Water in Hydration Shell of an Iodide Ion: Structure and Dynamics of Solute-Water Hydrogen Bonds and Vibrational Spectral Diffusion from First-Principles Simulations

The dynamics of hydrogen bonds and vibrational spectral diffusion of water in the hydration shell of an iodide ion and in bulk have been investigated for aqueous iodide solutions of two different concentrations by using ab initio molecular dynamics simulations. The effects of dispersion interactions on the dynamics have also been investigated by using a dispersion corrected density functional. For the dilute solution containing a single iodide ion, three time scales are found for the spectral diffusion of solvation shell water: A short time scale of similar to 150 fs, a slower time scale of similar to 2-3 ps and a yet longer time scale of similar to 14-16 ps. The long time scale of similar to 14-16 ps is not noticed when calculations are done for all OD modes for both the dilute and concentrated solutions. It is found that a clear separation between the solvation shell and bulk water does not exist in terms of OD stretching frequencies for the concentrated solution. The dynamics of vibrational spectral diffusion is explained in terms of hydrogen bond dynamics, other dynamical modes such as orientational relaxation and molecular diffusion, and also structural aspects of water molecules in the solvation shells. The current results are compared with those of available experimental and other theoretical studies.