Elucidating Protactinium Hydrolysis: The Relative Stabilities of PaO2(H2O)+ and PaO(OH)2+

Stability of the protactinium(V) mono-oxo cation probed by first-principle calculations

This study explores the distinctive behavior of protactinium (Z=91) within the actinide series. In contrast to neighboring elements like uranium or plutonium, protactinium in the pentavalent state diverges by not forming the typical dioxo protactinyl moiety PaO2 + in aqueous phase. Instead, it manifests as a monooxo PaO3+ cation or a Pa5+ . Employing first-principle calculations with implicit and explicit solvation, we investigate two stoichiometrically equivalent neutral complexes: PaO(OH)2 (X)(H2 O) and Pa(OH)4 (X), where X represents various monodentate and bidentate ligands. Calculating the Gibbs free energy for the reaction PaO(OH)2 (X)(H2 O)→Pa(OH)4 (X), we find that the PaO(OH)2 (X)(H2 O) complex is stabilized with Cl- , Br- , I- , NCS- , NO3 - , and SO4 2- ligands, while it is not favored with OH- , F- , and C2 O4 2- ligands. Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) methods reveal the Pa mono-oxo bond as a triple bond, with significant contributions from the 5f and 6d shells. Covalency of the Pa mono-oxo bond increases with certain ligands, such as Cl- , Br- , I- , NCS- , and NO3 - . These findings elucidate protactinium's unique chemical attributes and provide insights into the conditions supporting the stability of relevant complexes.

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.

A terminal neptunium(V)-mono(oxo) complex

Neptunium was the first actinide element to be artificially synthesized, yet, compared with its more famous neighbours uranium and plutonium, is less conspicuously studied. Most neptunium chemistry involves the neptunyl di(oxo)-motif, and transuranic compounds with one metal-ligand multiple bond are rare, being found only in extended-structure oxide, fluoride or oxyhalide materials. These combinations stabilize the required high oxidation states, which are otherwise challenging to realize for transuranic ions. Here we report the synthesis, isolation and characterization of a stable molecular neptunium(V)-mono(oxo) triamidoamine complex. We describe a strong Np≡O triple bond with dominant 5f-orbital contributions and σ_u > π_u energy ordering, akin to terminal uranium-nitrides and di(oxo)-actinyls, but not the uranium-mono(oxo) triple bonds or other actinide multiple bonds reported so far. This work demonstrates that molecular high-oxidation-state transuranic complexes with a single metal-ligand bond can be stabilized and studied in isolation.

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.

Density functional theory investigations on the coordination of Pa(v) with N,N-dialkylamide

The density functional theory (DFT) method was used to study the coordination of a series of N,N-dialkylamides with Pa(v) to shed light on the inherent principles for screening amide extractants of Pa(v) from aqueous solution.

Protactinium(V) in aqueous solution: a light actinide without actinyl moiety

This review highlights recent data on the complexation of Pa(V) with inorganic (fluoride and sulphate) and organic (oxalate, nitrilotriacetate, diethylenetriaminepentaacetate) ligands in solution. New thermodynamic parameters relative to the complexation of Pa(V) with sulphate are presented. The review also includes gas phase and theoretical studies focused on the interaction of Pa(V) in the dioxo and oxo forms with water.

Density Functional Theory Investigations on the Mechanism of Formation of Pa(V) Ion in Hydrous Solutions

Due to the enormous threat of protactinium to the environment and human health, its disposal and chemistry have long been important topics in nuclear science. [PaO(H₂O)₆]3+ is proposed as the predominant species in hydrous and acidic solutions, but little is known about its formation mechanism. In this study, density functional theory (DFT) calculations demonstrate a water coordination-proton transfer-water dissociation mechanism for the formation of PaO3+ in hydrous solutions. First, Pa(V) ion preferentially forms hydrated complexes with a coordination number of 10. Through hydrogen bonding, water molecules in the second coordination sphere easily capture two protons on the same coordinated H₂O ligand to form [PaO(H₂O)₉]3+. Water dissociation then occurs to generate the final [PaO(H₂O)₆]3+, which is the thermodynamic product of Pa(V) in hydrous solutions.

Organic Compounds of Actinyls: Systematic Computational Assessment of Structural and Topological Properties in [AnO2(C2O4) n](2 n-2)- (An = U, Np, Pu, Am; n = 1-3) Complexes

Exploring the bonding features between organics and actinide elements is a fundamental topic in nuclear waste separation. In this work, [AnO₂(C₂O₄) _n]^{(2 n-2)-} (An = U, Np, Pu, and Am; n = 1-3) complexes have been characterized by density functional theory. The actinyl oxalate complexes are found to exhibit the typical An-O_yl, An-O_eq bonds and O_yl-An-O_yl angles. Interatomic interaction analyzed by electron density difference, charge decomposition analysis, charges population, bond order, electron localization function, and quantum theory of atom in molecules indicates that An-O_eq bonds are ionic (closed-shell) bonding interaction with a small degree of covalent character. The similarities and differences between isomers have been discussed in the actinide coordination chemistry, and the orbital interactions also have been investigated through total, partial, and overlap population density of state diagrams. Besides, the electrostatic potential was used to predict the adsorption sites on the molecular vdW surface.

Separation of Protactinium Employing Sulfur-Based Extraction Chromatographic Resins

Protactinium-230 ( t_1/2 = 17.4 d) is the parent isotope of ²³⁰U ( t_1/2 = 20.8 d), a radionuclide of interest for targeted alpha therapy (TAT). Column chromatographic methods have been developed to separate no-carrier-added ²³⁰Pa from proton irradiated thorium targets and accompanying fission products. Results reported within demonstrate the use of novel sulfur bearing chromatographic extraction resins for the selective separation of protactinium. The recovery yield of ²³⁰Pa was 93 ± 4% employing a R₃P═S type commercially available resin and 88 ± 4% employing a DGTA (diglycothioamide) containing custom synthesized extraction chromatographic resin. The radiochemical purity of the recovered ²³⁰Pa was measured via high purity germanium γ-ray spectroscopy to be >99.5% with the remaining radioactive contaminant being ⁹⁵Nb due to its similar chemistry to protactinium. Measured equilibrium distribution coefficients for protactinium, thorium, uranium, niobium, radium, and actinium on both the R₃P═S type and the DGTA resin in hydrochloric acid media are reported, to the best of our knowledge, for the first time.

Protactinium and the intersection of actinide and transition metal chemistry

The role of the 5f and 6d orbitals in the chemistry of the actinide elements has been of considerable interest since their discovery and synthesis. Relativistic effects cause the energetics of the 5f and 6d orbitals to change as the actinide series is traversed left to right imparting a rich and complex chemistry. The 5f and 6d atomic states cross in energy at protactinium (Pa), making it a potential intersection between transition metal and actinide chemistries. Herein, we report the synthesis of a Pa-peroxo cluster, A₆(Pa₄O(O₂)₆F₁₂) [A = Rb, Cs, (CH₃)₄N], formed in pursuit of an actinide polyoxometalate. Quantum chemical calculations at the density functional theory level demonstrate equal 5f and 6d orbital participation in the chemistry of Pa and increasing 5f orbital participation for the heavier actinides. Periodic changes in orbital character to the bonding in the early actinides highlights the influence of the 5f orbitals in their reactivity and chemical structure.

Synthesis and radiometric evaluation of diglycolamide functionalized mesoporous silica for the chromatographic separation of actinides Th, Pa and U

Grafting of DGA on mesoporous silica affords a sorbent with the potential to separate U from Pa and Th.

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.

Synthesis, structure and bonding of actinide disulphide dications in the gas phase

CASPT2 computations reveal that gas-phase AnS₂²⁺ ions have ground states with triangular geometries and linear thio-actinyl structures are higher in energy, with a difference that increases upon moving from U to Pu.

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.

Thermodynamic study of the complexation between Nd3+ and functionalized diacetamide ligands in solution

Three amine-functionalized diamide ligands form tridentate complexes with Nd³⁺ in aqueous solutions. The stability constants of the complexes follow the order of the ligand basicity that can be tuned by different substitutional groups.