Spectroscopic investigations of the electronic structure of neptunyl ions.

Molecular electronic structures are innately sensitive to the geometric and chemical environments around the metal center of coordination compounds . However, the interrelationships between the electronic structures and molecular geometries of actinide species, which often contain more than one electron in the Sf valence shell, are quite complex due to the large numbers of possible electronic states and high densities of vibronically enabled transitions .1'2 Investigations of the optical signatures of simple, well-defined molecular systems should provide the most straightforward approach for unharnessing these fundamental relationships, and in particular, systems with a single electron in the valence Sf shell, such as the neptunyl ion (Np0 22+), should provide the most viable means for characte rizing actinide electronic structure. Furthermore, Sf orbital-occupied actinide systems exhibit not only visible and ultraviolet ligand-to-metal charge-transfer spectral bands, but also near-infrared Sf-Sf transitions resulting from promotion of a Sf electron to an orbital of primarily Sf character

Energy-Degeneracy-Driven Covalency in Actinide Bonding

Evaluating the nature of chemical bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic density functional theory study that quantitatively evaluates An–Cl covalency in AnCl62– (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2g*/eg*), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amount of M–Cl orbital mixing, such that AnIV 6d- and Cl 3p-mixing decreased and metal 5f- and Cl 3p-orbital mixing increased across this series

The dominant Anopheles vectors of human malaria in the Asia-Pacific region: occurrence data, distribution maps and bionomic précis

<p>Abstract</p> <p>Background</p> <p>The final article in a series of three publications examining the global distribution of 41 dominant vector species (DVS) of malaria is presented here. The first publication examined the DVS from the Americas, with the second covering those species present in Africa, Europe and the Middle East. Here we discuss the 19 DVS of the Asian-Pacific region. This region experiences a high diversity of vector species, many occurring sympatrically, which, combined with the occurrence of a high number of species complexes and suspected species complexes, and behavioural plasticity of many of these major vectors, adds a level of entomological complexity not comparable elsewhere globally. To try and untangle the intricacy of the vectors of this region and to increase the effectiveness of vector control interventions, an understanding of the contemporary distribution of each species, combined with a synthesis of the current knowledge of their behaviour and ecology is needed.</p> <p>Results</p> <p>Expert opinion (EO) range maps, created with the most up-to-date expert knowledge of each DVS distribution, were combined with a contemporary database of occurrence data and a suite of open access, environmental and climatic variables. Using the Boosted Regression Tree (BRT) modelling method, distribution maps of each DVS were produced. The occurrence data were abstracted from the formal, published literature, plus other relevant sources, resulting in the collation of DVS occurrence at 10116 locations across 31 countries, of which 8853 were successfully geo-referenced and 7430 were resolved to spatial areas that could be included in the BRT model. A detailed summary of the information on the bionomics of each species and species complex is also presented.</p> <p>Conclusions</p> <p>This article concludes a project aimed to establish the contemporary global distribution of the DVS of malaria. The three articles produced are intended as a detailed reference for scientists continuing research into the aspects of taxonomy, biology and ecology relevant to species-specific vector control. This research is particularly relevant to help unravel the complicated taxonomic status, ecology and epidemiology of the vectors of the Asia-Pacific region. All the occurrence data, predictive maps and EO-shape files generated during the production of these publications will be made available in the public domain. We hope that this will encourage data sharing to improve future iterations of the distribution maps.</p

Cerium(III) Carbonate Formation from {CeCp*₂H}₂ and Carbon Dioxide: Structure and Mechanistic Insights

The reaction between {CeCp*₂H}₂ with CO₂ in thf resulted in a new cerium­(III) carbonate species, {Ce­(Cp*)₂(thf)}₂(μ-η²:η¹-CO₃). Investigations into the mechanism suggest the intermediacy of a bridging oxo-complex, {CeCp*₂(thf)_<i>x</i>}₂(μ-O), which could either be formed from the ring opening and oxygen abstraction from an oxygenated solvent such as thf or via insertion of CO₂ and elimination of formaldehyde from the resulting methylenediolate intermediate

The multiple phenylpropene synthases in both Clarkia breweri and Petunia hybrida represent two distinct protein lineages

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74442/1/j.1365-313X.2008.03412.x.pd

Unexpected Actinyl Cation-Directed Structural Variation in Neptunyl(VI) A‑Type Tri-lacunary Heteropolyoxotungstate Complexes

A-type tri-lacunary heteropolyoxotungstate anions (e.g., [PW₉O₃₄]^9–, [AsW₉O₃₄]^9–, [SiW₉O₃₄]^10–, and [GeW₉O₃₄]^10–) are multidentate oxygen donor ligands that readily form sandwich complexes with actinyl cations ({UO₂}²⁺, {NpO₂}⁺, {NpO₂}²⁺, and {PuO₂}²⁺) in near-neutral/slightly alkaline aqueous solutions. Two or three actinyl cations are sandwiched between two tri-lacunary anions, with additional cations (Na⁺, K⁺, or NH₄⁺) also often held within the cluster. Studies thus far have indicated that it is these additional +1 cations, rather than the specific actinyl cation, that direct the structural variation in the complexes formed. We now report the structural characterization of the neptunyl­(VI) cluster complex (NH₄)₁₃[Na­(NpO₂)₂(A-α-PW₉O₃₄)₂]·12H₂O. The anion in this complex, [Na­(NpO₂)₂(PW₉O₃₄)₂]^13–, contains one Na⁺ cation and two {NpO₂}²⁺ cations held between two [PW₉O₃₄]^9– anions, with an additional partial occupancy NH₄⁺ or {NpO₂}²⁺ cation also present. In the analogous uranium­(VI) system, under similar reaction conditions that include an excess of NH₄Cl in the parent solution, it was previously shown that [(NH₄)₂(U^VIO₂)₂(A-PW₉O₃₄)₂]^12– is the dominant species in both solution and the crystallized salt. Spectroscopic studies provide further proof of differences in the observed chemistry for the {NpO₂}²⁺/[PW₉O₃₄]^9– and {UO₂}²⁺/[PW₉O₃₄]^9– systems, both in solution and in solid state complexes crystallized from comparable salt solutions. This work reveals that varying the actinide element (Np vs U) can indeed measurably impact structure and complex stability in the cluster chemistry of actinyl­(VI) cations with A-type tri-lacunary heteropolyoxotungstate anions

Tetrahalide Complexes of the [U(NR)₂]²⁺ Ion: Synthesis, Theory, and Chlorine K‑Edge X‑ray Absorption Spectroscopy

Synthetic routes to salts containing uranium bis-imido tetrahalide anions [U­(NR)₂X₄]^2– (X = Cl^–, Br^–) and non-coordinating NEt₄⁺ and PPh₄⁺ countercations are reported. In general, these compounds can be prepared from U(NR)₂I₂(THF)_<i>x</i> (<i>x</i> = 2 and R = ^<i>t</i>Bu, Ph; <i>x</i> = 3 and R = Me) upon addition of excess halide. In addition to providing stable coordination complexes with Cl^–, the [U­(NMe)₂]²⁺ cation also reacts with Br^– to form stable [NEt₄]₂[U­(NMe)₂Br₄] complexes. These materials were used as a platform to compare electronic structure and bonding in [U­(NR)₂]²⁺ with [UO₂]²⁺. Specifically, Cl K-edge X-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid density functional theory (DFT and TDDFT) were used to probe U–Cl bonding interactions in [PPh₄]₂[U­(N^<i>t</i>Bu)₂Cl₄] and [PPh₄]₂[UO₂Cl₄]. The DFT and XAS results show the total amount of Cl 3p character mixed with the U 5f orbitals was roughly 7–10% per U–Cl bond for both compounds, which shows that moving from oxo to imido has little effect on orbital mixing between the U 5f and equatorial Cl 3p orbitals. The results are presented in the context of recent Cl K-edge XAS and DFT studies on other hexavalent uranium chloride systems with fewer oxo or imido ligands

Tetrahalide Complexes of the [U(NR)₂]²⁺ Ion: Synthesis, Theory, and Chlorine K‑Edge X‑ray Absorption Spectroscopy

Synthetic routes to salts containing uranium bis-imido tetrahalide anions [U­(NR)₂X₄]^2– (X = Cl^–, Br^–) and non-coordinating NEt₄⁺ and PPh₄⁺ countercations are reported. In general, these compounds can be prepared from U(NR)₂I₂(THF)_<i>x</i> (<i>x</i> = 2 and R = ^<i>t</i>Bu, Ph; <i>x</i> = 3 and R = Me) upon addition of excess halide. In addition to providing stable coordination complexes with Cl^–, the [U­(NMe)₂]²⁺ cation also reacts with Br^– to form stable [NEt₄]₂[U­(NMe)₂Br₄] complexes. These materials were used as a platform to compare electronic structure and bonding in [U­(NR)₂]²⁺ with [UO₂]²⁺. Specifically, Cl K-edge X-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid density functional theory (DFT and TDDFT) were used to probe U–Cl bonding interactions in [PPh₄]₂[U­(N^<i>t</i>Bu)₂Cl₄] and [PPh₄]₂[UO₂Cl₄]. The DFT and XAS results show the total amount of Cl 3p character mixed with the U 5f orbitals was roughly 7–10% per U–Cl bond for both compounds, which shows that moving from oxo to imido has little effect on orbital mixing between the U 5f and equatorial Cl 3p orbitals. The results are presented in the context of recent Cl K-edge XAS and DFT studies on other hexavalent uranium chloride systems with fewer oxo or imido ligands

Determining Relative f and d Orbital Contributions to M–Cl Covalency in MCl₆^2– (M = Ti, Zr, Hf, U) and UOCl₅^– Using Cl K-Edge X‑ray Absorption Spectroscopy and Time-Dependent Density Functional Theory

Chlorine K-edge X-ray absorption spectroscopy (XAS) and ground-state and time-dependent hybrid density functional theory (DFT) were used to probe the electronic structures of <i>O</i>_<i>h</i>-MCl₆^2– (M = Ti, Zr, Hf, U) and <i>C</i>_4<i>v</i>-UOCl₅^–, and to determine the relative contributions of valence 3d, 4d, 5d, 6d, and 5f orbitals in M–Cl bonding. Spectral interpretations were guided by time-dependent DFT calculated transition energies and oscillator strengths, which agree well with the experimental XAS spectra. The data provide new spectroscopic evidence for the involvement of both 5f and 6d orbitals in actinide–ligand bonding in UCl₆^2–. For the MCl₆^2–, where transitions into d orbitals of <i>t</i>_2<i>g</i> symmetry are spectroscopically resolved for all four complexes, the experimentally determined Cl 3p character per M–Cl bond increases from 8.3(4)% (TiCl₆^2–) to 10.3(5)% (ZrCl₆^2–), 12(1)% (HfCl₆^2–), and 18(1)% (UCl₆^2–). Chlorine K-edge XAS spectra of UOCl₅^– provide additional insights into the transition assignments by lowering the symmetry to <i>C</i>_4<i>v</i>, where five pre-edge transitions into both 5f and 6d orbitals are observed. For UCl₆^2–, the XAS data suggest that orbital mixing associated with the U 5f orbitals is considerably lower than that of the U 6d orbitals. For both UCl₆^2– and UOCl₅^–, the ground-state DFT calculations predict a larger 5f contribution to bonding than is determined experimentally. These findings are discussed in the context of conventional theories of covalent bonding for d- and f-block metal complexes