Mass Dependence of Uranium Isotope Effects in the U(IV)−U(VI) Exchange Reaction

Nomura, Masao; Higuchi, Nobuhiko; Fujii, Yasuhiko

doi:10.1021/ja954075s

Cited by 101 publications

(93 citation statements)

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“…The anomalous 235 U-238 U isotope effect in U(IV)-U(VI) chemical exchange shown by the present authors (M. N. and Y. F.) could not be explained by the molecular vibration theory [9]. Later on, the same trend was found in the case of 233 U isotope effects [10]. It was suggested that the isotope effects in the U(IV)-U(VI) exchange system arise mainly from the interaction between the nuclei and the electrons in the isotopes.…”

Section: Introductionsupporting

confidence: 71%

The Effect of Temperature on Uranium Isotope Effects Studied by Cation Exchange Displacement Chromatography

Ismail

Nomura

Fujii

et al. 2002

Zeitschrift Für Naturforschung A

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IntroductionAfter the lithium isotope separation by ion exchange chromatography by Taylor and Urey [1], different systems, such as solvent extraction [2], amalgam/aqueous solution and amalgam/organic solution [3,4], ion exchange electromigration and ion exchange displacement chromatography [5][6][7], were applied to study the isotope effects by chemical exchange reactions. Regardless the chemical exchange system used, the molecular vibrations were believed to be the main, factor that affect the isotope exchange reactions of heavy elements. This belief was based on the relationship between the equilibrium constant of the isotopic exchange reactions and the reduced partition function ratios, RPFR, shown by Waldmann and by Bigeleisen and Mayer [8] in 1943 and in 1947, respectively. But during the systematic experimental and theoretical investigations on the isotope effects in chemical exchange systems carried out through the last two decades this belief became doubtful. The anomalous 235 U-238 U isotope effect in U(IV)-U(VI) chemical exchange shown by the present authors (M. N. and Y. F.) could not be explained by the molecular vibration theory [9]. Later on, the same trend was found in the case of 233 U isotope effects [10]. It was suggested that the isotope effects in the U(IV)-U(VI) exchange system arise mainly from the interaction between the nuclei and the electrons in the isotopes. Nishizawa has suggested anomalous mass effects in lighter elements such as Zn [11]. This was attributed to the isotope shift in the orbital energy. Bigeleisen has shown that the field shift becomes the major effect in the isotopic chemical exchange systems of uranium [12]. Recently, the anomaly of 155 Gd isotope effects in ligand exchange reactions observed by ion exchange chromatography were studied and found to be due to the shape and size of the nucleus [13].Studying the temperature effect on the equilibrium constant of isotopic chemical exchange reactions is a powerful tool for analyzing the isotope effects. It can help in determining which is the major factor that affects the isotope effects, molecular vibrations or the field shift. If the equilibrium constant was found to be inversely proportional to the square of temperature, the molecular vibration is considered as the main factor. On the other hand, the field shift would be considered as one of the main factors if the equilibrium constant was found to increase with the temperature. This trend was observed in the case of europium [7] and barium [3].By studying the temperature effect, a better understanding of the origin of the isotope effects would be achieved. So, this work was carried out to study the effect of temperature on the separation of uranium isotopes by using ion exchange chromatography based on a uranium-ligand exchange system. The uranium isotope effect in the exchange system uranyl(VI)-malate ligand at 288-343 K has been studied by ion exchange displacement chromatography. At all temperatures 235 U is enriched at the front of the uranium band. The single st...

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Section: Introductionsupporting

confidence: 71%

The Effect of Temperature on Uranium Isotope Effects Studied by Cation Exchange Displacement Chromatography

Ismail

Nomura

Fujii

et al. 2002

Zeitschrift Für Naturforschung A

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“…Nuclear field shifts have long been known in optical spectra of atoms and ions, but Bigeleisen (1) and Fujii and coworkers (8) were the first to recognize that the same phenomenon could lead to chemical fractionation of isotopes. They showed that uranium isotope separation via ion exchange columns (the Asahi process) tracked nuclear charge radii (or more specifically, the effective meansquare charge radius), rather than nuclear masses (1,8,9). Field shift effects are also expected to scale in proportion to the difference in electron densities inside the nucleus of the relevant atoms in the fractionating species (10).…”

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confidence: 99%

Modeling nuclear volume isotope effects in crystals

Schauble

2013

Proc. Natl. Acad. Sci. U.S.A.

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Mass-independent isotope fractionations driven by differences in volumes and shapes of nuclei (the field shift effect) are known in several elements and are likely to be found in more. All-electron relativistic electronic structure calculations can predict this effect but at present are computationally intensive and limited to modeling small gas phase molecules and clusters. Density functional theory, using the projector augmented wave method (DFT-PAW), has advantages in greater speed and compatibility with a three-dimensional periodic boundary condition while preserving information about the effects of chemistry on electron densities within nuclei. These electron density variations determine the volume component of the field shift effect. In this study, DFT-PAW calculations are calibrated against all-electron, relativistic DiracHartree-Fock, and coupled-cluster with single, double (triple) excitation methods for estimating nuclear volume isotope effects. DFT-PAW calculations accurately reproduce changes in electron densities within nuclei in typical molecules, when PAW datasets constructed with finite nuclei are used. Nuclear volume contributions to vapor-crystal isotope fractionation are calculated for elemental cadmium and mercury, showing good agreement with experiments. The nuclear-volume component of mercury and cadmium isotope fractionations between atomic vapor and montroydite (HgO), cinnabar (HgS), calomel (Hg 2 Cl 2 ), monteponite (CdO), and the CdS polymorphs hawleyite and greenockite are calculated, indicating preferential incorporation of neutron-rich isotopes in more oxidized, ionically bonded phases. Finally, field shift energies are related to Mössbauer isomer shifts, and equilibrium massindependent fractionations for several tin-bearing crystals are calculated from 119 Sn spectra. Isomer shift data should simplify calculations of mass-independent isotope fractionations in other elements with Mössbauer isotopes, such as platinum and uranium.Mössbauer spectroscopy | nuclear field shift | mass independent fractionation A mong the various causes of mass-independent isotope fractionation discovered over the past 40 years, the nuclear field shift effect (1) is unique in that it is an equilibrium thermodynamic phenomenon (and may also impart an identifiable and distinct signature in disequilibrium conditions) (2, 3). It has also proven amenable to prediction by electronic structure methods, at least in simple molecular systems (e.g., refs. 4-7). The goal of the present study is to extend the scope of systems that can be modeled to include condensed phases and crystals, and to speed up calculations for complex materials while retaining enough accuracy to be useful.Nuclear field shift effects result from the finite volume and sometimes nonspherical shapes of atomic nuclei, which are slightly different from one isotope to another. These differences in size and shape affect the coulomb potential felt by bound electrons, and thus the electronic structures of atoms and molecules. Nuclear field shifts have lo...

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“…In 1989, one of the present authors Y. F. and co-workers reported an anomalous mass-independence in the U (IV)-U (VI) electron-exchange reaction [4]. Nomura and Y. F. et al [5] again analyzed the U (IV)-U (VI) isotope fractionation among 232 U, 233 U, 234 U, 235 U, 236 U, and 238 U. Figure 1 shows the correlation between mass difference of isotopes and the slope of three-isotope plot, θ, which is the value to be proportional to isotope fractionation coefficients.…”

Section: Introductionmentioning

confidence: 63%

“…Typical examples are the uranium isotope fractionations in oxidation-reduction equilibrium states, such as the U (III)-U (IV) [2,3] and U (IV)-U (VI) [4,5] exchange systems. These systems show large isotope effects that are inexplicable by the standard theory.…”

Section: Introductionmentioning

confidence: 99%

Theoretical Study of Isotope Enrichment Caused by Nuclear Volume Effect

Abe

Hada

Suzuki

et al. 2014

J. Comput. Chem. Jpn.

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Isotope separation is important in industry, such as uranium enrichment. It is also very important in geochemistry and cosmo-chemistry because heavy and light isotopes are gradually fractionated in global equilibrium processes. For isotopes of heavy elements such as uranium and gadolinium, this fractionation occurs mainly owing to the difference of nuclear charge radii between the isotopes. This is because electronic states of isotopologues are different because of the difference of nuclear charge radii. Nuclear volume terms (lnKnv) in the equilibrium constants of isotope fractionations can be calculated from the electronic energy difference of isotopologues, obtained by relativistic quantum chemical calculations with finite-size nuclear models. In this review, we summarize our theoretical results in uranium isotope separations, using U (III)-U (IV) and U (IV)-U (VI) redox reaction systems. Calculated nuclear volume term (lnKnv) is reasonably close to the ones estimated from experimental analysis of temperature dependence in equilibrium constants.We used the four-component Dirac-Coulomb Hartree-Fock method with the Fermi and Gaussian-type nuclear models in our calculations. We also briefly mention how the nuclear volume effect is important in geochemistry or cosmochemistry referring to recent relevant works.

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Mass Dependence of Uranium Isotope Effects in the U(IV)−U(VI) Exchange Reaction

Cited by 101 publications

References 16 publications

The Effect of Temperature on Uranium Isotope Effects Studied by Cation Exchange Displacement Chromatography

The Effect of Temperature on Uranium Isotope Effects Studied by Cation Exchange Displacement Chromatography

Modeling nuclear volume isotope effects in crystals

Theoretical Study of Isotope Enrichment Caused by Nuclear Volume Effect

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