ScienceWeek

CHEMISTRY: ON THE INFRARED SPECTROSCOPY OF WATER

The following points are made by C.J. Fecko et al (Science 2003 301:1698):

1) The properties of liquid water that govern processes such as aqueous solvation and the transport of protons arise from the motions of water molecules within a constantly changing network of hydrogen bonds. The dynamics of this network occur over a range of time scales, from femtosecond fluctuations that involve a few molecules to picosecond diffusive motions that involve the breaking and forming of hydrogen bonds (1–5).

2) Time-resolved infrared (IR) spectroscopy is an ideal technique to investigate these dynamics, because the frequency of the intramolecular OH stretching vibration is particularly sensitive to a molecule's hydrogen-bond environment. Relative to the gas phase, this frequency red-shifts and the line shape broadens significantly because of hydrogen bonding in the liquid. Therefore, time-dependent shifts in the OH vibrational frequency, or spectral diffusion, can characterize changes in hydrogen bonding and intermolecular configuration.

3) Previous time-resolved IR studies have concentrated on time scales from a few hundred femtoseconds to several picoseconds, but have lacked sufficient time resolution to investigate the dynamics on the shorter time scales observed in femtosecond optical spectroscopies and predicted from simulation. Additionally, interpreting dynamics observed in IR experiments in terms of intermolecular structure requires an assignment of OH frequencies to structural variables of the liquid, which is still a topic of considerable debate. Previous simulation studies have highlighted the role of local hydrogen-bonding coordinates, but have not provided a simple, unifying picture of dephasing for all time scales. The authors address both of these issues by performing time-resolved IR spectroscopy on water with 52-fs IR pulses, allowing detection of faster dynamics, and by developing an atomistic model that qualitatively reproduces experimental results, exposing the essential molecular features that control spectral diffusion.

4) In summary: The authors investigated rearrangements of the hydrogen-bond network in water by measuring fluctuations in the OH-stretching frequency of HOD in liquid D2O with femtosecond infrared spectroscopy. Using simulations of an atomistic model of water, the authors relate these frequency fluctuations to intermolecular dynamics. The model reveals that OH frequency shifts arise from changes in the molecular electric field that acts on the proton. At short times, vibrational dephasing reflects an underdamped oscillation of the hydrogen bond with a period of 170 femtoseconds. At longer times, vibrational correlations decay on a 1.2-picosecond time scale because of collective structural reorganizations.

References (abridged):

1. W. Jarzeba, G. C. Walker, A. E. Johnson, M. A. Kahlow, P. F. Barbara, J. Phys. Chem. 92, 7039 (1988)

2. R. Jimenez, G. R. Fleming, P. V. Kumar, M. Maroncelli, Nature 369, 471 (1994)

3. M. J. Lang, X. J. Jordanides, X. Song, G. R. Fleming, J. Chem. Phys. 110, 5884 (1999)

4. F. H. Stillinger, Adv. Chem. Phys. 31, 1 (1975)

5. I. Ohmine, H. Tanaka, Chem. Rev. 93, 2545 (1993)

Science http://www.sciencemag.org

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ON WATER STRUCTURE

The following points are made by T. Head-Gordon and G. Hura (Chem. Rev. 2002 102:2651):

1) The fundamental unit of water structure is the hydrogen bond. In ice I a given water molecule is hydrogen bonded to four water neighbors in a tetrahedral structure that gives rise to a crystal made up of connected hexagonal rings. In the case of crystalline materials such as ice I, X-rays and neutrons are scattered by atomic centers at discrete angles represented as sinusoidal (Fourier) components of the electron density and nuclear scattering potential of the specimen, respectively. The scattering angle is determined by the spatial period of the Fourier component that is responsible for the scattering. The spatial period of each Fourier component of the electron density is determined by the lengths of the unit cell vectors of the crystal.

2) Representation of the electron density as a sum of Fourier components is equally applicable to noncrystalline materials, however, such as the water liquid. As a result it is still true that the spatial period of the Fourier component can be calculated from the measured scattering angle. As with crystalline materials, the amplitude of each Fourier component of the electron density is given by the square root of the scattered intensity. Information about the vector direction of the Fourier component is lost in scattering from liquids, however, unlike the case of crystals.

3) In the case of liquid water, the strict adherence to hydrogen-bonded hexagons of the ice crystal gives way to greater translational and rotational motion of waters and a broader distribution of hydrogen-bonded configurations, including a variety of polygons of varying sizes and degrees of puckering or distortion, all of which result in a more compact arrangement of water molecules. The electron density of the liquid is now characterized by the scattering as a diffuse water ring rather than a discrete distribution of Fourier components. Furthermore, the scattering intensity is peaked at a distance that remains larger than the center-to-center distance between individual water molecules, which is typically approximately 0.28 nm. Thus, it is clear that the most prominent Fourier components of the scattering density of pure liquid water have a repeat distance that is larger than the oxygen-oxygen nearest neighbor distance. This tells us that the fundamental scattering unit in liquid water must be something bigger than pairs of hydrogen-bonded water molecules. In fact, it is a measure of the highly associated three-dimensional hydrogen-bonded network of the water liquid. The importance of accurate experimental information and classical and emerging ab initio simulation methodologies is their ability to characterize this fundamental unit of scattering to help us to understand the topology of the hydrogen-bonded network over the full phase diagram.

References (abridged):

1. Water, a comprehensive treatise; Franks, F., Ed.; Plenum Press: New York, 1972.

2. Dore, J. C.; Teixeira, J. Hydrogen-bonded liquids: proceedings of the NATO Advanced Study Institute on Hydrogen-bonded Liquids, Institut Scientifique de Cargese, Corsica, April 3-15, 1989; Kluwer Academic Publishers: Dordrecht,; Boston, 1991.

3. Bellissent-Funel, M. C.; Dore, J. C. Hydrogen bond networks; Kluwer Academic Publishers: Dordrecht, Boston, 1994.

4. Franks, F. Water: A Matrix of Life, 2nd ed.; Royal Society of Chemistry: Cambridge, 2000.

5. Stillinger, F. H. Science (USA) 1980, 209, 451-7.

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