Soil Iron Complex Molecular Structure and Oxidation State Identification using Quantum Chemistry Simulated XAS Spectra to Interpret X-ray Absorption Measurements

Iron is ubiquitous in the soil environment from mineral and biological sources. Iron can readily form organometallic complexes with soil organic matter. Likely ligands for iron complexation include carboxylate, amine, thiol, and catechol-containing organic molecules. In our work, we perform detailed density functional theory (DFT) calculations to predict the molecular structures of model Fe organometallic complexes for soil organic matter and time-dependent density functional theory (TDDFT) calculations on these complexes to simulate the Fe K-edge x-ray absorption near-edge spectra (XANES) and extended x-ray absorption fine structure spectra (EXAFS). We use our calculations to interpret experimental data as well as to shed light on the local chemistry and molecular structure surrounding the Fe centers and the Fe oxidation state. The calculated spectra are also used to predict the x-ray absorption spectra (XAS) of composites of multiple Fe complexation sites within the organic matter.