Computational approaches for estimating iron complexation constants in environmental systems

Jason M. St. Clair, D. Whitney King, and Thomas W. Shattuck. Departent of Chemistry, Colby College, 5700 Mayflower Hill, Colby College, Waterville, ME 04901, fax: 207-872-3804, jmstclai@colby.edu

The complexation of Fe(III) in natural waters plays a critical role in defining the biogeochemistry of this element. Numerous investigators have used a range of spectroscopic, potentiometric, and solubility based techniques to measure Fe(III) complexation constants in the laboratory. These experiments, however, are very difficult to perform due to the low solubility of most Fe(III) solid phases. Consequently, many solution phase complexation constants for Fe(III) are poorly defined or have not been investigated. Computational techniques may provide an alternative approach for the determination of Fe(III) equilibrium constants. We report the calculated free energies of a series of Fe(III) complexes determined at multiple levels of theory. Calculated DGs for Fe(III)-ligand exchange equilibria are remarkably well correlated with the experimentally determined values. This linear free energy relationship provides an opportunity to predict complexation constants of other Fe(III)-ligand species.