TY - JOUR

T1 - Theoretical investigation of iron isotope fractionation between Fe(H2O)63+ and Fe(H2O) 62+

T2 - Implications for iron stable isotope geochemistry

AU - Anbar, Ariel

AU - Jarzecki, A. A.

AU - Spiro, T. G.

N1 - Funding Information:
The authors thank Pat Holland for suggesting this joint effort, the Gordon Research Conferences for providing an atmosphere conducive to collaboration, and François Morel for early encouragement. The thorough comments of Edwin Schauble and an anonymous reviewer are appreciated. Funding was provided by the Center for Environmental Bioinorganic Chemistry (NSF CHE 0221978) and by grants from the NASA Astrobiology Institute and NSF (EAR 0003565) to A. D. A.

PY - 2005/2/15

Y1 - 2005/2/15

N2 - The magnitude of equilibrium iron isotope fractionation between Fe(H2O)63+ and Fe(H2O) 62+ is calculated using density functional theory (DFT) and compared to prior theoretical and experimental results. DFT is a quantum chemical approach that permits a priori estimation of all vibrational modes and frequencies of these complexes and the effects of isotopic substitution. This information is used to calculate reduced partition function ratios of the complexes (103. ln(β)), and hence, the equilibrium isotope fractionation factor (103 ·ln(α)). Solvent effects are considered using the polarization continuum model (PCM). DFT calculations predict fractionations of several per mil in 56Fe/54Fe favoring partitioning of heavy isotopes in the ferric complex. Quantitatively, 103·ln(α) predicted at 22°C, ∼ 3 ‰, agrees with experimental determinations but is roughly half the size predicted by prior theoretical results using the Modified Urey-Bradley Force Field (MUBFF) model. Similar comparisons are seen at other temperatures. MUBFF makes a number of simplifying assumptions about molecular geometry and requires as input IR spectroscopic data. The difference between DFT and MUBFF results is primarily due to the difference between the DFT-predicted frequency for the ν4 mode (O-Fe-O deformation) of Fe(H2O) 63+ and spectroscopic determinations of this frequency used as input for MUBFF models (185-190 cm-1 vs. 304 cm-1, respectively). Hence, DFT-PCM estimates of 103·ln(β) for this complex are ∼ 20% smaller than MUBFF estimates. The DFT derived values can be used to refine predictions of equilibrium fractionation between ferric minerals and dissolved ferric iron, important for the interpretation of Fe isotope variations in ancient sediments. Our findings increase confidence in experimental determinations of the Fe(H2O)6 3+ - Fe(H2O)62+ fractionation factor and demonstrate the utility of DFT for applications in "heavy" stable isotope geochemistry.

AB - The magnitude of equilibrium iron isotope fractionation between Fe(H2O)63+ and Fe(H2O) 62+ is calculated using density functional theory (DFT) and compared to prior theoretical and experimental results. DFT is a quantum chemical approach that permits a priori estimation of all vibrational modes and frequencies of these complexes and the effects of isotopic substitution. This information is used to calculate reduced partition function ratios of the complexes (103. ln(β)), and hence, the equilibrium isotope fractionation factor (103 ·ln(α)). Solvent effects are considered using the polarization continuum model (PCM). DFT calculations predict fractionations of several per mil in 56Fe/54Fe favoring partitioning of heavy isotopes in the ferric complex. Quantitatively, 103·ln(α) predicted at 22°C, ∼ 3 ‰, agrees with experimental determinations but is roughly half the size predicted by prior theoretical results using the Modified Urey-Bradley Force Field (MUBFF) model. Similar comparisons are seen at other temperatures. MUBFF makes a number of simplifying assumptions about molecular geometry and requires as input IR spectroscopic data. The difference between DFT and MUBFF results is primarily due to the difference between the DFT-predicted frequency for the ν4 mode (O-Fe-O deformation) of Fe(H2O) 63+ and spectroscopic determinations of this frequency used as input for MUBFF models (185-190 cm-1 vs. 304 cm-1, respectively). Hence, DFT-PCM estimates of 103·ln(β) for this complex are ∼ 20% smaller than MUBFF estimates. The DFT derived values can be used to refine predictions of equilibrium fractionation between ferric minerals and dissolved ferric iron, important for the interpretation of Fe isotope variations in ancient sediments. Our findings increase confidence in experimental determinations of the Fe(H2O)6 3+ - Fe(H2O)62+ fractionation factor and demonstrate the utility of DFT for applications in "heavy" stable isotope geochemistry.

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U2 - 10.1016/j.gca.2004.06.012

DO - 10.1016/j.gca.2004.06.012

M3 - Article

AN - SCOPUS:84962441480

VL - 69

SP - 825

EP - 837

JO - Geochmica et Cosmochimica Acta

JF - Geochmica et Cosmochimica Acta

SN - 0016-7037

IS - 4

ER -