Huib J. Bakker

VIBRATIONAL SPECTROSCOPY AS A PROBE OF STRUCTURE AND DYNAMICS IN LIQUID WATER

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.

Resonant intermolecular transfer of vibrational energy in liquid water

Many biological, chemical and physical processes involve the transfer of energy. In the case of electronic excitations, transfer between molecules is rapid, whereas for vibrations in the condensed phase, resonant energy transfer is an unlikely process because the typical timescale of vibrational relaxation (a few picoseconds) is much shorter than that of resonant intermolecular vibrational energy transfer. For the OH-stretch vibration in liquid water, which is of particular importance due to its coupling to the hydrogen bond, extensive investigations have shown that vibrational relaxation takes place with a time constant of 740 ± 25 femtoseconds (ref. 7). So for resonant intermolecular energy transfer to occur in liquid water, the interaction between the OH-stretch modes of different water molecules needs to be extremely strong. Here we report time-resolved pump-probe laser spectroscopy measurements that reveal the occurrence of fast resonant intermolecular transfer of OH-stretch excitations over many water molecules before the excitation energy is dissipated. We find that the transfer process is mediated by dipole–dipole interactions (the Förster transfer mechanism) and additional mechanisms that are possibly based on intermolecular anharmonic interactions involving hydrogen bonds. Our findings suggest that liquid water may play an important role in transporting vibrational energy between OH groups located on either different biomolecules or along extended biological structures. OH groups in a hydrophobic environment should accordingly be able to remain in a vibrationally excited state longer than OH groups in a hydrophilic environment.

Cooperativity in Ion Hydration

Wet Twists and Turns When salts dissolve in water, their constituent positively and negatively charged ions are pulled apart and surrounded by shells of H2O molecules (see the Perspective by Skinner). Ji et al. (p. 1003) looked closely at the motion in these shells, using a type of vibrational spectroscopy sensitive to both the orientation and to the neighbors of the targeted molecules. In agreement with recent theoretical predictions, the individual water molecules shifted orientation between an anion and the surrounding liquid in sudden discrete steps, rather than by making smooth incremental rotations. Tielrooij et al. (p. 1006) compared the relative impacts of cations and anions on the rigidity of the wider water network, using spectroscopic techniques sensitive to the role of each ion. Certain cation/anion combinations, such as magnesium sulfate, appeared to act together to restrict water motion beyond the boundaries of individual shells. When salts dissolve in water, the separated cations and anions can still collectively impact the liquid structure. Despite prolonged scientific efforts to unravel the effects of ions on the structure and dynamics of water, many open questions remain, in particular concerning the spatial extent of this effect (i.e., the number of water molecules affected) and the origin of ion-specific effects. A combined terahertz and femtosecond infrared spectroscopic study of water dynamics around different ions (specifically magnesium, lithium, sodium, and cesium cations, as well as sulfate, chloride, iodide, and perchlorate anions) reveals that the effect of ions and counterions on water can be strongly interdependent and nonadditive, and in certain cases extends well beyond the first solvation shell of water molecules directly surrounding the ion.

Structural dynamics of aqueous salt solutions.

1. General Introduction 1456 2. Structural Dynamics of Ionic Hydrations Shells 1457 2.

Mechanism for vibrational relaxation in water investigated by femtosecond infrared spectroscopy

We present a study on the relaxation of the O–H stretch vibration in a dilute HDO:D2O solution using femtosecond mid-infrared pump-probe spectroscopy. We performed one-color experiments in which the 0→1 vibrational transition is probed at different frequencies, and two-color experiments in which the 1→2 transition is probed. In the one-color experiments, it is observed that the relaxation is faster at the blue side than at the center of the absorption band. Furthermore, it is observed that the vibrational relaxation time T1 shows an anomalous temperature dependence and increases from 0.74±0.01 ps at 298 K to 0.90±0.02 ps at 363 K. These results indicate that the O–H⋯O hydrogen bond forms the dominant accepting mode in the vibrational relaxation of the O–H stretch vibration.

Ultrafast vibrational energy transfer at the water/air interface revealed by two-dimensional surface vibrational spectroscopy

Water is very different from liquids of similar molecular weight, and one of its unique properties is the very efficient transfer of vibrational energy between molecules, which arises as a result of strong dipole-dipole interactions between the O-H oscillators. Although we have a sound understanding of such energy transfer in bulk water, we know less about how, and how quickly, transfer occurs at its interface with a hydrophobic phase, because specifically addressing the outermost monolayer is difficult. Here, we use ultrafast two-dimensional surface-specific vibrational spectroscopy to probe the interfacial energy dynamics of heavy water (D(2)O) at the water/air interface. The measurements reveal the presence of surprisingly rapid energy transfer, both between hydrogen-bonded interfacial water molecules (intermolecular), and between O-D groups sticking out from the water surface and those located on the same molecule and pointing towards the water bulk (intramolecular). Vibrational energy transfer occurs on sub-picosecond timescales, and its rates and pathways can be quantified directly.

Inhomogeneous dynamics in confined water nanodroplets

The effect of confinement on the dynamical properties of liquid water was studied by mid-infrared ultrafast pump–probe spectroscopy on HDO:D2O in reverse micelles. By preparing water-containing reverse micelles of different well defined sizes, we varied the degree of geometric confinement in water nanodroplets with radii ranging from 0.2 to 4.5 nm. We find that water molecules located near the interface confining the droplet exhibit slower vibrational energy relaxation and have a different spectral absorption than those located in the droplet core. As a result, we can measure the orientational dynamics of these different types of water with high selectivity. We observe that the water molecules in the core show similar orientational dynamics as bulk water and that the water layer solvating the interface is highly immobile.

Influence of ions on the hydrogen-bond structure in liquid water

The orientational-correlation time of water molecules in ionic solutions has been measured with femtosecond pump–probe spectroscopy. It is found that the addition of ions has no influence on the rotational dynamics of water molecules outside the first solvation shells of the ions. This shows that the presence of ions does not lead to an enhancement or a breakdown of the hydrogen-bond network in liquid water.

Protons and Hydroxide Ions in Aqueous Systems.

Understanding the structure and dynamics of waters constituent ions, proton and hydroxide, has been a subject of numerous experimental and theoretical studies over the last century. Besides their obvious importance in acid-base chemistry, these ions play an important role in numerous applications ranging from enzyme catalysis to environmental chemistry. Despite a long history of research, many fundamental issues regarding their properties continue to be an active area of research. Here, we provide a review of the experimental and theoretical advances made in the last several decades in understanding the structure, dynamics, and transport of the proton and hydroxide ions in different aqueous environments, ranging from water clusters to the bulk liquid and its interfaces with hydrophobic surfaces. The propensity of these ions to accumulate at hydrophobic surfaces has been a subject of intense debate, and we highlight the open issues and challenges in this area. Biological applications reviewed include proton transport along the hydration layer of various membranes and through channel proteins, problems that are at the core of cellular bioenergetics.

Hydrophobic Molecules Slow Down the Hydrogen-Bond Dynamics of Water

We study the spectral and orientational dynamics of HDO molecules in solutions of tertiary-butyl-alcohol (TBA), trimethyl-amine-oxide (TMAO), and tetramethylurea (TMU) in isotopically diluted water (HDO:D(2)O and HDO:H(2)O). The spectral dynamics are studied with femtosecond two-dimensional infrared spectroscopy and the orientational dynamics with femtosecond polarization-resolved vibrational pump-probe spectroscopy. We observe a strong slowing down of the spectral diffusion around the central part of the absorption line that increases with increasing solute concentration. At low concentrations, the fraction of water showing slow spectral dynamics is observed to scale with the number of methyl groups, indicating that this effect is due to slow hydrogen-bond dynamics in the hydration shell of the methyl groups of the solute molecules. The slowing down of the vibrational frequency dynamics is strongly correlated with the slowing down of the orientational mobility of the water molecules. This correlation indicates that these effects have a common origin in the effect of hydrophobic molecular groups on the hydrogen-bond dynamics of water.