Actinide Contraction Research Articles

Features of Actinide Contraction in Crystals AnN, AnP, and AnAs (An = Th, Pa, U, Np, Pu, Am, Cm, Bk)

Features of Actinide Contraction in Crystals AnN, AnP, and AnAs (An = Th, Pa, U, Np, Pu, Am, Cm, Bk)

Features of actinide contraction in AnХ crystals (X = S, Se or Te)

Using Voronoi–Dirichlet polyhedra, the features of actinide contraction in chalcogenides AnX, where An is Th, U, Np, Pu, Am, or Cm, and X is S, Se, or Te, were revealed and discussed. The characteristics of AnX and isostructural chalcogenides LnX, where Ln is lanthanide, were compared. It was found that, in contrast to LnX, which exhibit the effect of lanthanide contraction, in AnX crystals with increasing atomic number of An some characteristics change nonlinearly: in many cases the volume of Voronoi–Dirichlet polyhedra of An atoms as well as the length of An–X bonds and An–An contacts increase instead of decreasing. It was shown that the detected anomalies can be considered a consequence of the bonding 5f interactions, which are characteristic of U, Np, and Pu atoms, but are absent in the case of Th and transplutonium actinides. Using AnX and LnX as examples, a crystal-chemical method for evaluating the covalence of An–X and Ln–X bonds has been tested.

New Complexes of Actinides with Monobromoacetate Ions: Synthesis and Structures.

Synthesis, FTIR spectral study, and X-ray diffraction analysis of single crystals of (CH3)4N[UO2(mba)3] (I), (CH3)4N[NpO2(mba)2(NO3)] (II), (CH3)4N[PuO2(mba)2(NO3)] (III), and (CH3)4N[NpO2(mba)(NO3)2] (IV), where mba is a monobromoacetate ion (CH2BrCOO–), were conducted. The main structural units of crystals I–IV are mononuclear anionic complexes of the [AnO2(mba)3]−, [AnO2(mba)2(NO3)]−, or [AnO2(mba)(NO3)2]− composition. All these complex units are characterized with the same crystal-chemical formula AB013 (A = AnO22+ and B01 = CH2BrCOO– or NO3–). Using the method of molecular Voronoi–Dirichlet polyhedra, the contributions of various types of noncovalent interactions into the formation of supramolecular structures of the obtained complexes were characterized. The analysis of coordination modes of all monobromoacetate-containing compounds from the Cambridge Structural Database was accomplished. Actinide contraction in the studied compounds is discussed.

Evaluation of chemical bonding and electronic structures in trisodium actinate for actinide series from thorium to curium

Despite growing interest in actinide (An) complexes and their bonding chemistry in recent years, the characteristics of the chemical bonding of actinides in solid-state compounds remain to be relatively unclear compared with those of molecular complexes. Herein, we systematically characterized the structural and electronic properties of trisodium actinates [Na3AnO4 (An = Th to Cm)] using first-principles theoretical methods. In general, the band structures exhibit a transition from charge-transfer insulator with the p-d band gap type to Mott-insulator with the f-f band gap type at Na3PuO4. The conjugation effects of actinide contraction and the decreased energy of the 5f bands in these compounds are responsible for the transition. Consistently, the pentavalent An ions can be achieved from Pa to Pu, and then upon the reduction of the 5f atomic orbitals, strong spin polarization leads to tetravalent Am(f5)IV and Cm(f6)IV. These results improve our knowledge of the OS of actinides in solid-state compounds and establish the connection among local structure, chemical composition, and properties for understanding the underlying mechanism of An chemical state in actinide materials for specific applications.

\u03b4 and \u03c6 back-donation in AnIV metallacycles

In all known examples of metal–ligand (M–L) δ and φ bonds, the metal orbitals are aligned to the ligand orbitals in a “head-to-head” or “side-to-head” fashion. Here, we report two fundamentally new types of M–L δ and φ interactions; “head-to-side” δ and “side-to-side” φ back-bonding, found in complexes of metallacyclopropenes and metallacyclocumulenes of actinides (Pa–Pu) that makes them distinct from their corresponding Group 4 analogues. In addition to the known Th and U complexes, our calculations include complexes of Pa, Np, and Pu. In contrast with conventional An–C bond decreasing, due to the actinide contraction, the An–C distance increases from Pa to Pu. We demonstrate that the direct L–An σ and π donations combined with the An–L δ or φ back-donations are crucial in explaining this non-classical trend of the An–L bond lengths in both series, underscoring the significance of these δ/φ back-donation interactions, and their importance for complexes of Pa and U in particular.

Lanthanum and Lanthanide Trifluorides: Lanthanide Contraction and Volume of Fluorine Ion

Lanthanide contraction of lanthanum and 14 lanthanides (Ln), leading to a decrease in R3+ ionic radii by ~15% (with respect to the smallest one) with an increase in the element atomic number Z, is a way to implement a close packing of F1– ions in trifluorides of heavy 4f elements of the periodic table of elements. The efficiency of lanthanide (Ln) and actinide (An) contraction in the formation of inorganic fluoride crystals with a minimum unit-cell volume per fluorine ion (VF) is estimated. For AnF3 (An = Ac–Am), the VF value decreases from ~20 to ~17.4 A3 with an increase in Z from 89 to 95. The unavailability of heavy An makes this value final. The close packing of the fluorine ion in fluorides could not be obtained using the effects of lanthanide and actinide contraction and increase in the cation valence. Fluorides LnF3 (structure type β-YF3) from the end of the series from ErF3 (15.86 A3) to LuF3 (15.48 A3) are nearest to the anion close packing. The latter VF value is minimum among all studied LnF3 compounds and actinide fluorides (both simple and complex) and close to the saturation value. It corresponds to the estimated fluorine ionic radius of 1.246 A.

Guided Ion Beam and Quantum Chemical Investigation of the Thermochemistry of Thorium Dioxide Cations: Thermodynamic Evidence for Participation of f Orbitals in Bonding.

Kinetic energy dependent reactions of ThO+ with O2 are studied using a guided ion beam tandem mass spectrometer. The formation of ThO2+ in the reaction of ThO+ with O2 is observed to be slightly endothermic and also exhibits two obvious features in the cross section. These kinetic energy dependent cross sections were modeled to determine a 0 K bond dissociation energy of D0(OTh+-O) = 4.94 ± 0.06 eV. This value is slightly larger but within experimental uncertainty of less precise previously reported experimental values. The higher energy feature in the ThO2+ cross section was also analyzed and suggests formation of an excited state of the product ion lying 3.1 ± 0.2 eV above the ground state. Additionally, the thermochemistry of ThO2+ was explored by quantum chemical calculations, including a full Feller-Peterson-Dixon (FPD) composite approach with correlation contributions up to CCSDT(Q) and four-component spin-orbit corrections, as well as more approximate CCSD(T) calculations including semiempirical estimates of spin-orbit energy contributions. The FPD approach predicts D0(OTh+-O) = 4.87 ± 0.04 eV, in good agreement with the experimental value. Analogous FPD results for ThO+, ThO, and ThO2 are also presented, including ionization energies for both ThO and ThO2. The ThO2+ bond energy is larger than those of its transition metal congeners, TiO2+ and ZrO2+, which can be attributed partially to an actinide contraction, but also to contributions from the participation of f orbitals on thorium that are unavailable to the transition metal systems.

SPECTROSCOPIC AND THERMOCHEMICAL PROPERTIES OF ACTINIDE-CONTAINING SPECIES FROM FIRST PRINCIPLES: THORIUM AND AMERICIUM MONOXIDE MOLECULES

A relativistic version of a composite ab initio treatment of molecular spectroscopy and thermochemistry is developed, focusing on high-accuracy description of the properties of actinide (An) containing species. It is based on combining the calculation results at levels of theory with sufficiently full account of electron correlation, e.g., at the CCSDT(Q) level, but tackling only scalar relativity, with those obtained from more rigorous four-component relativistic calculations with the Dirac–Coulomb Hamiltonian. High accuracy achievable via this approach is revealed taking the examples of thorium and americium monoxide molecules. The errors in ab initio values for the bond length re, vibrational frequency ωe, and atomization energy D0 of the ThO molecule did not exceed 0.001 Å, 2.5 cm–1, and 0.5 kcal/mol, respectively. The composite numerical values for the first ionization potentials of the AmO molecule and the Am atom deviate from the experimental data just by 0.03 eV and 1 cm–1, respectively. For the first time, the proposed approach enabled high-accuracy evaluation of the molecular constants re, ωe and D0 for AmO and AmO+, as well as the second and third ionization potentials of the Am atom. The calculation results are indicative of a minor actinide contraction of the An–O bonds on going through the molecular series ThO → UO → AmO: the bond length in AmO is by 0.0073 Å shorter than that in ThO. The re(An–O) value is shown to be linearly dependent on the actinide atomic number in the periodic table. The results obtained may be used as benchmarks for parametrizing and calibrating the DFT functionals designed for treating An-containing molecules.

Monte Carlo simulation of thermophysical properties of liquid Ni-15%X (Co, Cu, Yb) alloys

In this paper, the embedded-atom method was used to study the influence of different solute elements (Co, Cu, Yb) on the thermophysical properties of liquid Ni-based alloys. By exploring the relationship between surface tension, viscosity, diffusion coefficient and temperature of liquid Ni-based alloys with three solutes in the range of 1500 ~ 1900 K, we found that under certain components the surface tension of liquid Ni-based alloys with three solutes decreased as temperature increased. Except the case that the atomic radius of the same period decreased with the increase of the atomic number and influence of lanthanide and actinide contraction, the surface tension of liquid Ni-based alloys decreased with the increase of atomic number. The viscosity of liquid alloy showed a downward trend with increasing temperature, and the viscosity decreased exponentially with the temperature increasing under certain components. The variation trend of diffusion coefficient of the three kinds of atoms was similar to that of the temperature, which increased with the increase of temperature under the same component. Especially, the diffusion coefficient increases exponentially with the increase of temperature under certain components. These simulated data provided necessary thermophysical parameters for non-equilibrium dynamics analysis of liquid-phase alloys and also supplied a sound theoretical basis for exploring the internal physical mechanism of “liquid microstructures–thermophysical properties–preferred phase of nucleation”.

Peculiarities of the Supramolecular Assembly of Tetraethylammonium and 3-Bromopropionate Ions in Uranyl, Neptunyl, and Plutonyl Coordination Compounds.

Synthesis and X-ray diffraction studies of {N(C2H5)4}[AnO2(C2H4BrCOO)3] [An = U (I), Np (II), or Pu (III)] and C2H4BrCOOH (IV), where C2H4BrCOO- is an anion of the 3-bromopropionic acid, are reported. The isostructural coordination compounds I-III contain mononuclear anionic complexes [AnO2(C2H4BrCOO)3]- belonging to the crystal chemical group AB013 (A = AnO22+; B01 = C2H4BrCOO-). In the crystal structure of IV, the C2H4BrCOOH molecules are hydrogen bonded into centrosymmetric dimers R22(8). Using the method of molecular Voronoi-Dirichlet (VD) polyhedra, the features of intermolecular interactions in crystals of I-IV are discussed in support of the results of IR and UV spectroscopy experiments. Actinide contraction in I-III manifests itself in a regular reduction of the average length of the axial and equatorial bonds in hexagonal bipyramids AnO8, in an increase in νas(AnO22+) wavenumbers, and in a simultaneous decrease in the volume and sphericity degree of VD polyhedra of An atoms in the U-Np-Pu series. The title compounds represent an interesting architecture, where 3-bromopropionate ions penetrate through the square 44 net of tetraethylammonium ions and thus bind adjacent nets via the "locking effect".

Bond energy of ThN+: A guided ion beam and quantum chemical investigation of the reactions of thorium cation with N2 and NO

Kinetic-energy dependent reactions of Th+ with N2 and NO are studied using a guided ion beam tandem mass spectrometer. The formation of ThO+ in the reaction of Th+ with NO is observed to be exothermic and barrierless with a reaction efficiency at low energies of 0.91 ± 0.18. Formation of ThN+ in the reactions of Th+ with N2 and NO is endothermic in both cases. The kinetic-energy dependent cross sections for formation of this product ion were evaluated to determine a 0 K bond dissociation energy (BDE) of D0(Th+-N) = 6.51 ± 0.08 eV, the first direct measurement of this BDE. Additionally, the reactions were explored by quantum chemical calculations, including a full Feller-Peterson-Dixon composite approach with correlation contributions up to CCSDTQ for ThN and ThN+, as well as more approximate CCSD(T) calculations where a semiempirical model was used to estimate spin-orbit energy contributions. The ThN+ BDE is found to be larger than those of the transition metal congeners, TiN+ along with estimated values for ZrN+ and HfN+, believed to be a result of the actinide contraction.

Actinide Contraction in Oxygen-Containing An(VI) Compounds

The actinide contraction effect in the structures of 55 crystals containing $$\rm{AnO}_2^{2+}$$ dioxocations (An = U, Np, Pu, Am) and belonging to 18 series of isostructural An(VI) compounds was analyzed using Voronoi-Dirichlet (VD) polyhedra. Analysis shows that the actinide contraction, as a rule, is accompanied by a regular decrease in the radii of the spherical domains (Rsd) of the volume equal to that of the VD polyhedra of the An(VI) atoms in the crystal structures. In the series U-Np-Pu, such contraction is accompanied by a regular increase in the second moment of inertia (G3) of the VD polyhedra, suggesting a decrease in the extent of their sphericity (due to stronger flattening of the coordination polyhedron in the form of bipyramid), whereas in going from Pu to Am the extent of sphericity of the VD polyhedron increases instead of decreasing. The results obtained confirm the viewpoint that the contraction and localization of the 5f shell start specifically from Am. The lack of correlation between the actinide contraction and relative strength of An-O bonds in the series U-Np-Pu-Am is noted.

Synthesis and Structure of U(VI), Np(VI), and Pu(VI) 2-Fluorobenzoates

The compounds NH4[AnO2(C6H4FCOO3], where An = U (I), Np (II), or Pu (III), CgH4COO− is the 2-fluorobenzoate anion, were synthesized and studied by single crystal X-ray diffraction. Compounds I–III are isostructural and crystallize in the cubic system, space group P213, Z = 4. The main structural units of I–III are mononuclear complexes [AnO2(C6H4COO)3]− belonging to crystal-chemical group AB31 (A = AnO22+, B01 = C6H4FCOO−). The actinide contraction in the structures of I–III is manifested in a regular decrease in the lengths of the An=0 bonds in the AnO22+ cations and in the volumes of the Voronoi-Dirichlet polyhedra (VDPs) of the An atoms in the series U-Np-Pu. The intermolecular interactions in crystal structures of I–III were analyzed by the method of molecular VDPs.

Uncovering Heavy Actinide Covalency: Implications for Minor Actinide Partitioning.

Across the actinide period, the stability of the trivalent oxidation state predominates in the heavy actinides, making their chemical nature close to that of rare earth elements. The resemblance in their chemistry poses difficulties in separating heavy actinides from lanthanides, which is a vital separation in the minor actinide partitioning process. Actinide contraction has conventionally implied electrostatic actinide-ligand interactions among the heavy actinides. The present study challenges this conventional understanding and reveals increasing covalency in the actinide-ligand bond across Am to Cf. Complexes of Am, Cm, Bk, and Cf have been examined for their electronic structure with a focus on the nature of their interactions with different ligands within the framework of density functional theory, where the relativistic effects have been incorporated by using zero-order regular approximation and spin-orbit coupling. The choice of ligands selected for this study facilitates the effect of the donor atom as well as denticity to be accounted for. Hence, heavy actinide complexes of the N- and O-donor ligand dipicolinic acid, S and O mixed donor ligands of the Cyanex type, and an octadentate ligand N, N, N' N'-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine have been optimized and evaluated. In each case energy decomposition analysis has been used to explicitly decompose the metal-ligand interaction energy into components which have then been analyzed. Irrespective of the hard-soft characteristics of donor atoms or the denticity of the ligands, steadily increased covalency has been observed across Am to Cf. Inspection of the ligand highest energy occupied molecular orbitals and metal orbitals sheds light on the origin of the unexpected covalency. An overall increase in bonding and also the orbital contribution along the Am-Cf series is clearly due to the enhancement in covalency, which is complementary to the orbital degeneracy induced covalency proposed very recently by Batista and co-workers.

Actinide Organometallic Complexes with π-Ligands.

Recent developments and results from the organometallic chemistry of the actinides are reviewed. In the last one and a half years the structural data of about 15 organometallic complexes of transuranium actinides (Np or Pu) have been published, all involving π‐ligands in the coordination sphere of the metal ion. On the basis of these data, a comparison of these molecules is presented. Depending on the steric demands of the ligands, effects like the actinide contraction seem to be stronger or weaker in the structural features. This indicates that the interplay between the actinide ion and the π‐ligand is rather flexible, enabling the formation of stable bonds over a broad range of actinide ion oxidation states.

Stereochemistry of Bk, Cf, and Es in Oxygen-Containing Compounds

Specific features of the coordination of the Bk, Cf, and Es atoms in the crystal structures of oxygencontaining compounds were considered using the Voronoi–Dirichlet polyhedra (VDPs). These actinides (An) form coordination polyhedra AnOn (6 ≤ n ≤ 9) of five types. At a fixed An oxidation state, the An VDP volume is virtually independent of the coordination number n. Some parameters of the atomic VDPs in the An sublattices were characterized, and compounds in which binding An–An 5f interactions occur in the crystal structures were revealed. As demonstrated by the example of compounds containing coordination polyhedra AnO9 (An = U–Cf) in the form of tricapped trigonal prisms, the VDP parameters can be used for evaluation of the actinide contraction of An(III) atoms in the crystal structures.

Halogen bonding, actinide contraction and coordination modes of ligands in uranyl, neptunyl and plutonyl trichloroacetates with ammonium cations

Halogen bonding, actinide contraction and coordination modes of ligands in uranyl, neptunyl and plutonyl trichloroacetates with ammonium cations

Thorium(IV) and Uranium(IV) Complexes with Cucurbit[5]uril.

Tetravalent thorium and uranium complexes with cucurbit[5]uril (Q[5]) were investigated with eight new complexes being synthesized and structurally characterized. [Th(Q[5])(OH)(H2O)2]6·18NO3· nH2O (1) has a hexagonal nanowheel structure with each of the six Th4+ ions being cap-coordinated by a Q[5] and monodentate-coordinated to the nearby Q[5]. [Th(Q[5])(HCOO)(H2O)4][Th(NO3)5(H2O)2]2[Th(NO3)3(HCOO)(H2O)2]0.5·NO3· nH2O (2) has a heteroleptic mononuclear structure with a Th4+ ion cap-coordinated on one side of the Q[5] portal and monodentate-coordinated to a formate anion inside the Q[5] cavity. [KTh1.5(Q[5])Cl(NO3)3][Th(NO3)5(H2O)2]·2NO3·2.5H2O (3) has a heterometallic structure with both Th4+ and K+ ions each occupying one side of the two Q[5] portals forming a capsule. [CsTh(Q[5])Cl(NO3)2(H2O)3]·2NO3· nH2O (4) has a heterometallic 1D polymeric structure with both Th4+ and Cs+ ions each occupying one side of the two Q[5] portals, forming monomers which are linked together by sharing two water molecules and one carbonyl oxygen atom between Th4+ and Cs+ ions. [Th(Q[5])Cl(H2O)][CdCl3][CdCl4]·0.5HCl·4H2O (5), [Th(Q[5])Cl(H2O)][Ru2OCl9(H2O)]·0.5HCl·9.5H2O (6), [Th(Q[5])Cl(H2O)][IrCl6]1.5·3H2O (7), and [U(Q[5])Cl(H2O)][ZnCl3(H2O)][(ZnCl4)]·8H2O (8) have similar 1D polymeric structures with Th4+/U4+ ions cap-coordinated on one side of a Q[5] and bidentate coordinated to the nearby Q[5]. The transition metal chlorides act as anions for charge compensation as well as structure inducers via cation-anion interactions forming various anion patterns around the 1D polymers. Actinide contraction has been observed in the early actinide series.

Synthesis and Structure of Mixed Cromate–Nitrate Complexes of Hexavalent Actinides, [(CH3)4N][(AnO2)(CrO4)(NO3)] (An = U, Np, Pu)

Mixed-anion compounds of the general composition [(CH3)4N][(AnO2)(CrO4)(NO3)], where An = U, Np, and Pu, were synthesized and structurally characterized. The structural motif of these compounds is based on anionic ribbons of the composition [AnO2(NO3)(CrO4)]nn–, consisting of seven-vertex An polyhedra linked via oppositely oriented CrO4 tetrahedra and NO3 groups acting in the An polyhedra as terminal bidentate ligands. Every four anionic ribbons form channels oriented along b-axis. Tetramethylammonium cations linked with the O atoms of the ribbons by hydrogen bonds are located in these channels. Actinide contraction is observed in the series U–Np–Pu. It is manifested in a regular decrease in the interatomic distances in the An polyhedra, in parameters b and c, and in the unit cell volume.

Synthesis, Structure, and Vibrational Properties of [Ph4P]2NpO2Cl4 and [Ph4P]2PuO2Cl4 Complexes

The synthesis, structure, and vibrational properties are presented for an isostructural series of Np(VI) and Pu(VI) complexes of the form [Ph4P]2AnO2Cl4, where An = Np(VI) or Pu(VI). The reported complexes are readily synthesized in ambient laboratory conditions, and their molecular structures were determined using single crystal X-ray diffraction. Their vibrational spectra were studied using a combination of Raman and FT-IR vibrational spectroscopies. Analysis of the vibrational spectra and force constants highlight the periodic properties associated with the actinide contraction and filling of the 5f electronic shells. Additionally, we have assessed the utility of these complexes as conveniently synthesized starting materials for non-aqueous synthesis of transuranium molecules and materials.