15 research outputs found
Representative Communication: Web Site Interactivity and Distributional Path Dependence in the U.S. Congress
Deliberação online em consultas pĂșblicas? O caso da assembleia legislativa de Minas Gerais
Understanding the Scarcity of Thorium Peroxide Clusters
The reaction of ThÂ(NO<sub>3</sub>)<sub>4</sub>·5H<sub>2</sub>O with 3 equiv of 2,2âČ,6âČ,2âł-terpyridine (terpy) in a mixture of acetonitrile and methanol results in formation of the trinuclear thorium peroxide cluster [ThÂ(O<sub>2</sub>)Â(terpy)Â(NO<sub>3</sub>)<sub>2</sub>]<sub>3</sub>. This cluster is assembled via bridging by ÎŒâη<sup>2</sup>:η<sup>2</sup> peroxide anions between thorium centers. It decomposes upon removal from the mother liquor to yield ThÂ(terpy)Â(NO<sub>3</sub>)<sub>4</sub> and ThÂ(terpy)Â(NO<sub>3</sub>)<sub>4</sub>(EtOH). The peroxide formation appears to be radiolytic in origin and is, most likely, generated from radiolysis of water by short-lived daughters generated from <sup>232</sup>Th decay. This cluster does not form when freshly recrystallized ThÂ(NO<sub>3</sub>)<sub>4</sub>·5H<sub>2</sub>O is used as the starting material and requires an aged source of thorium. Analysis of the bonding in these clusters shows that, unlike uraniumÂ(VI) peroxide interactions, thoriumÂ(IV) complexation by peroxide is quite weak and largely ionic. This explains its much lower stability, which is more comparable to that observed in similar zirconiumÂ(IV) peroxide clusters
Characterization of Lanthanide Complexes with Bis-1,2,3-triazole-bipyridine Ligands Involved in Actinide/Lanthanide Separation
The
complexation of selected trivalent lanthanide ions with derivatives
of the tetranitrogen donor ligands 6,6âČ-bis-1R,1<i>H</i>-1,2,3-triazol-4-yl-2,2âČ-bipyridines (BTzBPs, R = alkyl or
aryl) was investigated in solid state and in solution. An anhydrous
solid [CeÂ(Bn-BTzBP)Â(NO<sub>3</sub>)<sub>3</sub>] (Bn = benzene) complex
was synthesized and characterized by single-crystal X-ray diffraction.
EuÂ(III) complexes with the 2-ethylÂ(hexyl) derivative EH-BTzBP in methanol
were studied by time-resolved fluorescence spectroscopy. Earlier studies
have identified the EH-BTzBP as a potentially useful solvent extraction
reagent for the separation of americium from lanthanide metal ions,
a challenging component of advanced nuclear fuel cycles for actinide
transmutation. To help identify species formed in the extraction process,
the influence of 2-bromohexanoic acid (identified as an essential
component of the separation system) on EuÂ(III) complexes was investigated.
Comparison with an organic phase after extraction of EuÂ(III) by EH-BTzBP
and 2-bromohexanoic acid showed that both 1:1 and 1:2 (Eu/EH-BTzBP)
complexes are involved in the extraction. UVâvisible spectrophotometry
was used to compare EuÂ(III) stability constants with those of other
LnÂ(III) complexes
Uncovering the Origin of Divergence in the CsM(CrO4)2 (M = La, Pr, Nd, Sm, Eu; Am) Family through Examination of the Chemical Bonding in a Molecular Cluster and by Band Structure Analysis
A series
of f-block chromates, CsMÂ(CrO<sub>4</sub>)<sub>2</sub> (M = La, Pr,
Nd, Sm, Eu; Am), were prepared revealing notable differences
between the Am<sup>III</sup> derivatives and their lanthanide analogs.
While all compounds form similar layered structures, the americium
compound exhibits polymorphism and adopts both a structure isomorphous
with the early lanthanides as well as one that possesses lower symmetry.
Both polymorphs are dark red and possess band gaps that are smaller
than the Ln<sup>III</sup> compounds. In order to probe the origin
of these differences, the electronic structure of α-CsSmÂ(CrO<sub>4</sub>)<sub>2</sub>, α-CsEuÂ(CrO<sub>4</sub>)<sub>2</sub>,
and α-CsAmÂ(CrO<sub>4</sub>)<sub>2</sub> were studied using both
a molecular cluster approach featuring hybrid density functional theory
and QTAIM analysis and by the periodic LDA+GA and LDA+DMFT methods.
Notably, the covalent contributions to bonding by the f orbitals were
found to be more than twice as large in the Am<sup>III</sup> chromate
than in the Sm<sup>III</sup> and Eu<sup>III</sup> compounds, and even
larger in magnitude than the Am-5f spinâorbit splitting in
this system. Our analysis indicates also that the AmâO covalency
in α-CsAmÂ(CrO<sub>4</sub>)<sub>2</sub> is driven by the degeneracy
of the 5f and 2p orbitals, and not by orbital overlap