Water Dynamics:  Vibrational Echo Correlation Spectroscopy and Comparison to Molecular Dynamics Simulations

Abstract

The dynamics of water are examined using ultrafast IR stimulated vibrational echo correlation spectroscopy. The OD hydroxyl stretch of HOD in H2O is probed with 45-fs pulses that have sufficient bandwidth (>400 cm-1) to span the entire broad spectrum. High-quality 2D correlation spectra are obtained having the correct phase relations across the broad hydroxyl band. The correlation spectra are found to evolve on multiple time scales. The time evolution of the vibrational echo correlation spectrum reflects the structural evolution of the hydrogen bond networks. The extended vibrational lifetime of the OD hydroxyl stretch of HOD in H2O facilitates the measurement of hydrogen bond dynamics for longer times than possible in previous studies of the OH stretch. Molecular dynamics simulations/electronic structure calculations are used to obtain the time correlation functions (TCF) for two water models, TIP4P and SPC/E. The TCFs are inputs to full time-dependent diagrammatic perturbation theory calculations, which yield theoretical correlation spectra. Quantitative comparison with the data demonstrates that the two water models somewhat overemphasize the fast fluctuations in water and do not contain a slow enough component to account for the slowest fluctuations. Fits to the data using a phenomenological triexponential TCF yield a slowest component of ∼2 ps, and TIP4P and SPC/E have slowest components of <1 ps. The TCF obtained from the water models and the triexponential TCF reproduce the linear absorption line shape equally well, but all miss to some extent the asymmetric “wing” on the low-energy side of the line. Therefore, the time dependence of the vibrational echo correlation spectra provides a good test for the TCF, but the absorption spectrum does not.

Collective Solvent Coordinates for the Infrared Spectrum of HOD in D2O Based on an ab Initio Electrostatic Map

“…7,44 This correlation is reasonable because the hydrogen atom moves primarily along the O-H stretch. We have therefore examined the actual contribution of the electric field on the hydrogen atom in that direction to the frequency fluctuation.…”

“…The vibrational properties of liquid water have received considerable attention. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]31,[43][44][45] Much experimental [9][10][11][12][13][14][15][16][17] and theoretical [18][19][20][21]43,44 effort has focused on the OH stretch of HOD in D 2 O. This is a simpler system than neat water since that mode is spectrally isolated so that the rapid resonant intermolecular vibrational energy transfer 46 is eliminated.…”

Dynamics of Nanoscopic Water: Vibrational Echo and Infrared Pump−Probe Studies of Reverse Micelles

“…20,69 In the simulations of water, this fastest component is identified as the very local fluctuations of the lengths of the hydrogen bonds. 18,[20][21][22] The slowest component has been associated with more global structural evolution of the hydrogen-bonding network, i.e., the making and breaking of hydrogen bonds (hydrogen-bond equilibration). 18,[20][21][22] Simulations of the FFCF of nanoscopic water in reverse micelles have not yet been performed.…”

“…49 Here, we present a detailed account of the vibrational echo experiments on AOT reverse micelles that have been reported previously, 49 including new frequency-dependent data, as well as vibrational pump-probe experiments that examine the population dynamics of the hydroxyl stretch. Vibrational echo spectroscopy [16][17][18]20,[50][51][52][53][54][55][56] is uniquely capable of determining system dynamics by removing the inhomogeneous broadening contribution from the line shape 16,[56][57][58][59][60][61][62] and tracking the underlying dynamics. 63, 64 The properties of hydrogen-bond networks can be studied using IR spectroscopy of the hydroxyl stretching mode, because of the strong influence of the number and strength of hydrogen bonds on the hydroxyl stretch frequency.…”

Dynamics of a Multimode System Coupled to Multiple Heat Baths Probed by Two-Dimensional Infrared Spectroscopy

“…Consequently, the multidimensional spectroscopy has been proven to be valuable and versatile tools for diverse topics in condensed-phase chemical dynamics. Especially, objects under study in two-dimensional infrared (2D-IR) spectroscopy have definitely spread very wide: structures and dynamics of peptides, [9][10][11][12][13][14][15] conformational changes in protein, [16][17][18] water dynamics, 19 hydrogen-bond dynamics, 20-25 solute-solvent interactions, 26,27 quantum tunneling processes, [28][29][30] and fast chemical exchange in molecular complexes. [31][32][33] Multidimensional vibrational spectroscopy is the optical counterpart of multidimensional NMR such as COSY and NOESY, which are powerful tools for organic structural analysis and structural biology.…”