On the Origins of Faster Oxo Exchange for Uranyl(V) versus Plutonyl(V)

Reactions of gas-phase uranyl formate/acetate anions: reduction of carboxylate ligands to aldehydes by intra-complex hydride attack

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 (UVIO22+) 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 [UO2(O2C-CH3)2(O2C-H)]- and [UO2(O2C-CH3)(O2C-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 [UO2(H)(O2C-CH3)(O2C-H)]-. We also discovered that MSn CID of the acetate-formate complexes leads to generation of the oxyl-methide species, [UO2(O)(CH3)]-, which reacts with H2O to generate [UO2(O)(OH)]-. DFT calculations support the observation that formation of [UO2(O)(OH)]- by elimination of CH4 is favored over H2O addition and rearrangement to create [UO2(OH)2(CH3)]-.

Organolanthanide Complexes Containing Ln-CH3 σ-bonds: Unexpectedly Similar Hydrolysis Rates for Trivalent and Tetravalent Organocerium

We report the gas-phase preparation, isolation, and reactivity of a series of organolanthanides featuring the Ln-CH3 bond. The complexes are formed by decarboxylating anionic lanthanide acetates to form trivalent [LnIII(CH3)(CH3CO2)3]- (Ln = La, Ce, Pr, Nd, Sm, Tb, Tm, Yb, Lu), divalent [EuII(CH3)(CH3CO2)2]-, and the first examples of tetravalent organocerium complexes featuring CeIV-Calkyl σ-bonds: [CeIV(O)(CH3)(CH3CO2)2]- and [CeIV(O)(CH3)(NO3)2]-. Attempts to isolate PrIV-CH3 and TbIV-CH3 were unsuccessful; however, fragmentation patterns reveal that the oxidation of LnIII to a LnIV-oxo-acetate complex is more favorable for Ln = Pr than for Ln = Tb. The rate of Ln-CH3 hydrolysis is a measure of bond stability, and it decreases from LaIII-CH3 to LuIII-CH3, with increasing steric crowding for smaller Ln stabilizing the harder Ln-CH3 bond against hydrolysis. [EuII(CH3)(CH3CO2)2]- engages in a much faster hydrolysis versus LnIII-CH3. The surprising observation of similar hydrolysis rates for CeIV-CH3 and CeIII-CH3 is discussed with respect to sterics, the oxo ligand, and bond covalency in σ-bonded organolanthanides.

Intrinsic reactivity of [OUCH]+ : Apparent synthesis of [OUS]+ by reaction with CS2

RATIONALE

Building on our report that collision-induced dissociation (CID) can be used to create the highly reactive U-alkylidyne species [O=U≡CH]⁺ , our goal was to determine whether the species could be as an intermediate for synthesis of [OUS]⁺ by reaction with carbon disulfide (CS₂ ).

METHODS

Cationic uranyl-propiolate precursor ions were generated by electrospray ionization, and multiple-stage CID in a linear trap instrument was used to prepare [O=U≡CH]⁺ . Neutral CS₂ was admitted into the trap through a modified He inlet and precision leak valves.

RESULTS

The [O=U≡CH]⁺ ion reacts with CS₂ to generate [OUS]⁺ . CID of [OUS]⁺ causes elimination of the axial sulfide ligand to generate [OU]⁺ . Using isotopically labeled reagent, we found that [OUS]⁺ reacts with O₂ to create [UO₂ ]⁺ .

CONCLUSIONS

[O=U≡CH]⁺ proves to be a useful reagent ion for synthesis of [OUS]⁺ , a species that to date has only been created by gas-phase reactions of U⁺ and U²⁺ . Dissociation of [OUS]⁺ to create [OU]⁺ , but not [US]⁺ , and the efficient conversion of the species into [UO₂ ]⁺ , is consistent with the relative differences in U-O and U-S bond energies.

X-ray absorption spectra of f-element complexes: insight from relativistic multiconfigurational wavefunction theory

X-ray absorption near edge structure (XANES) spectroscopy, coupled with ab initio calculations, has emerged as the state-of-the-art tool for elucidating the metal-ligand bonding in f-element complexes. This highlight presents recent efforts in calculating XANES spectra of lanthanide and actinide compounds with relativistic multiconfiguration wavefunction approaches that account for differences in donation bonding in the ground state (GS) versus a core-excited state (ES), multiplet effects, and spin-orbit-coupling. With the GS and ES wavefunctions available, including spin-orbit effects, an arsenal of chemical bonding tools that are popular among chemists can be applied to rationalize the observed intensities in terms of covalent bonding.

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.

Characterization of Uranyl Coordinated by Equatorial Oxygen: Oxo in UO3 versus Oxyl in UO3. J Phys Chem A 2021;125:5544-5555. [PMID: 34138571 DOI: 10.1021/acs.jpca.1c03818]

Uranium trioxide, UO₃, has a T-shaped structure with bent uranyl, UO₂²⁺, coordinated by an equatorial oxo, O^2-. The structure of cation UO₃⁺ 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, [UO₂(NO₃)₃]^- (complex A1), eliminates NO₂ to produce nitrate-coordinated UO₃⁺, [UO₂(O^•)(NO₃)₂]^- (B1), which ejects NO₃ to yield UO₃ in [UO₂(O)(NO₃)]^- (C1). Finally, C1 associates with H₂O to afford uranyl hydroxide in [UO₂(OH)₂(NO₃)]^- (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 UO₃, which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl ν₃ IR frequencies indicate the following donor ordering: O^2-[best donor] ≫ O^•-> OH^-> NO₃^-.

Multinuclear NMR Studies on Lewis Acid-Lewis Base Interactions between Bis(pentafluorophenyl)borinic Acid and Uranyl β-Diketonato Complexes in Toluene

In order to examine the possibility of Lewis acid-Lewis base (LA-LB) interactions between the boron atom of B(C₆F₅)₂OH and the oxo groups ("yl" oxygen atoms) of uranyl β-diketonato complexes, we have measured the ¹H, ¹¹B, ¹⁷O, ¹⁹F NMR and IR spectra of toluene solutions containing β-diketonato complexes [UO₂(acac)₂DMSO or UO₂(dfh)₂DMSO, where acac = 2,4-pentanedionate, dfh = 1,1,1,2,2,6,6,7,7,7-decafluoroheptane-3,5-dionate, and DMSO = dimethyl sulfoxide] and B(C₆F₅)₂OH. ¹¹B and ¹⁷O NMR spectra of solutions containing UO₂(dfh)₂DMSO and B(C₆F₅)₂OH showed no change in their chemical shifts regardless of the [B(C₆F₅)₂OH]/[UO₂(dfh)₂DMSO] ratio. This indicates that there were no apparent interactions between B(C₆F₅)₂OH and UO₂(dfh)₂DMSO. On the other hand, in the corresponding NMR spectra of solutions containing UO₂(acac)₂DMSO and B(C₆F₅)₂OH, new signals were observed at a higher field than signals observed in the solutions containing only B(C₆F₅)₂OH or UO₂(acac)₂DMSO, and their intensity changed with the [B(C₆F₅)₂OH]/[UO₂(acac)₂DMSO] ratio. These results reveal that a complex with LA-LB interaction (B···O═U) between the boron atom of B(C₆F₅)₂OH and the "yl" oxygen atom of UO₂(acac)₂DMSO was formed. IR spectra also supported such complex formation; i.e., the asymmetric O═U═O stretching band of UO₂(acac)₂DMSO was observed to shift from 897 to 810 cm^-1 with the addition of B(C₆F₅)₂OH. Moreover, ¹⁹F NMR spectra indicated that 1:1 and 2:1 LA-LB complexes exist in equilibrium, UO{OB(C₆F₅)₂OH}(acac)₂DMSO + B(C₆F₅)₂OH = U{OB(C₆F₅)₂OH}₂(acac)₂DMSO. The thermodynamic parameters for this equilibrium were obtained as K = (2.5 ± 0.6) × 10² M^-1 (at 25 °C), ΔH = -42.4 ± 5.2 kJ mol^-1, and ΔS = -96.7 ± 19.4 J K^-1 mol^-1. In ¹H NMR spectra, the signal due to -CH groups of UO₂(acac)₂DMSO disappeared, and three signals due to the corresponding -CH groups newly appeared with an increase in the [B(C₆F₅)₂OH]/[UO₂(acac)₂DMSO] ratio. From these phenomena, it is proposed that 1:1 and 2:1 LA-LB complexes having interactions between the -CH groups of acac and the -OH group of coordinated B(C₆F₅)₂OH are formed depending on the [B(C₆F₅)₂OH]/[UO₂(acac)₂DMSO] ratio.

Abiotic reduction of uranium(VI) with humic acid at mineral surfaces: Competing mechanisms, ligand and substituent effects, and electronic structure and vibrational properties

Abiotic reduction represents an attractive technology to control U(VI) contamination. In this work, an abiotic route of U(VI) reduction with humic acid at mineral surfaces is proposed and reaction mechanisms are addressed by periodic density functional theory calculations. Different influencing factors such as ligand effect, content of CO₃^2- ligands and substituent effect are inspected. The coordination chemistry of uranyl(VI) surface complexes relies strongly on substrates and ligands, and the calculated results are in good agreements with experimental observations available. For the OH^- ligand, two competitive mechanisms co-exist that respectively produce the U(IV) and U(V) species, and the former is significantly preferred because of lower energy barriers. Instead, the NO₃^- ligand leads to the formation of U(V) while for the Cl^- ligand, the U(VI) surface complex remains very stable and is not likely to be reduced because of very high energy barriers. The U(V) and U(IV) complexes are the predominant products for low and high CO₃^2- contents, respectively. Accordingly, the abiotic reduction processes with humic acid are efficient to manage U(VI) contamination and become preferred under basic conditions or at higher CO₃^2- contents. The U(VI) reduction is further promoted by introduction of electron-donating rather than electron-withdrawing substituents to humic acid. Electronic structure analyses and vibrational frequency assignments are calculated for the various uranium surface complexes of the reduction processes, serving as a guide for future experimental and engineered studies. The molecular-level understanding given in this work offers an abiotic route for efficient reduction of U(VI) and remediation of U(VI)-contaminated sites at ambient conditions.

Formation and hydrolysis of gas-phase [UO2 (R)]+ : R═CH3 , CH2 CH3 , CH═CH2 , and C6 H5

The goals of the present study were (a) to create positively charged organo-uranyl complexes with general formula [UO₂ (R)]⁺ (eg, R═CH₃ and CH₂ CH₃ ) by decarboxylation of [UO₂ (O₂ C─R)]⁺ precursors and (b) to identify the pathways by which the complexes, if formed, dissociate by collisional activation or otherwise react when exposed to gas-phase H₂ O. Collision-induced dissociation (CID) of both [UO₂ (O₂ C─CH₃ )]⁺ and [UO₂ (O₂ C─CH₂ CH₃ )]⁺ causes H⁺ transfer and elimination of a ketene to leave [UO₂ (OH)]⁺ . However, CID of the alkoxides [UO₂ (OCH₂ CH₃ )]⁺ and [UO₂ (OCH₂ CH₂ CH₃ )]⁺ produced [UO₂ (CH₃ )]⁺ and [UO₂ (CH₂ CH₃ )]⁺ , respectively. Isolation of [UO₂ (CH₃ )]⁺ and [UO₂ (CH₂ CH₃ )]⁺ for reaction with H₂ O caused formation of [UO₂ (H₂ O)]⁺ by elimination of ·CH₃ and ·CH₂ CH₃ : Hydrolysis was not observed. CID of the acrylate and benzoate versions of the complexes, [UO₂ (O₂ C─CH═CH₂ )]⁺ and [UO₂ (O₂ C─C₆ H₅ )]⁺ , caused decarboxylation to leave [UO₂ (CH═CH₂ )]⁺ and [UO₂ (C₆ H₅ )]⁺ , respectively. These organometallic species do react with H₂ O to produce [UO₂ (OH)]⁺ , and loss of the respective radicals to leave [UO₂ (H₂ O)]⁺ was not detected. Density functional theory calculations suggest that formation of [UO₂ (OH)]⁺ , rather than the hydrated U^V O₂ ⁺ , cation is energetically favored regardless of the precursor ion. However, for the [UO₂ (CH₃ )]⁺ and [UO₂ (CH₂ CH₃ )]⁺ precursors, the transition state energy for proton transfer to generate [UO₂ (OH)]⁺ and the associated neutral alkanes is higher than the path involving direct elimination of the organic neutral to form [UO₂ (H₂ O)]⁺ . The situation is reversed for the [UO₂ (CH═CH₂ )]⁺ and [UO₂ (C₆ H₅ )]⁺ precursors: The transition state for proton transfer is lower than the energy required for creation of [UO₂ (H₂ O)]⁺ by elimination of CH═CH₂ or C₆ H₅ radical.

Ab Initio Study of Covalency in the Ground versus Core-Excited States and X-ray Absorption Spectra of Actinide Complexes

Relativistic multireference ab initio wave function calculations within the restricted active space (RAS) framework were performed to calculate metal and ligand X-ray absorption (XAS) near-edge spectroscopy (XANES) intensities for the metal M_4,5 edges of [PuO₂(H₂O)₅]²⁺, [An^VIO₂]²⁺ (An = U, Np, Pu), and [AmCl₆]^3- and the Cl K edge of the Am complex. The extent of An(5f)-ligand bonding was determined via natural localized molecular orbital analyses of the relevant spin-orbit coupled multireference states. The calculated spectra are in good agreement with experiments and allow a detailed assignment of the observed spectral features. The XANES M_4,5-edge spectra are representative of the actinide orbital covalency in the probed core-excited states, which may be different from the ground-state covalency. An assignment of ground-state An orbital covalency based on XAS spectra should therefore be made with caution.

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.

Transuranic Computational Chemistry

Recent developments in the chemistry of the transuranic elements are surveyed, with particular emphasis on computational contributions. Examples are drawn from molecular coordination and organometallic chemistry, and from the study of extended solid systems. The role of the metal valence orbitals in covalent bonding is a particular focus, especially the consequences of the stabilization of the 5f orbitals as the actinide series is traversed. The fledgling chemistry of transuranic elements in the +II oxidation state is highlighted. Throughout, the symbiotic interplay of experimental and computational studies is emphasized; the extraordinary challenges of experimental transuranic chemistry afford computational chemistry a particularly valuable role at the frontier of the periodic table.

The role of the 5f valence orbitals of early actinides in chemical bonding

One of the long standing debates in actinide chemistry is the level of localization and participation of the actinide 5f valence orbitals in covalent bonds across the actinide series. Here we illuminate the role of the 5f valence orbitals of uranium, neptunium and plutonium in chemical bonding using advanced spectroscopies: actinide M4,5 HR-XANES and 3d4f RIXS. Results reveal that the 5f orbitals are active in the chemical bonding for uranium and neptunium, shown by significant variations in the level of their localization evidenced in the spectra. In contrast, the 5f orbitals of plutonium appear localized and surprisingly insensitive to different bonding environments. We envisage that this report of using relative energy differences between the 5fδ/φ and 5fπ*/5fσ* orbitals as a qualitative measure of overlap-driven actinyl bond covalency will spark activity, and extend to numerous applications of RIXS and HR-XANES to gain new insights into the electronic structures of the actinide elements.

Gas Phase Reactions of Ions Derived from Anionic Uranyl Formate and Uranyl Acetate Complexes

The speciation and reactivity of uranium are topics of sustained interest because of their importance to the development of nuclear fuel processing methods, and a more complete understanding of the factors that govern the mobility and fate of the element in the environment. Tandem mass spectrometry can be used to examine the intrinsic reactivity (i.e., free from influence of solvent and other condensed phase effects) of a wide range of metal ion complexes in a species-specific fashion. Here, electrospray ionization, collision-induced dissociation, and gas-phase ion-molecule reactions were used to create and characterize ions derived from precursors composed of uranyl cation (U^VIO₂²⁺) coordinated by formate or acetate ligands. Anionic complexes containing U^VIO₂²⁺ and formate ligands fragment by decarboxylation and elimination of CH₂=O, ultimately to produce an oxo-hydride species [U^VIO₂(O)(H)]^-. Cationic species ultimately dissociate to make [U^VIO₂(OH)]⁺. Anionic complexes containing acetate ligands exhibit an initial loss of acetyloxyl radical, CH₃CO₂•, with associated reduction of uranyl to U^VO₂⁺. Subsequent CID steps cause elimination of CO₂ and CH₄, ultimately to produce [U^VO₂(O)]^-. Loss of CH₄ occurs by an intra-complex H⁺ transfer process that leaves U^VO₂⁺ coordinated by acetate and acetate enolate ligands. A subsequent dissociation step causes elimination of CH₂=C=O to leave [U^VO₂(O)]^-. Elimination of CH₄ is also observed as a result of hydrolysis caused by ion-molecule reaction with H₂O. The reactions of other anionic species with gas-phase H₂O create hydroxyl products, presumably through the elimination of H₂. Graphical Abstract ᅟ.

Collision-induced dissociation of uranyl-methoxide and uranyl-ethoxide cations: Formation of UO2 H(+) and uranyl-alkyl product ions

RATIONALE

The lower levels of adventitious H2 O in a linear ion trap allow the fragmentation reactions of [UO2 OCH3 ](+) and [UO2 OCH2 CH3 ](+) to be examined in detail.

METHODS

Methanol- and ethanol-coordinated UO2 (2+) -alkoxide precursors were generated by electrospray ionization (ESI). Multiple-stage tandem mass spectrometry (MS(n) ) and collision-induced dissociation (CID) were performed using a linear ion trap mass spectrometer.

RESULTS

CID of [UO2 OCH3 (CH3 OH)n ](+) and [UO2 OCH2 CH3 (CH3 CH2 OH)n ](+) , n = 3 and 2, causes loss of neutral alcohol ligands, leading ultimately to bare uranyl-alkoxide species. Comparison of 'native' to deuterium-labeled precursors reveals dissociation pathways not previously observed in 3-D ion trap experiments.

CONCLUSIONS

UO2 H(+) is generated from [UO2 OCH3 ](+) by transfer of H from the methyl group. Variable-energy and variable-time CID experiments suggest that the apparent threshold for production of UO2 H(+) is lower than for UO2 (+) , but the pathway is kinetically less favored for the former than for the latter. CID experiments reveal that [UO2 OCH2 CH3 ](+) dissociates to generate [UO2 CH3 ](+) , a relatively rare species with a U-C bond, and [UO2 (O = CH2 )](+) .

Covalency hinders AnO2(H2O)(+)→ AnO(OH)2(+) isomerisation (An = Pa-Pu)

The enthalpies of the reactions AnO2(+)→ AnO(+) + O and AnO2(+) + H2O → AnO2(H2O)(+), and those of the isomerisation of the latter to AnO(OH)2(+), have been calculated for An = Pa-Pu. The data match previous experimental and computational values very closely, and the computed enthalpy for the isomerisation of PaO2(H2O)(+) to PaO(OH)2(+), requested by the authors of Inorg. Chem., 2015, 54, 7474, is found to be 0.8 kJ mol(-1). The NPA, NBO and QTAIM approaches are used to probe covalency in the An-Oyl bond of AnO2(H2O)(+), and all metrics agree that these bonds become increasingly covalent as the 5f series is crossed, providing rationalisation for the increasingly endothermic isomerisation reactions. QTAIM analysis indicates that the An=O and An-OH bonds in the oxide hydroxide isomers also become increasingly covalent as the 5f series is crossed.

Theoretical investigation on the mechanism and dynamics of oxo exchange of neptunyl(VI) hydroxide in aqueous solution

Four types of reaction mechanisms for the oxo ligand exchange of monomeric and dimeric neptunyl(VI) hydroxide in aqueous solution were explored computationally using density functional theory (DFT) and ab initio classical molecular dynamics. The obtained results were compared with previous studies on the oxo exchange of uranyl hydroxide, as well as with experiments. It is found that the stable T-shaped [NpO3(OH)3](3-) intermediate is a key species for oxo exchange in the proton transfer in mononuclear Path I and binuclear Path IV, similar to the case of uranyl(VI) hydroxide. Path I is thought to be the preferred oxo exchange mechanism for neptunyl(VI) hydroxide in our calculations, due to the lower activation energy (22.7 and 13.1 kcal mol(-1) for ΔG(‡) and ΔH(‡), respectively) of the overall reaction. Path II via a cis-neptunyl structure assisted by a water molecule might be a competitive channel against Path I with a mononuclear mechanism, owing to a rapid dynamical process occurring in Path II. In Path IV with the binuclear mechanism, oxo exchange is accomplished via the interaction between [NpO2(OH)4](2-) and T-shaped [NpO3(OH)3](3-) with a low activation energy for the rate-determining step, however, the overall energy required to fulfill the reaction is slightly higher than that in mononuclear Path I, suggesting a possible binuclear process in the higher energy region. The chemical bonding evolution along the reaction pathways was discussed by using topological methodologies of the electron localization function (ELF).

Promising density functional theory methods for predicting the structures of uranyl complexes

By examining the overall accuracy of different theoretical methods in predicting the U–X bond distances (of a series uranyl complexes), we found that both the global-hybrid meta-GGA functional of BB1K and the range-seperated LC-BLYP functional are fairly good (even better than the popular B3LYP method).

Oxidation states, geometries, and electronic structures of plutonium tetroxide PuO4 isomers: is octavalent Pu viable?

In neutral chemical compounds, the highest known oxidation state of all elements in the Periodic Table is +VIII. While PuO4 is viewed as an exotic Pu(+VIII) complex, we have shown here that no stable electronic homologue of octavalent RuO4 and OsO4 exists for PuO4, even though Pu has the same number of eight valence electrons as Ru and Os. Using quantum chemical approaches at the levels of quasi-relativistic DFT, MP2, CCSD(T), and CASPT2, we find the ground state of PuO4 as a quintet (5)C2v-(PuO2)(+)(O2)(-) complex with the leading valence configuration of an (f(3))plutonyl(V) unit, loosely coupled to a superoxido (π*(3))O2(-) ligand. This stable isomer is likely detectable as a transient species, while the previously suggested planar (1)D4h-Pu(VIII)O4 isomer is only metastable. Through electronic structure analyses, the bonding and the oxidation states are explained and rationalized. We have predicted the characteristics of the electronic and vibrational spectra to assist future experimental identification of (PuO2)(+)(O2)(-) by IR, UV-vis, and ionization spectroscopy.