Using MD Snapshots in ab Initio and DFT Calculations:  OH Vibrations in the First Hydration Shell around Li⁺(aq)

Abstract

The average OH stretching vibrational frequency for the water molecules in the first hydration shell around a Li(+) ion in a dilute aqueous solution was calculated by a hybrid molecular dynamics + quantum-mechanical ("MD + QM") approach. Using geometry configurations from a series of snapshots from an MD simulation, the anharmonic, uncoupled OH stretching frequencies were calculated for 100 first-shell OH oscillators at the B3LYP and HF/6-31G(d,p) levels of theory, explicitly including the first shell and the relevant second shell water molecules into charge-embedded supermolecular QM calculations. Infrared intensity-weighting of the density-of-states (DOS) distributions by means of the squared dipole moment derivatives (which vary by a factor of 20 over the OH stretching frequency band at the B3LYP level), changes the downshift from approximately -205 to -275 cm(-1) at the B3LYP level. Explicit inclusion of relevant third-shell water molecules in the supermolecular cluster leads to a further downshift by approximately -30 cm(-1). Our final estimated average downshift is approximately -305 cm(-1). The experimental value lies somewhere in the range between -290 and -420 cm(-1). Also, the absolute nu(OH) frequency is well reproduced in our calculations. "In-liquid" instantaneous correlation curves between nu(OH) and various typical H-bond strength parameters such as R(O...O), R(H...O), the intramolecular OH bond length, and the IR intensity are presented. Some of these correlations are robust and persist also for the rather distorted instantaneous geometries in the liquid; others are less so.

Lithium Ion–Water Clusters in Strong Electric Fields: A Quantum Chemical Study

“…20,[37][38][39] Explicit polarizability may be important for understanding this, but in this case it is the polarization of the solvent molecules at short separations from the small Li + ion that is crucial. 40,41 The sensitivity of the Li + -water solvation structure can also be inferred from the analysis of extensive experimental studies, which have raised questions about the relative stability of water around Li + . 39,[42][43][44] In particular, it is still unclear whether the Li + -water coordination number is closer to 4 or 6 at moderate concentrations, ∼ 1 M.…”

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

“…Further, there are several simulation studies, which have been able to identify residence times in anionic solvation shells, rotational anisotropy apart from ion‐solvent structures . Yet, only a few of them have focused on theoretical calculations of vibrational lineshape and spectral diffusion timescales . Skinner and coworkers employed an approach where an empirical relationship between instantaneous vibrational frequency was developed and subsequently used to calculate the electric field applied along the vibrational mode of interest and thus obtaining vibrational lineshapes and vibrational spectral diffusion timescales .…”

“…[16,[34][35][36] Yet, only a few of them have focused on theoretical calculations of vibrational lineshape and spectral diffusion timescales. [37][38][39][40] Skinner and coworkers employed an approach where an empirical relationship between instantaneous vibrational frequency was developed and subsequently used to calculate the electric field applied along the vibrational mode of interest and thus obtaining vibrational lineshapes and vibrational spectral diffusion timescales. [4,[20][21][22]38] In another classical simulation study, the solvation shell dynamics around anion was investigated using theoretically simulated pump-probe spectrum.…”

Vibrational mode assignment of finite temperature infrared spectra using the AMOEBA polarizable force field

“…13, Wheeler et al have suggested principal mode analysis (PMA) 11,27 in which the average massweighted normal modes and associated frequencies are calculated from a classical MD trajectory by evaluating the eigenvectors and eigenvalues of the covariance matrix associated with the mass-weighted displacements of the molecular coordinates from their equilibrium positions. [49][50][51] The instantaneous normal mode analysis (INMA) and its variants [31][32][33][34][35][36] are obtained by diagonalizing the Hessian matrix at each instantaneous configuration of the molecule. when the potential energy surface of the molecule is anharmonic or when the thermal equilibrium is not guaranteed.…”