Actinide Atoms Research Articles

Gas‐phase ammonia activation by Th, Th+, and Th2+: Reaction mechanisms, bonding analysis, and rate constant calculations

The gas‐phase reactions of Th, Th+, and Th2+ with NH3 were systematically investigated using different approaches of density functional theory. A detailed description of the reaction mechanisms along with the bonding character analysis offers deep insights into the reaction of Th species. Different possible spin multiplicities were considered as well as the effect of spin‐orbit interactions. The analyses of results indicate that the reactions of Th species with NH3 have two types of reaction channel: isomerization and dehydrogenation. These dehydrogenation reactions are found to be exothermic, with the best thermochemical conditions observed for Th2++NH3. The bonding properties of the species involved in the reaction pathways were investigated by means of diverse analyses including electron localization function, atoms in molecules, and natural bond orbital. Reaction rate constants were computed between 298 and 2000 K at levels of variational transition state theory with the Wigner and Eckart tunneling correction. The computed results are compared with the available experimental values. A comparison with previously studied reactions of Th species with H2O and CH4 as well as U species with NH3 is performed to obtain similarities and differences of the insertion reactions of actinide atom and cations. © 2014 Wiley Periodicals, Inc.

High-Resolution Solid-State Oxygen-17 NMR of Actinide-Bearing Compounds: An Insight into the 5f Chemistry

A massive interest has been generated lately by the improvement of solid-state magic-angle spinning (MAS) NMR methods for the study of a broad range of paramagnetic organic and inorganic materials. The open-shell cations at the origin of this paramagnetism can be metals, transition metals, or rare-earth elements. Actinide-bearing compounds and their 5f unpaired electrons remain elusive in this intensive research area due to their well-known high radiotoxicity. A dedicated effort enabling the handling of these highly radioactive materials now allows their analysis using high-resolution MAS NMR (>55 kHz). Here, the study of the local structure of a series of actinide dioxides, namely, ThO2, UO2, NpO2, PuO2, and AmO2, using solid-state (17)O MAS NMR is reported. An important increase of the spectral resolution is found due to the removal of the dipolar broadening proving the efficiency of this technique for structural analysis. The NMR parameters in these systems with numerous and unpaired 5f electrons were interpreted using an empirical approach. Single-ion model calculations were performed for the first time to determine the z component of electron spin on each of the actinide atoms, which is proportional to the shifts. A similar variation thereof was observed only for the heavier actinides of this study.

Theoretical study of Pu and Am tetracarbide molecules

The electronic structure and ground‐state molecular properties of Pu and Am tetracarbides have been investigated by relativistic multireference calculations using CASSCF/CASPT2 theory as well as by density functional theory in conjunction with relativistic pseudopotentials. The CASSCF/CASPT2 treatment has been extended by spin–orbit coupling effects for selected species using the CAS state‐interaction method. The five atoms can form various structural isomers, from which 12 ones have been identified in our study. The electronic ground state in both molecules corresponds to a planar fan‐type structure of C2v symmetry, in which the actinide atom is connected to a bent C4 moiety. The other structures are much higher in energy, the ones computed in this study appear between 250 and 1050 kJ/mol. The bonding characteristics in the most relevant structures have been analyzed on the basis of the valence molecular orbitals and natural bond orbital analysis. The most stable structures have been characterized by their spectroscopic (vibrational and electron) properties. © 2014 Wiley Periodicals, Inc.

Theoretical Study of Thorium and Uranium Tetracarbide Molecules

AbstractThe electronic structure and ground‐state molecular properties of neutral Th and U tetracarbides were investigated by relativistic multireference calculations using complete active space self‐consistent field/multiconfiguration second‐order perturbation theory (CASSCF/CASPT2) with an all‐electron basis set as well as by density functional theory (B3LYP) in conjunction with relativistic pseudopotentials. The former calculations were extended with complete active space state interaction (CASSI) calculations accounting for spin–orbit coupling. The four carbon atoms can be arranged in various forms around the actinide atom to result in ten structural isomers found altogether in our study. The electronic ground state in both molecules corresponds to a planar fan‐type structure of C2v symmetry, in which the actinide atom is connected to a bent C4 moiety. Three structures arise from various arrangements of C2 moieties around the Th and U, whereas six isomers contain a C3 moiety and a lone C bonded to the actinide. The latter CAnC3‐type structures are very high‐energy structures (over 200 kJ mol–1 with respect to the ground state). The bonding characteristics in the most relevant structures were analysed on the basis of the valence molecular orbitals and the natural bond orbital method. We evaluated and interpreted the vibrational and electronic spectra. The present computed data for UC4 suggest a revision of the assignment of a previously observed IR absorption band to a different isomer.

Structure and Other Molecular Properties of Actinide Trichlorides AnCl3 (An = Th–Cm)

The ground-state molecular properties of the trichlorides of light actinides (An = Th-Cm) have been predicted by state-of-the-art quantum chemical calculations. The ground electronic states have been determined by multireference calculations at the CASPT2 level including both scalar and spin-orbit relativistic effects. These studies supported the expected single-configuration character of ThCl3 and CmCl3 with their well-defined 6dσ/7s hybrid and 5f(7) configurations, respectively. In contrast, the intermediate actinides (PaCl3-AmCl3) with partly filled 5f shells have numerous very low-lying excited states and consequently a mixed character of the spin-orbit ground states. Apart from the planar ThCl3 the ground-state molecular geometries proved to be pyramidal with C(3v) symmetry. The gradually decreasing An-Cl bond distances reveal the actinide contraction known for the atomic and ionic radii of these actinide atoms. Other ground-state molecular properties as vibrational frequencies and natural charges have been obtained by density functional theory calculations using the B3LYP exchange-correlation functional in conjunction with small-core relativistic energy-consistent pseudopotentials for the actinides.

Antenna Effect by Organometallic Chromophores in Bimetallic d–f Complexes

The nature of the intermetallic bond in a series of complexes of the type [Cp2-TM-M-Cp2] (where TM = Re and M = Y, La, Lu, Yb, Ac; also TM = Os and M = Th; Cp = cyclopentadienyl ligand) have been studied by relativistic two-component density functional theory (DFT) calculations. The results obtained in this work show that the interaction between the transition metal and lanthanide atoms is mainly ionic in all cases, while for the case of actinide atoms this interaction becomes significantly more covalent. The effective direction of the electron transfer between the Re→Ac or Os→Th centers allows us to propose that the [Cp2ReAcCp2] and [Cp2OsThCp2] complexes are ideal candidates for near-infrared (NIR) technologies since their absorption spectra show some transitions over 600 nm. We also observed a shifting of the absorption spectrum of around 100 nm of the [Cp2Re] fragment when is compared against the absorption spectrum of the entire complex. This behavior allows us to argue that the [Cp2Re] fragment is a good antenna chromophore due to the possibility of charge transfer transitions from this fragment to the f shell in lanthanide or actinide elements studied here.

Crystal Growth and First Crystallographic Characterization of Mixed Uranium(IV)–Plutonium(III) Oxalates

The mixed-actinide uranium(IV)-plutonium(III) oxalate single crystals (NH4)0.5[Pu(III)0.5U(IV)0.5(C2O4)2·H2O]·nH2O (1) and (NH4)2.7Pu(III)0.7U(IV)1.3(C2O4)5·nH2O (2) have been prepared by the diffusion of different ions through membranes separating compartments of a triple cell. UV-vis, Raman, and thermal ionization mass spectrometry analyses demonstrate the presence of both uranium and plutonium metal cations with conservation of the initial oxidation state, U(IV) and Pu(III), and the formation of mixed-valence, mixed-actinide oxalate compounds. The structure of 1 and an average structure of 2 were determined by single-crystal X-ray diffraction and were solved by direct methods and Fourier difference techniques. Compounds 1 and 2 are the first mixed uranium(IV)-plutonium(III) compounds to be structurally characterized by single-crystal X-ray diffraction. The structure of 1, space group P4/n, a = 8.8558(3) Å, b = 7.8963(2) Å, Z = 2, consists of layers formed by four-membered rings of the two actinide metals occupying the same crystallographic site connected through oxalate ions. The actinide atoms are nine-coordinated by oxygen atoms from four bidentate oxalate ligands and one water molecule, which alternates up and down the layer. The single-charged cations and nonbonded water molecules are disordered in the same crystallographic site. For compound 2, an average structure has been determined in space group P6/mmm with a = 11.158(2) Å and c = 6.400(1) Å. The honeycomb-like framework [Pu(III)0.7U(IV)1.3(C2O4)5](2.7-) results from a three-dimensional arrangement of mixed (U0.65Pu0.35)O10 polyhedra connected by five bis-bidentate μ(2)-oxalate ions in a trigonal-bipyramidal configuration.

Electronic structure of predicted endohedral fullerenes An@C40 (An = Th–Md)

The geometry optimization of the neutral molecules An@C40 (An=Th–Md) was carried out using the DFT based DMol3 method. For the calculations of electronic structure of these complexes we used the fully relativistic discrete variational method (RDV). The systems containing Th, Pa, U and Np have considerable energetic stability, which is noticeably greater than that of corresponding exohedral and “networked” complexes. The clusters containing actinide atom from Pu to Fm are also more stable than empty C40 cage. It was found, that the 5f-orbitals contribution to chemical bonding is noticeably less than that of the 6d-states even for the complexes at the beginning of An@C40 row. The effective charges on the atoms were calculated using integral scheme incorporated in RDV and Hirshfeld procedure of DMol3.

Multi-edge X-ray absorption spectroscopy of thorium, neptunium and plutonium hexacyanoferrate compounds

Although transition metal cyano bimetallic compounds have been well known for decades for their interesting optical and magnetic properties, reports on actinide hexacyanoferrate compounds are scarce. For instance, a thorough structural description is still lacking. Another question is the possible covalency or charge transfer effects in these materials that are known to foster electron delocalization with a large variety of transition metal cations. In this paper, new members of the actinide(IV) hexacyanoferrates have been synthesized with Th, Np and Pu. This is the first review of thorium to plutonium hexacyanoferrate compounds since the early investigations during the Manhattan Project some 70 years ago. We have carried out an extensive structural characterization using powder X-ray Diffraction (XRD), X-ray Absorption Spectroscopy (XAS) and X-ray microscopy for the plutonium adduct. The crystallographic space group of microcrystalline Th, Np and Pu hexacyanoferrate compounds appears to be very similar to that of the early lanthanide adducts, suggesting that the tetravalent actinides are arranged in a tricapped trigonal prismatic polyhedron of coordination number 9, in which the actinide atom is bonded to six nitrogen atoms and to three water molecules. Further combined analysis of the iron K-edge and actinide LIII-edge EXAFS data and XRD data provided the basis for a three-dimensional molecular model. Structural data in terms of actinide–ligand bond lengths have been compared to those reported for the parent lanthanide(III) compounds, confirming the structural similarities. In addition, two new structures with the thorium cation have been obtained and described using single-crystal XRD: (H5O2)[Th(DMF)5(H2O)]2[Fe(CN)6]3 and [Th(DMF)4(H2O)3][Fe(CN)6](NO3)·2H2O. This structural description of the Th, Np and Pu hexacyanoferrate system will be followed by a semi-quantitative electronic description of the actinide–cyano bond using NEXAFS data analysis in a coming paper.

Theoretical study of the Pu and Am dicarbide molecules

The electronic structure and ground-state molecular properties of Pu and Am dicarbides were investigated in this study by relativistic multireference calculations using CASSCF/CASPT2 theory. The electronic ground states in both the molecules correspond to a symmetrical triangular structure, in which the actinide atom is connected to a C2 moiety. The other experimentally relevant structure, the symmetrical linear arrangement, is higher than 100 kJ/mol in energy. The bonding characteristics in the two structures were analysed on the basis of the valence molecular orbitals. Besides the usual two-electron bonds, a few one-electron bonding orbitals were also identified. We evaluated the electronic spectra with complete active space state interaction (CASSI) calculations taking into account spin–orbit coupling. The infrared and Raman spectral characteristics were obtained using DFT calculations. Assessment of the present results with the previously reported data on Th and U dicarbides provides information on the change of various molecular parameters in the actinide row.

Linear quantum capacitance scaling for lanthanide and actinide atoms: Analysis of two differing sets of electron-affinity predictions

Scaling of quantum capacitances is explored for lanthanide and actinide atoms. For lighter atoms, quantum capacitances have been seen to scale linearly with mean radii of the atoms' outermost occupied orbitals. This scaling law is used to analyze two recent, differing sets of theoretical calculations for lanthanide electron affinities $A$. Consistent with the scaling law, $A$ values predicted by O'Malley and Beck for lanthanides [Phys. Rev. A 78, 012510 (2008)], using a relativistic configuration interaction (RCI) method, produce capacitances that scale with the atoms' mean radii along a single line to a high degree of confidence. Similar linear scaling behavior also results for the actinides from O'Malley and Beck's RCI calculations of their $A$ values. However, lanthanide $A$ values predicted by Felfli et al. [Phys. Rev. A 81, 042707 (2010)], using a Regge-pole approach, unexpectedly produce capacitance scaling along two different lines for atoms with similar neutral electron configurations. Both types of linear capacitance scaling are internally consistent, though, and do not serve to determine definitively which set of electron affinity predictions for the lanthanides is likely to be more accurate. Still, evidence from this and prior capacitance scaling investigations tends to favor the O'Malley and Beck results. In addition, linear capacitance scaling for the actinides is applied to estimate the previously unknown $A$ values for Fm and Md as 0.007 eV and $\ensuremath{-}0.006$ eV, respectively.

Infrared Spectra of the η2-M(NC)-CH3, CH3-MNC, and CH2═M(H)NC Complexes Prepared by Reactions of Thorium and Uranium Atoms with Acetonitrile

The η2-M(NC)-CH3, CH3-MNC, and CH2═M(H)NC complexes are observed in the matrix IR spectra from reactions of laser-ablated Th and U atoms with acetonitrile. These actinide products are in line with those from reactions of group 4–6 metals with acetonitrile, and they are again the most stable steps in the previously proposed reaction path. The N-coordination and methylidyne products (M←NCCH3 and HCM(H)2NC) are not observed probably due to their high energies. The actinide methylidenes show agostic distortion, and the nitrile π-complexes (η2-M(NC)-CH3) support strong back-donations to the π*-orbitals. NBO analysis reveals single M–N and M–C bonds with full σ- and π-bonds in the N═C subunit. The unidentified N-coordinated-acetonitrile complexes show strong calculated interactions with these early actinide atoms.

Dimethyl sulfoxide as an O-donor ligand in tetravalent actinide complexes

Four new perchlorate complexes of tetravalent actinides with dimethyl sulfoxide (DMSO) molecules (An4+ = Th, U, Np, Pu) are synthesized and studied. According to the X-ray diffraction data, compounds [Th(DMSO)9](ClO4)4 · 2CH3CN (I), [U(DMSO)8](ClO4)4 · CH3CN (II), [Np(DMSO)8](ClO4)4 · CH3CN (III), and [Pu(DMSO)8](ClO4)4 · CH3CN (IV) crystallize in the triclinic crystal system (space group P1). The crystals of compounds II–IV are isostructural. The absorption spectra of the complexes in the IR and visible regions are measured. All compounds exhibit a decrease in the frequencies of stretching vibrations ν(SO) over the spectrum of free DMSO, indicating the formation of the O-bonded complexes of An4+. The optical spectra of the crystalline compounds exhibit shifts of the bands of electronic f-f transitions of the An4+ ions relative to the hydrated ions: the bathochromic shifts for the U and Np complexes and the hypsochromic shift for the Pu complex. The first coordination sphere of the actinide atoms in the studied complexes is highly stable.

Bulk properties and photoelectron spectroscopy of the ζ-U–Pu phase

The ζ-phase, existing between 35% and 70% U in Pu, belongs to the high-density phases seen from the point of view of systematics of allotropic modifications of Pu metal. Despite the volume per actinide atom only slightly higher than for α-Pu, it magnetic susceptibility is much higher than for α-Pu and exceeds even the δ-Pu value. Similarly, the Sommerfeld coefficient γ > 40 mJ/mol Pu K 2 exceeds the experimental δ-Pu value. The data confirm that the volume is not the primary control parameter affecting the situation around the Fermi level of common Pu phases and they point against the traditional belief that they are essentially narrow 5 f band systems. Electronic structure calculations suggest that the 5 f states of Pu have slightly lower occupancy comparing with δ-Pu. A tendency to the 5 f localization can be seen in the Pu dilution situation in the photoelectron spectra.

First-principles investigation of higher oxides of uranium and neptunium:U3O8and Np2O5

A computational study is presented of the structural, electronic, and magnetic properties of ${\mathrm{U}}_{3}$O${}_{8}$ and Np${}_{2}$O${}_{5}$, which are actinide oxides in a higher oxidation state than the tetravalent state of the common dioxide phases, UO${}_{2}$ and NpO${}_{2}$. The calculations are based on the density functional theory+$U$ approach, in which additional Coulomb correlations on the actinide atom are taken into account. The calculated properties of these two higher oxidized actinide oxides are analyzed and compared to those of their tetravalent analogs. The optimized structural parameters of these noncubic oxides are found to be in reasonable agreement with available experimental data. ${\mathrm{U}}_{3}$O${}_{8}$ is predicted to be a magnetic insulator, having one U atom in a hexavalent oxidation state and two U atoms in a pentavalent oxidation state. For Np${}_{2}$O${}_{5}$, which is also predicted to be an insulator, a complicated noncollinear magnetic structure is computed, leading to a nonzero overall magnetization with a slight antiferromagnetic canting. The calculated electronic structures are presented and the variation of the U $5f$ or Np $5f$--O $2p$ hybridization with the oxidation state is analyzed. With increasing oxygen content, the nearly localized 5$f$ electrons of the actinide elements are more positioned near the Fermi level and the hybridization between 5$f$ and 2$p$ states is markedly increased.

Crystal structure of 2,2′-bipyridine complexes of Np(V) and Pu(V) m-nitrobenzoates

Abstract Single crystals of [AnO2(bipy)(OOCC6H4NO2)(H2O)]·0.5bipy (An=Np, Pu, bipy=2,2′-bipyridine) have been synthesized and their structures have been determined by X-ray analysis. The compounds are isomorphous and contain electroneutral complexes [AnO2(bipy)(OOCC6H4NO2)(H2O)] and solvate bipy molecules. Actinide atoms in both compounds are seven coordinated. Actinide contraction results in a shortening of all bonds in the coordination spheres at the transition from Np to Pu. The structures are compared with those of other An(V) benzoate compounds. In contrast to earlier structurally characterized An(V) benzoates, cation–cation interaction in [AnO2(bipy)(OOCC6H4NO2)(H2O)] complexes is absent. Lower complexing ability of m-nitrobenzoate is compensated by engagement of water molecule into the coordination sphere of An(V). This is accompanied by formation of strong H-bonds between AnO2 + cations and coordinated water molecules.

Actinide sulfides in the gas phase: experimental and theoretical studies of the thermochemistry of AnS (An = Ac, Th, Pa, U, Np, Pu, Am and Cm)

The gas-phase thermochemistry of actinide monosulfides, AnS, was investigated experimentally and theoretically. Fourier transform ion cyclotron resonance mass spectrometry was employed to study the reactivity of An(+) and AnO(+) (An = Th, Pa, U, Np, Pu, Am and Cm) with CS(2) and COS, as well as the reactivity of the produced AnS(+) with oxidants (COS, CO(2), CH(2)O and NO). From these experiments, An(+)-S bond dissociation energies could be bracketed. Density functional theory studies of the energetics of neutral and monocationic AnS (An = Ac, Th, Pa, U, Np, Pu, Am and Cm) provided values for bond dissociation energies and ionization energies; the computed energetics of neutral and monocationic AnO were also obtained for comparison. The theoretical data, together with comparisons with known An(+)-O bond dissociation energies and M(+)-S and M(+)-O dissociation energies for the early transition metals, allowed for the refining of the An(+)-S bond dissociation energy ranges obtained from experiment. Examination of the reactivity of AnS(+) with dienes, coupled to comparisons with reactivities of the AnO(+) analogues, systematic considerations and the theoretical results, allowed for the estimation of the ionization energies of the AnS; the bond dissociation energies of neutral AnS were consequently derived. Estimates for the case of AcS were also made, based on correlations of the data for the other An and the electronic energetics of neutral and ionic An. The nature of the bonding in the elementary molecular actinide chalcogenides (oxides and sulfides) is discussed, based on both the experimental data and the computed electronic structures. DFT calculations of ionization energies for the actinide atoms and the diatomic sulfides and oxides are relatively reliable, but the calculation of bond dissociation energies is not uniformly satisfactory, either with DFT or CCSD(T). A key conclusion from both the experimental and theoretical results is that the 5f electrons do not substantially participate in actinide-sulfur bonding. We emphasize that actinides form strikingly strong bonds with both oxygen and sulfur.

The Homoleptic U(NCSe)84– Anion in (Pr4N)4U(NCSe)8·2CFCl3 and Th(NCSe)4(OP(NMe2)3)4·0.5CH3CN·0.5H2O: First Structurally Characterised Actinide Isoselenocyanates

AbstractThe neutral thorium complex Th(NCSe)4(OP(NMe2)3)4 and homoleptic octa(isoselenocyanato)uranate anion U(NCSe)84– in (Pr4N)4U(NCSe)8·2CFCl3 (1) were synthesised and structurally characterised. (Pr4N)4U(NCSe)8·2CFCl3 contains the UIV anion U(NCSe)84– and was characterised using IR spectroscopy and single‐crystal X‐ray diffraction. Th(NCSe)4(OP(N(CH3)2)3)4·0.5CH3CN·0.5H2O (2) was characterised using IR and Raman spectroscopy, as well as 31P{1H}, 15N{1H}, 14N{1H}, 13C{1H}, 1H and 77Se NMR spectroscopy and structurally characterised using single‐crystal X‐ray diffraction. The U(NCSe)84– anion and Th(NCSe)4(OP(N(CH3)2)3)4·0.5CH3CN·0.5H2O complex are the first structurally characterised actinide‐isoselenocyanates. The crystal structures shows an approximate square antiprismatic arrangement of the ligands around the actinide(IV) atoms.

Synthesis and crystal structure of new U(VI) and Np(VI) benzoates, K11(AnO2)23(OOCC6H5)57(H2O)18+x

The structure of new double benzoates of hexavalent actinides, K11(AnO2)23(O2C7H5)57(H2O)18+x (An = U, Np), was studied. There are five crystallographically independent actinide atoms in the crystals. The coordination polyhedra of An(1), An(3), An(4), and An(5) are distorted hexagonal bipyramids, and that of An(2) is a distorted pentagonal bipyramid. Ten crystallographically independent benzoate ions have been found in the crystals. All but one anions are bidentate chelating and form with AnO 2 2+ cations either electrically neutral [AnO2(O2C7H5)2(H2O)2][An(1)O 2 2+ cations] or negatively charged [AnO2(O2C7H5)3]−[An(3)O 2 2+ , An(4)O 2 2+ , An(5)O 2 2+ cations] complexes. The bidentate bridging benzoate ion links two adjacent An(2)O 2 2+ cations with the formation of peculiar electrically neutral cyclic fragments [AnO2(O2C7H5)2(H2O)]6 arranged perpendicular to the c-axis. There are three independent K+ cations in the structure. The coordination surrounding of K(1)+ (coordination number CN 6) is formed by oxygen atoms of two electrically neutral [AnO2(O2C7H5)2(H2O)2] fragments and of the complex anion [AnO2(O2C7H5)3]−. The coordination surrounding of K(2)+ (CN 6) consists of oxygen atoms of three complex anions [AnO2(O2C7H5)3]−. The disordered K(3)+ cation is arranged near the sixfold axis between the cyclic fragments [AnO2(O2C7H5)2(H2O)]6. An important structure-forming factor in the crystals is hydrogen bonding involving coordinated water molecules. In the coordination polyhedra of actinides, the An-O bond lengths regularly decrease in going from U to Np owing to actinide contraction.

Electronic Properties of ζ-U-Pu Alloys

AbstractThe ζ-phase, existing between 35 and 70% U in Pu, belongs to the high density phases seen from the point of view of systematics of allotropic modifications of Pu metal. Despite the volume per actinide atom only slightly higher than for α-Pu, it magnetic susceptibility is much higher than for α-Pu and exceeds even the δ-Pu value. Similarly, the Sommerfeld coefficient γ > 40 mJ/mol Pu K2 exceeds the experimental δ-Pu value. The data confirm that the volume is not the primary control parameter affecting the situation around the Fermi level of common Pu phases and they point against the traditional belief that they are essentially narrow 5f band systems.