Vibrational echo spectral observables and frequency fluctuations of hydration shell water around a fluoride ion from first principles simulations

Abstract

Aqueous solution of a fluoride ion at 300 K is studied using the method of ab initio molecular dynamics simulation. Instantaneous fluctuations in vibrational frequencies of local OD stretch modes of deuterated water are calculated using a time-series analysis of the simulated trajectory. The vibrational spectral diffusion of OD modes in the first and second solvation shells and also in bulk of the aqueous fluoride ionic solution are studied through calculations of the frequency time correlation function (FTCF), joint probability distributions, slope of three pulse photon echo (S3PE) and two dimensional infrared spectrum (2D-IR). The vibrational spectral dynamics in the first solvation shell shows decay with three components which can be correlated with the dynamics of intact ion-water hydrogen bonds, ion-water hydrogen bond lifetime and the escape dynamics of water molecules from the solvation shell. The vibrational spectral diffusion of OD modes in the second solvation shell and in the bulk show very similar decay behavior. The timescales obtained from FTCF, S3PE and the slope of nodal line (SNL) of 2D-IR are found to be in reasonable agreement with each others. Solvation shell dynamics of water molecules in different solvation shells of fluoride ion is studied using vibrational echo spectral observables from first principles simulations. The decay of FTCF, S3PE and SNL for water molecules in first solvation shell show multiple timescales which can be associated with ion-water hydrogen bond lifetime and residence time of water in the solvation shell.