Molecular Structure and Vibrational Spectra of Mixed MDyX₄ (M = Li, Na, K, Rb, Cs; X = F, Cl, Br, I) Vapor Complexes: A Computational and Matrix-Isolation Infrared Spectroscopic Study

The structures, energetic, and vibrational properties of MDyX₄ (M = Li, Na, K, Rb, Cs; X = F, Cl, Br, I) mixed alkali halide/dysprosium halide complexes have been investigated by a joint computational and experimental, matrix-isolation Fourier-transform infrared spectroscopic (MI-IR), study. According to our DFT computations for the complexes with heavier halides and alkali metals the ground-state structure is the tridentate isomer; while at high temperatures the bidentate structural isomer dominates. The survey of various dissociation processes revealed the preference of the dissociation to neutral MX and DyX₃ fragments over ionic and radical dissociation products. Cationic complexes are considerably less stable at 1000 K than the neutral complexes, and they prefer to dissociate to M⁺ + DyX₄^• fragments. The vapor species of selected mixtures of NaBr and CsBr with DyBr₃ and of CsI with DyI₃ in the temperature range 900–1000 K have been isolated in krypton and xenon matrices and investigated by infrared spectroscopy. Besides the characteristic vibrational frequencies of the monomeric and dimeric alkali halide species and of the dysprosium trihalide molecules, certain signals indicated the formation of MDyX₄ (M = Na, Cs; X = Br, I) mixed complexes. Comparison with the computed vibrational and thermodynamic characteristics of the relevant species lead to the conclusion that these complexes appear in the vapor predominantly as the <i>C</i>_2v-symmetry bidentate isomer. This is the first time that this structure was identified in an experimental vibrational spectroscopic study. The signals appearing upon performing a thermal anneal cycle were tentatively assigned to the double complex M₂DyX₅ (M = Na, Cs; X = Br, I). A structure in which one alkali atom is bound to dysprosium by three and the other by two bridges is proposed for these double complexes

Theoretical Study of the Structure and Bonding in ThC₂ and UC₂

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 (<i>C</i>_2<i>v</i>) triangular structure. The similar and, also very close in energy, asymmetric (<i>C</i>_<i>s</i>) 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

A Uranyl Peroxide Dimer in the Gas Phase

The gas-phase uranyl peroxide dimer, [(UO₂)₂(O₂)­(L)₂]²⁺ where L = 2,2′-trifluoroethylazanediyl)­bis­(<i>N</i>,<i>N</i>′-dimethylacetamide), was synthesized by electrospray ionization of a solution of UO₂²⁺ and L. Collision-induced dissociation of this dimer resulted in endothermic O atom elimination to give [(UO₂)₂(O)­(L)₂]²⁺, which was found to spontaneously react with water via exothermic hydrolytic chemisorption to yield [(UO₂)₂(OH)₂(L)₂]²⁺. Density functional theory computations of the energies for the gas-phase reactions are in accord with observations. The structures of the observed uranyl dimer were computed, with that of the peroxide of particular interest, as a basis to evaluate the formation of condensed phase uranyl peroxides with bent structures. The computed dihedral angle in [(UO₂)₂(O₂)­(L)₂]²⁺ is 145°, indicating a substantial deviation from the planar structure with a dihedral angle of 180°. Energies needed to induce bending in the most elementary gas-phase uranyl peroxide complex, [(UO₂)₂(O₂)]²⁺, were computed. It was found that bending from the lowest-energy planar structure to dihedral angles up to 140° required energies of <10 kJ/mol. The gas-phase results demonstrate the inherent stability of the uranyl peroxide moiety and support the notion that the uranyl-peroxide-uranyl structural unit is intrinsically planar, with only minor energy perturbations needed to form the bent structures found in studtite and uranyl peroxide nanostructures