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Journal Articles

Structural study on 2,2'-(methylimino)bis($$N$$,$$N$$-dioctylacetamide) complex with Re(VII)O$$_{4}$$$$^{-}$$ and Tc(VII)O$$_{4}$$$$^{-}$$ by $$^{1}$$H-NMR, EXAFS and IR spectroscopy

Saeki, Morihisa; Sasaki, Yuji; Nakai, Ayaka*; Ohashi, Akira*; Banerjee, D.*; Scheinost, A. C.*; Foerstendorf, H.*

Inorganic Chemistry, 51(10), p.5814 - 5821, 2012/05

 Times Cited Count:23 Percentile:70.08(Chemistry, Inorganic & Nuclear)

The structures of the complex of 2,2'-(methylimino)bis($$N$$,$$N$$-dioctylacetamide) (MIDOA) with M(VII)O$$_{4}$$$$^{-}$$ (M=Re and Tc), which were prepared by liquid-liquid solvent extraction, were investigated by using $$^{1}$$H-NMR, EXAFS and IR spectroscopy. The results from $$^{1}$$H-NMR and EXAFS provide therefore evidence of M(VII)O$$_{4}$$$$^{-}$$...H$$^{+}$$MIDOA complex formation in the organic solution. The IR spectra of Re(VII)O$$_{4}$$$$^{-}$$...H$$^{+}$$MIDOA and Tc(VII)O$$_{4}$$$$^{-}$$...H$$^{+}$$MIDOA were analyzed based on the structures and the IR spectra calculated at the B3LYP/cc-pVDZ level. Comparison of the observed and calculated IR spectra demonstrates that an intramolecular hydrogen bond is formed in H$$^{+}$$MIDOA and the M(VII)O$$_{4}$$$$^{-}$$ ion interact with H$$^{+}$$MIDOA through multiple C-H$$_{n}$$...O hydrogen bonds.

Journal Articles

Redox-dependant sulphate coordination of neptunium in aqueous solutions

Hennig, C.*; Ikeda, Atsushi; Tsushima, Satoru*; Scheinost, A. C.*

ESRF Highlights 2009, p.84 - 85, 2010/02

The sulphate coordination to neptunium ion has been investigated in aqueous solution by X-ray absorption spectroscopy. The results suggest that the coordination mode of sulphate ion strongly depends the oxidation state of neptunium, and changes from uni- to bidentate modes.

Journal Articles

Neptunium carbonato complexes in aqueous solution; An Electrochemical, spectroscopic, and quantum chemical study

Ikeda, Atsushi; Tsushima, Satoru*; Takao, Koichiro*; Rossberg, A.*; Funke, H.*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi; Hennig, C.*

Inorganic Chemistry, 48(24), p.11779 - 11787, 2009/12

 Times Cited Count:33 Percentile:79.83(Chemistry, Inorganic & Nuclear)

The electrochemical behavior and complex structure of Np carbonato complexes have been investigated in aqueous Na$$_{2}$$CO$$_{3}$$ and Na$$_{2}$$CO$$_{3}$$/NaOH solutions at different oxidation states by using cyclic voltammetry, X-ray absorption spectroscopy, and density functional theory calculations. The end-member complexes of penta- and hexavalent Np in 1.5 M Na$$_{2}$$CO$$_{3}$$ with pH = 11.7 have been determined as a transdioxo neptunyl tricarbonato complex, [NpO$$_{2}$$(CO$$_{3}$$)$$_{3}$$]$$^{n-}$$ ($$n$$ = 5 for Np$$^{rm V}$$, and 4 for Np$$^{rm VI}$$). In contrast, the electrochemical oxidation of Np$$^{rm V}$$ in a highly basic carbonate solution of 2.0M Na$$_{2}$$CO$$_{3}$$/1.0M NaOH (pH $$>$$ 13) yielded a stable heptavalent Np complex of [Np$$^{rm VII}$$ O$$_{4}$$(OH)$$_{2}$$]$$^{3-}$$, indicating that the oxidation reaction from Np$$^{rm V}$$ to Np$$^{rm VII}$$ in the carbonate solution involves a drastic structural rearrangement from the transdioxo configuration to a square-planar-tetraoxo configuration, as well as exchanging the coordinating anions from carbonate ions (CO$$_{3}$$$$^{2-}$$) to hydroxide ions (OH$$^{-}$$).

Journal Articles

Speciation and structural study of U(IV) and -(VI) in perchloric and nitric acid solutions

Ikeda, Atsushi; Hennig, C.*; Tsushima, Satoru*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi

Inorganic Chemistry, 48(15), p.7201 - 7210, 2009/07

 Times Cited Count:72 Percentile:94.95(Chemistry, Inorganic & Nuclear)

Speciation and complex structure of U(IV) and U(VI) are studied in aqueous acidic solutions of HClO$$_{4}$$ and HNO$$_{3}$$ by means of UV-visible-NIR and X-ray absorption spectroscopies, and density functional theory (DFT) calculations. A spherical coordinating tetravalent cation of U$$^{4+}$$ is surrounded by 9-10 water molecules in the primary coordination sphere in 1.0 M HClO$$_{4}$$, while it forms a not-well-ordered colloidal compound of UO$$_{rm 2+x}$$ (x = 0.2) mixture in a lower acidic concentration of 0.1 M HClO$$_{4}$$. U(VI) exists as a transdioxo uranyl cation, UO$$_{2}$$$$^{2+}$$, and forms a 5-fold pure hydrate complex of [UO$$_{2}$$(H$$_{2}$$O)$$_{5}$$]$$^{2+}$$ in 1.0 M HClO$$_{4}$$. The water molecules in the pure hydrate complexes of U(IV) and U(VI) are successively replaced by planar bidentate coordinate nitrate ions (NO$$_{3}$$$$^{-}$$) as a function of increasing HNO$$_{3}$$ concentration. The presence of unidentate coordinate nitrate complexes or tetranitrato U(VI) complexes is less probable in the present HNO$$_{3}$$ system.

Journal Articles

The Sulfate coordination of Np(IV), Np(V), and Np(VI) in aqueous solution

Hennig, C.*; Ikeda, Atsushi; Tsushima, Satoru*; Scheinost, A. C.*

Inorganic Chemistry, 48(12), p.5350 - 5360, 2009/06

 Times Cited Count:41 Percentile:84.64(Chemistry, Inorganic & Nuclear)

Coordination and redox behavior of Np(IV), Np(V) and Np(VI) sulfate in aqueous solution has been investigated by Np L3-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, cyclic voltammetry and density functional theory (DFT) calculations. TNp(VI) is coordinated by sulfate in bidentate (RNp-S = 3.12 A) and monodentate (RNp-S = 3.61 A) modes at a low sulfate concentration of [SO$$_{4}$$$$^{2-}$$]/[NpO$$_{2}$$$$^{2+}$$] = 1. At higher [SO$$_{4}$$$$^{2-}$$]/[NpO$$_{2}$$$$^{2+}$$] ratios bidentate coordination prevails. Approximately two bidentate sulfate groups are coordinated to Np(VI) at 2.0 M SO$$_{4}$$$$^{2-}$$ and pH 1.1. Np(V) is coordinated by sulfate in bidentate (RNp-S = 3.16 A) and monodentate (RNp-S = 3.67 A) modes. However, sulfate coordination is less pronounced here and does not exceed in total one SO$$_{4}$$$$^{2-}$$ ligand in a solution of 2.0 M SO$$_{4}$$$$^{2-}$$. The redox couple Np(VI)/Np(V) is reversible at low [SO$$_{4}$$$$^{2-}$$]/[NpO$$_{2}$$$$^{2+}$$] ratio and becomes irreversible at high sulfate concentration due to structural rearrangement of the sulfate ligands.

Journal Articles

Structural determination of neptunium redox species in aqueous solutions

Ikeda, Atsushi; Hennig, C.*; Rossberg, A.*; Funke, H.*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi

ESRF Highlights 2008, p.99 - 100, 2009/02

Structural arrangement of Np species has been investigated in various aqueous solutions by synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy. The obtained results revealed that Np(IV) dominantly forms a spherically coordinated decahydrate complex, [Np(H$$_{2}$$O)$$_{20}$$]$$^{4+}$$, while Np(V) and -(VI) form a pentahydrate neptunyl complex, [NpO$$_{2}$$(H$$_{2}$$O)$$_{5}$$]$$^{n+}$$.

Journal Articles

Electrochemical and complexation behavior of neptunium in aqueous perchlorate and nitrate solutions

Ikeda, Atsushi; Hennig, C.*; Rossberg, A.*; Funke, H.*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi

Inorganic Chemistry, 47(18), p.8294 - 8305, 2008/09

 Times Cited Count:81 Percentile:68.53(Chemistry, Inorganic & Nuclear)

Electrochemical and complexation properties of neptunium (Np) are investigated in aqueous perchlorate and nitrate solutions by means of cyclic voltammetry, bulk electrolysis, UV-visible absorption and Np L$$_{rm III}$$-edge X-ray absorption spectroscopies. The redox reactions of Np(III)/Np(IV) and Np(V)/Np(VI) couples are reversible or quasi-reversible, while the electrochemical reaction between Np(III/IV) and Np(V/VI) is irreversible because they undergo the structural rearrangement from spherical coordinating ions (Np$$^{3+}$$ and Np$$^{4+}$$) to transdioxo neptunyl ions (NpO$$_{2}$$$$^{n+}$$, n = 1 for Np(V) and 2 for Np(VI)). A detailed analysis on extended X-ray absorption fine structure (EXAFS) spectra suggests that Np(IV) forms a decaaquo complex of [Np(H$$_{2}$$O)$$_{10}$$]$$^{4+}$$ in 1.0 M HClO$$_{4}$$, while Np(V) and -(VI) exist dominantly as pentaaquo neptunyl complexes, [NpO$$_{2}$$(H$$_{2}$$O)$$_{5}$$]$$^{n+}$$ (n = 1 for Np(V) and 2 for Np(VI)).

Journal Articles

Coordination of a uranium(IV) sulfate monomer in an aqueous solution and in the solid state

Hennig, C.*; Kraus, W.*; Emmerling, F.*; Ikeda, Atsushi; Scheinost, A. C.*

Inorganic Chemistry, 47(5), p.1634 - 1638, 2008/02

 Times Cited Count:28 Percentile:74.54(Chemistry, Inorganic & Nuclear)

Uranium(IV) sulfate species in an aqueous solution and in the solid state has been investigated by extended X-ray absorption fine structure and X-ray diffraction. The coordination polyhedron was found to comprise unidentate and bidentate sulfate ions, and water molecules both in the aqueous solution phase and in the solid state.

Journal Articles

Structural determination of individual chemical species in a mixed system by iterative transformation factor analysis-based X-ray absorption spectroscopy combined with UV-visible absorption and quantum chemical calculation

Ikeda, Atsushi; Hennig, C.*; Rossberg, A.*; Tsushima, Satoru*; Scheinost, A. C.*; Bernhard, G.*

Analytical Chemistry, 80(4), p.1102 - 1110, 2008/01

 Times Cited Count:25 Percentile:60.29(Chemistry, Analytical)

A multitechnique approach using extended X-ray absorption fine structure spectroscopy based on iterative transformation factor analysis, UV-visible absorption spectroscopy, and quantum chemical calculations has been performed for identifying the complex structure of individual U(VI) nitrate species.

Oral presentation

Structural determination of neptunium species in aqueous solutions by EXAFS and quantum chemical calculations

Ikeda, Atsushi; Hennig, C.*; Tsushima, Satoru*; Rossberg, A.*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi

no journal, , 

Neptunium (93Np) is one of the most problematic nuclides in the nuclear fuel reprocessing process and the following radioactive waste disposal because of its chemical similarity to the fissile nuclides of uranium (U) and plutonium (Pu). Proper understanding of the behavior of Np in the reprocessing process or in the migration process on the geological disposal of radioactive wastes requires vast fundamental information about the chemical properties of Np in solutions. In the present study, Np solution samples with different oxidation states are electrochemically prepared in aqueous perchlorate, nitrate, and carbonate solutions, and the complex structure of Np species in the sample solutions are determined by EXAFS spectroscopy and DFT calculations. The obtained results reveal the structural difference between different Np oxidation states, as well as the different coordination behavior in each solution system.

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