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.

Vibrational echo spectroscopy of aqueous sodium bromide solutions from first principles simulations

“…For calculations of the three‐pulse photon echo intensities, one also requires the vibrational frequencies corresponding to higher excitations. For this purpose following our recent work, we model the local OD oscillator as a Morse oscillator given as where D e is the bond dissociation energy, r e is the equilibrium bond length, and α is the anharmonic parameter that determines the curvature of the potential and ν is the harmonic oscillation frequency, which is related to the curvature of the potential at its minimum by the following equation …”

“…This is justifiable because the numerical value of the transition dipole used for the calculation of the spectral dynamics alters the spectral intensity parametrically but it would not modify any of the spectral relaxation features. The value of κ ( μ 21 / μ 10 ) was found out using the known Eigen functions of Morse oscillators, and its value was found to be 1.34 . We have performed simulations of sodium and bromide ions dissolved in pure D 2 O, which allow us to have 214 OD modes for the dilute solution and 176 OD modes for the concentrated solution, thus enhancing the statistical averaging.…”

“…The value of κ (μ 21 / μ 10 ) was found out using the known Eigen functions of Morse oscillators, and its value was found to be 1.34. [26][27][28][29] We have performed simulations of sodium and bromide ions dissolved in pure D 2 O, which allow us to have 214 OD modes for the dilute solution and 176 OD modes for the concentrated solution, thus enhancing the statistical averaging. Each OD mode is considered as a decoupled stretching mode, hence the present study may be considered as an investigation of isolated OD modes in dilute and concentrated aqueous solutions of NaBr at different temperatures.…”

“…Recently, there have been studies on the spectral diffusion in aqueous salt solutions from first principles molecular dynamics simulations. The rates of spectral diffusion in bulk and in the first solvation shell of the anions were predicted using transient hole‐burning calculations and frequency correlation functions …”

“…The rates of spectral diffusion in bulk and in the first solvation shell of the anions were predicted using transient hole-burning calculations and frequency correlation functions. [27,40] However, the existing first principles studies of ionic solutions have not investigated the relation between the frequency time correlation function (FTCF) and the nonlinear observables extracted from vibrational echo spectroscopy. In calculations of photon echo and nonlinear observables, many of the earlier studies employed second-order cumulant approximation.…”