PNNL

Abstract

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. XAFS spectra from the iodide K-, L1-, and L3- edges were co-refined to establish the complete structure of the first hydration shell about aqueous I-. The results show approximately 6.3 water molecules located at I-H and I-O distances of 2.65 Å and 3.50 Å, respectively. Whereas the I-O bond is moderately disordered (Debye Waller factor, s2 = 0.017 Å2) due to the relatively low charge-to-ion radius ratio, the I-H interaction shows even higher disorder (s2 = 0.036 Å2) due to the variable angular orientation of water at the ion surface. Molecular dynamics simulations employing both DFT (+dispersion) and classical potentials generate quite similar structures and they both agree to a large extent with the structure from the experimental XAFS. However the DFT-MD simulations provide a description of molecular structure that is more consistent with the XAFS experiment data. We employ a molecular anaylsis in which we incrementally evaluate the structural contributions from each of the nearest-neighbor water molecules about the iodide to provide a clear picture of the hydrated structure. For the DFT description of molecular interaction, a water molecule in the first shell has more freedom to rotate about the O atom when compared to motions resulting from a classical potential. Further, the hydrogen bonding of first-shell water with the second shell water establishes an strong ordering of the water about I- surface leading to characteristic O-I-O angles of 79 and 142°. This ordering, in addition to the higher coordination number leads to a more symmetric solvation from the DFT-MD configurations relative to the classical potential simulation.