Nuclear Size and Shape Effects in Chemical Reactions. Isotope Chemistry of the Heavy Elements

Bigeleisen, Jacob

doi:10.1021/ja954076k

Cited by 380 publications

(361 citation statements)

References 31 publications

Supporting

Mentioning

344

Contrasting

Unclassified

Order By: Relevance

“…The most important driver of isotopic fractionation for the heavy element U is the nuclear volume effect, which leads to preferential enrichment of heavy U isotopes in U(IV) during U(VI) reduction (Bigeleisen 1996;Schauble, 2007;Brennecka et al, 2010;Basu, 2013;Bopp et a., 2009Bopp et a., , 2010. However, in our experiments, U is always in the (VI) oxidation state.…”

Section: U Isotope Fractionation Mechanismmentioning

confidence: 63%

Uranium isotope fractionation during coprecipitation with aragonite and calcite

Chen

Romaniello

Herrmann

et al. 2016

Geochimica et Cosmochimica Acta

View full text Add to dashboard Cite

Natural variations in 238 U/ 235 U of marine carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ~7.5 and ~ 8.5 to study possible U isotope fractionation during incorporation into these minerals.Small but significant U isotope fractionation was observed in aragonite experiments at pH ~ 8.5, with heavier U in the solid phase. 238 U/ 235 U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007+0.00002/-0.00003, 1.00005 ± 0.00001, and 1.00003 ± 0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH ~7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca 2 UO 2 (CO 3 ) 3 (aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca 2 UO 2 (CO 3 ) 3 (aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism.ii These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, P CO2 , [Ca], or [Mg] concentrations. In general, these effects are likely to be small (<0.13 ‰), but are nevertheless potentially significant because of the small natural range of variation of 238 U/ 235 U.

show abstract

Section: U Isotope Fractionation Mechanismmentioning

confidence: 63%

Uranium isotope fractionation during coprecipitation with aragonite and calcite

Chen

Romaniello

Herrmann

et al. 2016

Geochimica et Cosmochimica Acta

View full text Add to dashboard Cite

show abstract

“…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).…”

mentioning

confidence: 99%

Modeling nuclear volume isotope effects in crystals

Schauble

2013

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

View full text Add to dashboard Cite

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...

show abstract

“…However, Fe/Ni ratios of all sample were measured, and corrected by peak strip- Thus, we feel we have extensively addressed the possibility of interferences affecting our data and 613 find no significant influence. It has been suggested (Fujii et al, 2006(Fujii et al, , 2009) that mass-independent anomalies may be the result 616 of mass-independent fractionation controlled by the nuclear field shift effect (Bigeleisen, 1996) and not As stated above, we chose iron meteorites for this study because they do not contain refractory com-627 ponents and so selective dissolution problems should be negligible. However, in general, iron meteorites 628 have longer cosmic ray exposure ages than the chondrites so we need to consider the possible perturba-629 tions due to spallation reactions.…”

mentioning

confidence: 99%

Confirmation of mass-independent Ni isotopic variability in iron meteorites

Steele

Elliott

Coath

et al. 2011

Geochimica et Cosmochimica Acta

103

View full text Add to dashboard Cite

We report high-precision analyses of internally-normalised Ni isotope ratios in 12 bulk iron meteorites. and E 64 Ni 58 61 , respectively, which is comparable to our sample reproducibility. We show that this 'massindependent' Ni isotope variability cannot be ascribed to interferences, inaccurate correction of instrumental or natural mass-dependent fractionation, fractionation controlled by nuclear field shift effects, nor the influence of cosmic ray spallation. These results thus document the presence of mass-independent Ni isotopic heterogeneity in bulk meteoritic samples, as previously proposed by Regelous et al. 2008 (EPSL 272, 330-338), but our new analyses are more precise and include determination of 64 Ni. Intriguingly, we find that terrestrial materials do not yield homogenous internally-normalised Ni isotope compositions, which, as pointed out by Young 2002 (GCA 66, 1095-1104, may be the expected result of using the exponential (kinetic) law and atomic masses to normalise all fractionation processes. The certified Ni isotope reference material NIST SRM 986 defines zero in this study, while appropriate ratios for the bulk silicate Earth are given by the peridotites JP-1 and DTS-2 and, relative to NIST SRM 986, yield devi-

show abstract

Nuclear Size and Shape Effects in Chemical Reactions. Isotope Chemistry of the Heavy Elements

Cited by 380 publications

References 31 publications

Uranium isotope fractionation during coprecipitation with aragonite and calcite

Uranium isotope fractionation during coprecipitation with aragonite and calcite

Modeling nuclear volume isotope effects in crystals

Confirmation of mass-independent Ni isotopic variability in iron meteorites

Contact Info

Product

Resources

About