Probing the hydration structure of polarizable halides: A multiedge XAFS and molecular dynamics study of the iodide anion

A comprehensive analysis of the H₂O structure about aqueous iodide (I^-) is reported from molecular dynamics (MD) simulation and X-ray absorption fine structure (XAFS) measurements. This study establishes the essential ingredients of an interaction potential that reproduces the experimentally determined first-solvation shell of aqueous iodide. XAFS spectra from the iodide K, L₁, and L₃ edges were corefined to establish the complete structure of the first hydration shell about aqueous iodide. Further, we have utilized molecular dynamics simulations employing both DFT (+dispersion) and empirical polarizable interaction potentials to generate an ensemble of structures that were directly compared to the XAFS data. Our results indicate that DFT-MD simulations provide a description of the molecular structure that is more consistent with the XAFS experimental data. The experimental data yield approximately 6.3 water molecules located at I - H and I - O distances of 2.65 and 3.50 Å, respectively. The differences in the two interaction potentials can be traced to the treatment of the electronic charge density in the vicinity of the iodide. The empirical polarizable interaction potential yields a significantly higher induced dipole for the aqueous iodide than the DFT study. The lower induced dipole moment from the DFT simulation produces a higher coordination number and leads to a more symmetric solvation environment than that produced by the empirical polarizable interaction potential. Furthermore, the hydrogen bonding of second-shell water with the first-shell water establishes a strong ordering of the water about the iodide surface.