Water is, of course, a fascinating and important substance. For such a simple molecule, its condensed phase properties are surprisingly complex. Here we might mention the many different solid phases, the higher density of the liquid as compared to ice Ih, and the density maximum (as a function of temperature) in the liquid phase. Moreover, for such a light molecule, many of the liquid-state properties are anomalous: the boiling point, freezing point, heat capacity, surface tension, and viscosity are all unusually high. Even so, it is perhaps surprising that we still do not fully understand the properties of the liquid state.1-3 From a theoretical point of view, this can probably be attributed to two features of liquid water: cooperative hydrogen bonding (H-bonding) and nuclear quantum effects. The former refers to the fact that the binding energy of two H-bonded molecules is modified by the presence of a third molecule.4-9 In terms of simulating the liquid, then, it follows that the potential energy cannot be written as a sum of twomolecule terms. This means that simple two-body simulation models cannot completely describe reality, and attempts to capture the effects of these many-body interactions with polarizable models are not fully satisfactory either.8,10 Nuclear quantum effects occur because the hydrogen nucleus is sufficiently light that classical mechanics for the nuclear motion is simply not adequate. Thus, classical mechanics cannot describe such important properties as spatial dispersion of the hydrogen positions, nuclear tunneling, zero-point energy, and quantization of nuclear motions. Much energy has recently been expended toward the simulation of liquid water using ab initio electronic-structure methods (which, in priniciple, will produce the correct Born-Oppenheimer potential surface, including the effects of many-body interactions),11-19 together with methods for quantum dynamics,19-25 but still more work needs to be done before we have a complete and accurate description.

Vibrational Spectroscopy as a Probe of Structure and Dynamics in Liquid Water

IR and Raman (parallel- and perpendicular-polarized) spectra in the OH stretch region for liquid water were measured some years ago, but their interpretation is still controversial. In part, this is because theoretical calculation of such spectra for a neat liquid presents a formidable challenge due to the coupling between vibrational chromophores and the effects of motional narrowing. Recently we proposed an electronic structure/molecular dynamics method for calculating spectra of dilute HOD in liquid D(2)O, which relied on ab initio calculations on clusters to provide a map from nuclear coordinates of the molecules in the liquid to OH stretch frequencies, transition dipoles, and polarizabilities. Here we extend this approach to the calculation of couplings between chromophores. From the trajectories of the fluctuating local-mode frequencies, transition moments, and couplings, we use our recently developed time-averaging approximation to calculate the line shapes. Our results are in good agreement with experiment for the IR and Raman line shapes, and capture the significant differences among them. Our analysis shows that while the coupling between chromophores is relatively modest, it nevertheless produces delocalization of the vibrational eigenstates over up to 12 chromophores, which has a profound effect on the spectroscopy. In particular, our results demonstrate that the peak in the parallel-polarized Raman spectrum at about 3250 wavenumbers is collective in nature.

IR and Raman spectra of liquid water: theory and interpretation.

Infrared spectroscopy measurements have been completed over a wide range of frequencies allowing to measure the evolution of both intramolecular and intermolecular vibrational modes in water as a function of temperature. Emphasis is made on the high frequency OH stretching band and the so-called connectivity band that lies in the far infrared region. The substructures of the two infrared bands are analyzed in terms of different levels of connectivity of the water molecules, along the statements of the percolation model. Both band profiles appear to be related to the different degrees of connectivity of water molecules. Comparison of the data with the predictions of the percolation model shows good agreement as for the temperature evolution of liquid water. This work provides additional support to the interpretation of water bands substructures as signatures of its very specific connectivity pattern.

Signatures of the hydrogen bonding in the infrared bands of water.

▪ Abstract The need to operate polymer electrolyte membrane (PEM) fuel cells at temperatures above 100°C, where the amount of water in the membrane is restricted, has provided much of the motivation for understanding the mechanisms of proton conduction at low degrees of hydration. Although experiments have not provided any direct information, numerous theoretical investigations have begun to provide the basis for understanding the mechanisms of proton conduction in these nano-phase-separated materials. Both the hydrated morphology and the nature of the confined water in the hydrophilic domains influence proton dissociation from the acidic sites (i.e., −SO3H), transfer to the water environment, and transport through the membrane. The following molecular processes are discussed in connection to their role in the conduction of protons in sulfonic acid–based polymer electrolyte membranes (PEMs): (a) local chemistry of the hydrophilic side chains; its effect on the dissociation of the proton and eventual stabi...

Proton Conduction Mechanisms at Low Degrees of Hydration in Sulfonic Acid–Based Polymer Electrolyte Membranes

The unique chemical and physical properties of liquid water are a direct result of its highly directional hydrogen-bond (HB) network structure and associated dynamics. However, despite intense experimental and theoretical scrutiny spanning more than four decades, a coherent description of this HB network remains elusive. The essential question of whether continuum or multicomponent (“intact,” “broken bond,” etc.) models best describe the HB interactions in liquid water has engendered particularly intense discussion. Most notably, the temperature dependence of water's Raman spectrum has long been considered to be among the strongest evidence for a multicomponent distribution. Using a combined experimental and theoretical approach, we show here that many of the features of the Raman spectrum that are considered to be hallmarks of a multistate system, including the asymmetric band profile, the isosbestic (temperature invariant) point, and van't Hoff behavior, actually result from a continuous distribution. Furthermore, the excellent agreement between our newly remeasured Raman spectra and our model system further supports the locally tetrahedral description of liquid water, which has recently been called into question [Wernet, P., et al. (2004) Science 304, 995-999].

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Unified description of temperature-dependent hydrogen-bond rearrangements in liquid water.

Water confined in nonionic fluorocarbon reverse micelles was investigated through mid-infrared spectroscopy (OH stretching and bending modes), in combination with quasielastic light scattering data. The characteristic OH stretching band is seen to exhibit significant changes upon decreasing water core size. An analysis in terms of three different levels of water connectivity has allowed the estimation of the extent of perturbation of water dynamics as a function of confining size.

A. Filabozzi

Papers

Signatures of the hydrogen bonding in the infrared bands of water.

Dependence of Water Dynamics upon Confinement Size