Hydration properties of the bromide aqua ion: the interplay of first principle and classical molecular dynamics, and X-ray absorption spectroscopy.

The hydration properties of the bromide aqua ion have been investigated using state of the art density functional theory (DFT) based molecular dynamics with dispersion-corrected atom-centered pseudopotentials for water and classical molecular dynamics simulations. The reliability of the theoretical results has been assessed by comparing the attained structural results with the extended X-ray absorption fine structure (EXAFS) experimental data. The EXAFS technique is mainly sensitive to short distances around the bromine atom, and it is a direct probe of the local solvation structure. The comparison shows that the DFT simulation delivers a good description of the EXAFS experimental signal, while classical simulation performs poorly. The main reason behind this is the neglect of polarization effects in the classical ion-water interaction potentials. By taking advantage of the reliable information on the Br(-) local hydration structure it has been possible to highlight the contribution of hydrogen atoms to the EXAFS spectra of halide aqueous systems.