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The article discusses quantum chemical calculations and experimental investigations of molecular actinide oxides. Topics covered include the body of theoretical studies on actinide oxide molecules, the primary methodologies used to include electron correlation in studying actinide oxides, and experimental determinations of the important physical properties for elementary actinide molecules such as rotational spectroscopy and electronic spectroscopy.

A comprehensive review of the thermodynamic properties of the oxide compounds of the lanthanide and actinide elements is presented. The available literature data for the solid, liquid, and gaseous state have been analysed and recommended values are presented. In case experimental data are missing, estimates have been made based on the trends in the two series, which are extensively discussed. [ABSTRACT FROM AUTHOR]

The 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 C2 v 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. [ABSTRACT FROM AUTHOR]

The vibrational frequencies of the actinide oxides AnO and AnO2 (An = Th, Pa, U, Np, Pu, Am, Cm) and of their mono- and dications have been calculated using advanced quantum chemical techniques. The stretching fundamental frequencies of the monoxides have been determined by fitting the potential function to single-point energies obtained by relativistic CASPT2 calculations along the stretching coordinate and on this basis solving numerically the ro-vibrational Schrödinger equation. To obtain reliable fundamental frequencies of the dioxides, we developed an empirical approach. In this approach the harmonic vibrational frequencies of the AnO20/+/2+ species were calculated using eight different exchange-correlation DFT functionals. On the basis of the good correlation found between the vibrational frequencies and computed bond distances, the final frequency values were derived for the CASPT2 reference bond distances from linear regression equations fitted to the DFT data of each species. As a test, the approach provided excellent agreement with accurate experimental data of ThO, ThO+, UO, and UO+. The joint analysis of literature experimental and our computed data facilitated the prediction of reliable gas-phase molecular properties for some oxides. They include the stretching frequencies of PuO, ThO2, UO2, and UO2+ and the bond distance of PuO (1.818 Å, being likely within 0.002 Å of the real value). Also the derived equilibrium bond distances of ThO2, UO2, and UO2+ (1.896, 1.790, and 1.758 Å, respectively) should approximate closely the (yet unknown) experimental values. On the basis of the present results, we suggest that the ground electronic state of PuO2 in Ar and Kr matrices is probably different from that in the gaseous phase, similarly to UO2 observed previously. [ABSTRACT FROM AUTHOR]

The mineral cheralite [CaTh(PO4)2] allows for the incorporation of tetravalent actinides in monazitebased crystalline phases. Experimental determination of its thermodynamic properties is crucial for defining its stability and subsequent long-term ability to immobilize radionuclides. Low-temperature heat capacity from 0.5 to 300 K, enthalpy increments from 485 to 1565 K, and the enthalpy of formation of cheralite from the oxides were measured and reported on for the first time. At 298.15 K, S° = (201.6 ± 2.6) J/(K·mol), which includes the configurational entropy of Ca and Th mixing, (This character cannot be represented in ASCII code). In aqueous environments, cheralite is able to form from whitlockite or apatite and thorianite. Under anhydrous conditions, cheralite can form by solid-state reaction only if the resultant product includes very stable Ca salts instead of CaO. [ABSTRACT FROM AUTHOR]

In the present study we evaluated trends in the bond distances and dissociation enthalpies of actinide oxides AnO and AnO2 (An = Th-Lr) on the basis of consistent computed data obtained by using density functional theory in conjunction with relativistic small-core pseudopotentials. Computations were carried out on AnO (An = Th-Lr) and AnO2 (An = Np, Pu, Bk-Lr) species, while for the remaining AnO2 species recent literature data (Theor. Chem. Acc. 2011, 129, 657) were utilized. The most important computed properties include the geometries, vibrational frequencies, dissociation enthalpies, and several excited electronic states. These molecular properties of the late actinide oxides (An = Bk-No) are reported here for the first time. We present detailed analyses of the bond distances, covalent bonding properties, and dissociation enthalpies. [ABSTRACT FROM AUTHOR]

The electronic structure and various molecular properties of the actinide (An) dicarbides ThC2 and UC2 were investigated by relativistic quantum chemical calculations. We probe five possible geometrical arrangements: two triangular structures including an acetylide (C2) moiety, as well as the linear AnCC, CAnC, and bent CAnC geometries. Our calculations at various levels of theory indicate that the triangular species are energetically more favorable, while the latter three arrangements proved to be higher-energy structures. Our SO-CASPT2 calculations give the ground-state molecular geometry for both ThC2 and UC2 as the symmetric (C2v) triangular structure. The similar and, also very close in energy, asymmetric (Cs) triangular geometry belongs to a different electronic state. DFT and single-determinant ab initio methods failed to distinguish between these two similar electronic states demonstrating the power of multiconfiguration ab initio methods to deal with such subtle and delicate problems. We report detailed data on the electronic structure and bonding properties of the most relevant structures. [ABSTRACT FROM AUTHOR]