Density functional theory predictions of equilibrium isotope fractionation of iron due to redox changes and organic complexation

作者：Domagal Goldman Shawn D^*; Kubicki James D

来源：Geochimica et Cosmochimica Acta, 2008, 72(21): 5201-5216.

DOI：10.1016/j.gca.2008.05.066

We present molecular orbital/density functional theory (MO/DFT) calculations that predict a greater isotopic fractionation in redox reactions than in reactions involving ligand exchange. The predicted fractionation factors, reported as 1000 center dot ln((56-54)alpha), associated with equilibrium between Fe-organic and Fe-H2O species were <1.6 parts per thousand in vacuo and <1.2 parts per thousand in solution when the oxidation state of the system was held constant. These fractionation factors were significantly smaller than those predicted for equilibrium between different oxidation states of Fe, for which 1000 center dot ln((56-54)alpha) was >2.7 parts per thousand. in vacuo and >2.2 parts per thousand in solution when the bound ligands were unchanged. The predicted Fe-56/Fe-54 ratio was greater in complexes containing Fe3+ and in complexes with shorter Fe-O bond lengths; both of these trends follow previous theoretical results. Our predictions also agree with previous experimental measurements that suggest that the largest biological fractionations will be associated with processes that change the oxidation state of Fe, and that identification of biologically controlled Fe isotope fractionation may be difficult when abiotic redox fractionations are present in the system. The models studied here also have important implications for future theoretical isotope calculations, because we have discovered the necessity of using vibrational frequencies instead of reduced masses when predicting reduced partition functions in aqueous-phase species.

• 出版日期2008-11-1