-examined here with the goal of determining whether additional, previously obscured dissociation pathways would be revealed under conditions in which the level of background H₂ O was lower.

METHODS

Water- and methanol-coordinated [U^VI O₂ (ClO₄ )]⁺ precursor ions were generated by electrospray ionization. Multiple-stage tandem mass spectrometry (MSⁿ ) for CID and ion-molecule reaction (IMR) studies was performed using a linear ion trap mass spectrometer.

RESULTS

Under conditions of low background H₂ O, CID of [U^VI O₂ (ClO₄ )]⁺ generates [U^VI O₂ (Cl)]⁺ , presumably by elimination of two O₂ molecules. Using low isolation/reaction times, we found that [U^VI O₂ (Cl)]⁺ will undergo an IMR with H₂ O to generate [U^VI O₂ (OH)]⁺ .

CONCLUSIONS

With lower levels of background H₂ O, CID experiments reveal that the intrinsic dissociation pathway for [U^VI O₂ (ClO₄ )]⁺ leads to [U^VI O₂ (Cl)]⁺ , apparently by loss of two O₂ molecules. We propose that the results reported in the earlier CID study reflected a two-step process: initial formation of [U^VI O₂ (Cl)]⁺ by CID, followed by a very rapid hydrolysis reaction to leave [U^VI O₂ (OH)]⁺ .

Influence of Background H2O on the Collision-Induced Dissociation Products Generated from [UO2NO3]<sup/>

Developing a comprehensive understanding of the reactivity of uranium-containing species remains an important goal in areas ranging from the development of nuclear fuel processing methods to studies of the migration and fate of the element in the environment. Electrospray ionization (ESI) is an effective way to generate gas-phase complexes containing uranium for subsequent studies of intrinsic structure and reactivity. Recent experiments by our group have demonstrated that the relatively low levels of residual H₂O in a 2-D, linear ion trap (LIT) make it possible to examine fragmentation pathways and reactions not observed in earlier studies conducted with 3-D ion traps (Van Stipdonk et al. J. Am. Soc. Mass Spectrom. 14, 1205-1214, 2003). In the present study, we revisited the dissociation of complexes composed of uranyl nitrate cation [U^VIO₂(NO₃)]⁺ coordinated by alcohol ligands (methanol and ethanol) using the 2-D LIT. With relatively low levels of background H₂O, collision-induced dissociation (CID) of [U^VIO₂(NO₃)]⁺ primarily creates [UO₂(O₂)]⁺ by the ejection of NO. However, CID (using He as collision gas) of [U^VIO₂(NO₃)]⁺ creates [UO₂(H₂O)]⁺ and UO₂⁺ when the 2-D LIT is used with higher levels of background H₂O. Based on the results presented here, we propose that product ion spectrum in the previous experiments was the result of a two-step process: initial formation of [U^VIO₂(O₂)]⁺ followed by rapid exchange of O₂ for H₂O by ion-molecule reaction. Our experiments illustrate the impact of residual H₂O in ion trap instruments on the product ions generated by CID and provide a more accurate description of the intrinsic dissociation pathway for [U^VIO₂(NO₃)]⁺. Graphical Abstract ᅟ.