Predissociation-based measurements of bond dissociation energies: US2, OUS, and USe

The uranium-containing molecules US2, OUS, and USe have been investigated using a pulsed laser ablation supersonic beam molecular source with time-of-flight mass spectrometric detection. Spectra have been recorded using the resonant two-photon ionization method over the spectroscopic range from 277 to 238 nm. These species have a myriad of excited electronic states in this spectroscopic region, leading to spectra that are highly congested and appear quasicontinuous. Sharp predissociation thresholds are observed, allowing precise bond dissociation energies to be measured. In the case of the triatomic molecules, it was necessary to use one laser for excitation and a delayed laser for ionization in order to observe a sharp predissociation threshold that allowed a precise bond dissociation energy to be measured. The resulting thermochemical values are D0(SU-S) = 4.910 ± 0.003 eV, D0(OU-S) = 5.035 ± 0.004 eV, and D0(USe) = 4.609 ± 0.009 eV. These results provide the first measurement of D0(USe) and reduce the error limits in the previous values of D0(SU-S) and D0(OU-S) by a factor of more than 70.

f-Block reactions of metal cations with carbon dioxide studied by inductively coupled plasma tandem mass spectrometry

f-Block chemistry offers an opportunity to test current knowledge of chemical reactivity. The energy dependence of lanthanide cation (Ln+ = Ce+, Pr+, Nd+-Eu+) and actinide cation (An+ = Th+, U+-Am+) oxidation reactions by CO2, was observed by inductively coupled plasma tandem mass spectrometry. This reaction is commonly spin-unallowed because the neutral reactant (CO2, 1Σ+g) and product (CO, 1Σ+) require the metal and metal oxide cations to have the same spin state. Correlation of the promotion energy (Ep) to the first state with two free d-electrons with the reaction efficiency indicates that spin conservation is not a primary factor in the reaction rate. The Ep likely influences the reaction rate by partially setting the crossing between the ground and reactive states. Comparison of Ln+ and An+ congener reactivity indicates that the 5f-orbitals play a small role in the An+ reactions.

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.

Carbene Complexes of Neptunium

Since the advent of organotransuranium chemistry six decades ago, structurally verified complexes remain restricted to π-bonded carbocycle and σ-bonded hydrocarbyl derivatives. Thus, transuranium-carbon multiple or dative bonds are yet to be reported. Here, utilizing diphosphoniomethanide precursors we report the synthesis and characterization of transuranium-carbene derivatives, namely, diphosphonio-alkylidene- and N-heterocyclic carbene–neptunium(III) complexes that exhibit polarized-covalent σ²π² multiple and dative σ² single transuranium-carbon bond interactions, respectively. The reaction of [Np^IIII₃(THF)₄] with [Rb(BIPM^TMSH)] (BIPM^TMSH = {HC(PPh₂NSiMe₃)₂}^1–) affords [(BIPM^TMSH)Np^III(I)₂(THF)] (3Np) in situ, and subsequent treatment with the N-heterocyclic carbene {C(NMeCMe)₂} (I^Me4) allows isolation of [(BIPM^TMSH)Np^III(I)₂(I^Me4)] (4Np). Separate treatment of in situ prepared 3Np with benzyl potassium in 1,2-dimethoxyethane (DME) affords [(BIPM^TMS)Np^III(I)(DME)] (5Np, BIPM^TMS = {C(PPh₂NSiMe₃)₂}^2–). Analogously, addition of benzyl potassium and I^Me4 to 4Np gives [(BIPM^TMS)Np^III(I)(I^Me4)₂] (6Np). The synthesis of 3Np–6Np was facilitated by adopting a scaled-down prechoreographed approach using cerium synthetic surrogates. The thorium(III) and uranium(III) analogues of these neptunium(III) complexes are currently unavailable, meaning that the synthesis of 4Np–6Np provides an example of experimental grounding of 5f- vs 5f- and 5f- vs 4f-element bonding and reactivity comparisons being led by nonaqueous transuranium chemistry rather than thorium and uranium congeners. Computational analysis suggests that these Np^III=C bonds are more covalent than U^III=C, Ce^III=C, and Pm^III=C congeners but comparable to analogous U^IV=C bonds in terms of bond orders and total metal contributions to the M=C bonds. A preliminary assessment of Np^III=C reactivity has introduced multiple bond metathesis to transuranium chemistry, extending the range of known metallo-Wittig reactions to encompass actinide oxidation states III-VI.

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.

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.

Quantum chemical study of reaction mechanism between plutonium and nitrogen

The study of the reaction between plutonium and nitrogen is helpful in further understanding the interaction between plutonium and air molecules. Currently, there is no research on the microscopic reaction mechanism of plutonium nitridation reactions. Therefore, the microscopic mechanism of the Pu with N₂ gas phase reaction is explored in this study, based on density functional theory (DFT) using different basis functions. In this paper, the geometry of stationary points on the potential energy surface is optimized. In addition, the transition states are verified by frequency analysis and intrinsic reaction coordination (IRC). Finally, we obtained the reaction potential energy curve and micro reaction pathways. Analysis of the reaction mechanism shows that the reaction of Pu with N₂ has two pathways. Pathway 1 (Pu + N₂ → R1 → TS1 → PuN₂) has a T-shaped transition state and pathway 2 (Pu + N₂ → R2 → TS2 → PuN + N) has an L-shaped transition state. Both transition states have only one imaginary frequency. According to the comparison of the energy at each stagnation point along the two pathways, and the heat energy emitted by the two reaction paths, we found that pathway 1 is the main reaction pathway. The nature of Pu-N bonding evolution along the pathways was studied by atoms in molecules (AIM) and electron localization function (ELF) topological approaches. In order to analyze the role of the plutonium atom 5f orbital in the reaction, the variation in density state along the pathways was measured. Results show that the 5f orbital mainly contributes to the formation of Pu-N bonds, and the influence of temperature on the reaction rate is revealed by calculating the rate constants of the two reaction pathways.

Selective oxo ligand functionalisation and substitution reactivity in an oxo/catecholate-bridged UIV/UIV Pacman complex

The oxo- and catecholate-bridged U^IV/U^IV Pacman complex [{(py)U^IVOU^IV(μ-O₂C₆H₄)(py)}(L^A)] A (L^A = a macrocyclic "Pacman" ligand; anthracenylene hinge between N₄-donor pockets, ethyl substituents on meso-carbon atom of each N₄-donor pocket) featuring a bent U^IV-O-U^IV oxo bridge readily reacts with small molecule substrates to undergo either oxo-atom functionalisation or substitution. Complex A reacts with H₂O or MeOH to afford [{(py)U^IV(μ-OH)₂U^IV(μ-O₂C₆H₄)(py)}(L^A)] (1) and [{(py)U^IV(μ-OH)(μ-OMe)U^IV(μ-O₂C₆H₄)(py)}(L^A)] (2), respectively, in which the bridging oxo ligand in A is substituted for two bridging hydroxo ligands or one bridging hydroxo and one bridging methoxy ligand, respectively. Alternatively, A reacts with either 0.5 equiv. of S₈ or 4 equiv. of Se to provide [{(py)U^IV(μ-η²:η²-E₂)U^IV(μ-O₂C₆H₄)(py)}(L^A)] (E = S (3), Se (4)) respectively, in which the [E₂]^2- ion bridges the two U^IV centres. To the best of our knowledge, complex A is the first example of either a d- or f-block bimetallic μ-oxo complex that activates elemental chalcogens. Complex A also reacts with XeF₂ or 2 equiv. of Me₃SiCl to provide [{(py)U^IV(μ-X)₂U^IV(μ-O₂C₆H₄)(py)}(L^A)] (X = F (5), Cl (6)), in which the oxo ligand has been substituted for two bridging halido ligands. Reacting A with either XeF₂ or Me₃SiCl in the presence of O(Bcat)₂ at room temperature forms [{(py)U^IV(μ-X)(μ-OBcat)U^IV(μ-O₂C₆H₄)(py)}(L^A)] (X = F (5A), Cl (6A)), which upon heating to 80 °C is converted to 5 and 6, respectively. In order to probe the importance of the bent U^IV-O-U^IV motif in A on the observed reactivity, the bis(boroxido)-U^IV/U^IV complex, [{(py)(pinBO)U^IVOU^IV(OBpin)(py)}(L^A)] (B), featuring a linear U^IV-O-U^IV bond angle was treated with H₂O and Me₃SiCl. Complex B reacts with two equiv. of either H₂O or Me₃SiCl to provide [{(py)HOU^IVOU^IVOH(py)}(L^A)] (7) and [{(py)ClU^IVOU^IVCl(py)}(L^A)] (8), respectively, in which reactions occur preferentially at the boroxido ligands, with the μ-oxo ligand unchanged. The formal U^IV oxidation state is retained in all of the products 1-8, and selective reactions at the bridging oxo ligand in A is facilitated by: (1) its highly nucleophilic character which is a result of a non-linear U^IV-O-U^IV bond angle causing an increase in U-O bond covalency and localisation of the lone pairs of electrons on the μ-oxo group, and (2) the presence of the bridging catecholate ligand, which destabilises a linear oxo-bridging geometry and stabilises the resulting products.

Reaction mechanism of synthetic thorium sulfides: theoretical calculation study

Actinide sulfides are especially significant in actinide chemistry because of their potentials that are used as nuclear fuel and the wide variety of their stoichiometries and physical properties. It is essential for studying the synthesis mechanism of actinide sulfides. In this study, the reactions of thorium cation Th²⁺ with the facile sulfur-atom donor OCS to produce thorium sulfides have been systematically explored by using density functional. The detailed insights of the primary reaction and secondary reaction paths are reported. We investigated that the multiple bonding characters and complexes involved in reaction exhibit significant covalent character. The reaction rate indicated that the tunneling effect is small compared with the effect of temperature on the rate. This study addresses some of the current limitation in understanding the detailed reaction information of Th²⁺+OCS.

Activation of Water by Thorium Cation: A Guided Ion Beam and Quantum Chemical Study

The reaction of atomic thorium cations with deuterated water as a function of kinetic energy from thermal to 10 eV was studied using guided ion beam tandem mass spectrometry. At thermal energies, both ThO⁺ + D₂ and DThO⁺ + D are formed in barrierless exothermic processes and reproduce results in the literature obtained using ion cyclotron resonance mass spectrometry. As the energy is increased, the branching ratio between these two channels changes such that the dominant product changes from ThO⁺ to DThO⁺ and back to ThO⁺, until ThD⁺ + OD is energetically available and is the dominant product channel. To help understand these experimental results, a variety of theoretical approaches were tried and used to establish a potential energy surface, which compares well with previous theoretical studies. Utilizing the theoretical results, the kinetic energy dependent branching ratio between the ThO⁺ + D₂ and DThO⁺ + D channels was calculated using both RRKM and phase space theory (PST). The results indicate that consideration of angular momentum conservation (as in PST) and spin-orbit corrected energies are needed to reproduce experimental results quantitatively. The PST modeling also provides relative energies for the two competing transition states that lead to the primary products, for which theory provides reasonable agreement. Graphical Abstract Note: This data is.

Electron correlation effects of the ThO and ThS molecules in the spinor basis. A relativistic coupled cluster study of ground and excited states properties

We present a comprehensive relativistic coupled cluster study of the electronic structures of the ThO and ThS molecules in the spinor basis. Specifically, we use the single-reference coupled cluster and the multi-reference Fock Space Coupled Cluster (FSCC) methods to model their ground and electronically-excited states. Two variants of the FSCC method have been investigated: (a) one where the electronic spectrum is obtained from sector (1,1) of the Fock space, and (b) another where the excited states come from the doubly attached electronic states to the doubly charged systems (ThO2+ and ThS2+), that is, from sector (0,2) of the Fock space. Our study provides a reliable set of spectroscopic parameters such as bond lengths, excitation energies, and vibrational frequencies, as well as a detailed analysis of the electron correlation effects in the ThO and ThS molecules. Finally, we examine the first ionization potential and electron affinity of the above mentioned molecules.

Gas-phase COS activation by U+: Reaction mechanisms and bonding analysis

Density functional theory (DFT) calculations were used to investigate the gas phase reaction of U[Formula: see text] with COS to produce US[Formula: see text]CO and UO[Formula: see text]CS. It is shown that the two reactions are exothermic and the formation of UO[Formula: see text]CS has the lower energy barrier which agrees with the experimental result that UO[Formula: see text] is the main product. The reaction mechanisms and the potential energy profiles (CPEPs) considering different spin states were presented in detail. Diverse analyses including atoms in molecules, natural bond orbital were used to study the bonding properties of all the involved species.

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.

Synthesis and reactivity of a terminal uranium(iv) sulfide supported by siloxide ligands

The S-transfer reaction from Ph₃PS to the tetrasiloxide U(iii) complex [U(OSi(O^tBu)₃)₄K] affords a stable U(iv) triply bonded terminal sulfide that can be protonated to yield a U(iv) doubly bonded terminal hydrosulfide.

The reactions of the tetrasiloxide U(iii) complexes [U(OSi(O^tBu)₃)₄K] and [U(OSi(O^tBu)₃)₄][K18c6] with 0.5 equiv. of triphenylphosphine sulfide led to reductive S-transfer reactions, affording the U(iv) sulfide complexes [SU(OSi(O^tBu)₃)₄K₂]₂, 1, and [{SU(OSi(O^tBu)₃)₄K₂}₂(μ-18c6)], 2, with concomitant formation of the U(iv) complex [U(OSi(O^tBu)₃)₄]. Addition of 1 equiv. of 2.2.2-cryptand to complex 1 resulted in the isolation of a terminal sulfide complex, [SU(OSi(O^tBu)₃)₄K][Kcryptand], 3. The crucial role of the K⁺ Lewis acid in these reductive sulfur transfer reactions was confirmed, since the formation of complex 3 from the reaction of the U(iii) complex [U(OSi(O^tBu)₃)₄][Kcryptand] and 0.5 equiv. of PPh₃S was not possible. Reactivity studies of the U(iv) sulfide complexes showed that the sulfide is easily transferred to CO₂ and CS₂ to afford S-functionalized products. Moreover, we have found that the sulfide provides a convenient precursor for the synthesis of the corresponding U(iv) hydrosulfide, {[(SH)U(OSi(O^tBu)₃)₄][K18c6]}, 5, after protonation with PyHCl. Finally, DFT calculations were performed to investigate the nature of the U–S bond in complexes 1, 3 and 5. Based on various analyses, triple-bond character was suggested for the U–S bond in complexes 1 and 3, while double-bond character was determined for the U–SH bond in complex 5.

The strength of actinide-element bonds from the quantum theory of atoms-in-molecules

[AnX(3)](2)(μ-η(2):η(2)-N(2)) (An = Th-Pu; X = F, Cl, Br, Me, H, OPh) have been studied using relativistic density functional theory. Geometric and vibrational data suggest that metal→N(2) charge transfer maximises at the protactinium systems, which feature the longest N-N bonds and the smallest σ(N-N), as a result of partial population of the N-N π* orbitals. There is very strong correlation of the standard quantum theory of atoms-in-molecules (QTAIM) metrics - bond critical point ρ, ∇(2)ρ and H and delocalisation indices - with An-N and N-N bond lengths and σ(N-N), but the correlation with An-N interaction energies is very poor. A similar situation exists for the other systems studied; neutral and cationic actinide monoxide and dioxides, and AnL(3+) and AnL(3)(3+) (L = pyridine (Py), pyrazine (Pz) and triazine (Tz)) with the exception of some of the ∇(2)ρ data, for which moderate to good correlations with energy data are sometimes seen. By contrast, in almost all cases there is very strong correlation of interaction and bond energies with |ΔQ(QTAIM)(An)|, a simple QTAIM metric which measures the amount of charge transferred to or from the actinide on compound formation.

Spectroscopy and structure of the simplest actinide bonds

Understanding the influence of electrons in partially filled f- and d-orbitals on bonding and reactivity is a key issue for actinide chemistry. This question can be investigated by using a combination of well-defined experimental measurements and theoretical calculations. Gas phase spectroscopic data are particularly valuable for the evaluation of theoretical models. Consequently, the primary objectives of our research have been to obtain gas phase spectra for small actinide molecules. To complement the experimental effort, we are investigating the potential for using relativistic ab initio calculations and semiempirical models to predict and interpret the electronic energy level patterns for f-element compounds. Multiple resonance spectroscopy and jet cooling techniques have been used to unravel the complex electronic spectra of Th and U compounds. Recent results for fluorides, sulfides, and nitrides are discussed.

Synthesis and properties of uranium sulfide cations. An evaluation of the stability of thiouranyl, {S═U═S}2+

Atomic uranium cations, U(+) and U(2+), reacted with the facile sulfur-atom donor OCS to produce several monopositive and dipositive uranium sulfide species containing up to four sulfur atoms. Sequential abstraction of two sulfur atoms by U(2+) resulted in US2(2+); density functional theory computations indicate that the ground-state structure for this species is side-on η(2)-S2 triangular US2(2+), with the linear thiouranyl isomer, {S═U(VI)═S}(2+), some 171 kJ mol(-1) higher in energy. The result that the linear thiouranyl structure is a local minimum at a moderate energy suggests that it should be feasible to stabilize this moiety in molecular compounds.