Modeling the effects of bond environment on equilibrium iron isotope fractionation in ferric aquo-chloro complexes

Abstract

Four or five sets of ab initio models, including Unrestricted Hartree Fock (UHF) and hybrid Density Functional Theory (DFT) are calculated for each species in a series of aqueous ferric aquo-chloro complexes: Fe ( H 2 O ) 6 3 + , FeCl ( H 2 O ) 5 2 + , FeCl 2 ( H 2 O ) 4 + , FeCl 3(H 2O) 3, FeCl 3(H 2O) 2, FeCl 4 - , FeCl 5H 2O 2−, FeCl 5 2 - , FeCl 6 3 - ) in order to determine the relative isotopic fractionation among the complexes, to compare the results of different models for the same complexes, to examine factors that influence the magnitude of the isotopic fractionation, and to compare bond-partner-driven fractionation with redox-driven fractionation. Relative to Fe ( H 2 O ) 6 3 + , all models show a nearly linear decrease in 56Fe/ 54Fe as the number of Cl − ions per Fe 3+ ion increases, with slopes of −0.8‰ to −1.0‰ per Cl − at 20 °C. At 20 °C, 1000 ln β ( β = 56Fe/ 54Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 8.93‰ to 9.73‰ for Fe ( H 2 O ) 6 3 + , 8.04–9.12‰ for FeCl ( H 2 O ) 5 2 + , 7.61–8.73‰ for FeCl 2 ( H 2 O ) 4 + , 7.14–8.25‰ for FeCl 4 - , and 3.09–4.41‰ for FeCl 6 3 - . The fractionation between Fe ( H 2 O ) 6 3 + and FeCl 4 - ranges from 1.5‰ to 2.6‰, depending on the model; this is comparable in magnitude to fractionation effects due to Fe 3+/Fe 2+ redox reactions. β values from the UHF models are consistently higher than those from the hybrid DFT models. Isotopic fractionation is shown to be sensitive to differences in ligand bond stiffness (above), coordination number, bond length, and the frequency of the asymmetric Fe–X stretching vibrational mode, as predicted by previous theoretical studies. Complexes with smaller coordination numbers have higher 1000 ln β (7.46‰, 5.25‰, and 3.48‰ for FeCl 4 - , FeCl 5 2 - , FeCl 6 3 - , respectively, from the B3LYP/6-31G(d) model). Species with the same number of chlorides but fewer waters also show the effect of coordination number on 1000 ln β: (7.46‰ vs. 7.05‰ for FeCl 3(H 2O) 2 vs. FeCl 3(H 2O) 3 and 5.25‰ vs. 4.94‰ for FeCl 5 2 - vs. FeCl 5H 2O 2− with the B3LYP/6-31G(d) model). As more Fe–Cl bonds substitute for Fe–OH 2 bonds (with a resulting decrease in β), the lengths of the Fe–Cl bonds and the Fe–O bonds increase. Preliminary modeling of Fe ( H 2 O ) 6 2 + shows an Fe 3+/Fe 2+ fractionation of 3.2‰ for the B3LYP/6-31G(d) model, in agreement with previous studies. The addition of an explicit outer hydration sphere of 12 H 2O molecules to models of Fe ( H 2 O ) 6 3 + improves agreement with measured vibrational frequencies and bond lengths; 1000 ln β increases by 0.8–1.0‰. An additional hydration sphere around FeCl 4 - increases 1000 ln β by only 0.1‰. Isotopic fractionations predicted for this simple system imply that ligands present in an aqueous iron environment are potentially important drivers of fractionation, and suggest that significant fractionation effects are likely in other aqueous systems containing sulfides or organic ligands. Fractionation effects due to both speciation and redox must be considered when interpreting iron isotope fractionations in the geological record.