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Energetics of Hydrogen Bond Network Rearrangements in Liquid Water Print

The unique chemical and physical properties of liquid water are thought to result from the highly directional hydrogen bonding (H-bonding) network structure and its associated dynamics. However, despite intense experimental and theoretical scrutiny, a complete description of this structure has been elusive. Recently, with the help of their novel liquid microjet apparatus, a University of California, Berkeley, group derived a new energy criterion for H-bonds based on experimental data. With this new criterion based on analysis of the temperature dependence of the x-ray absorption spectra of normal and supercooled liquid water, they concluded that the traditional structural model of water is valid.

The Bondedness of Water

For many of us, the main question we have about liquid water is whether to drink it straight from the tap or filter it first. For the scientific community, however, water remains a focus of research both because its unique physical and chemical properties make it critical to life, the environment, and industry and because some of its basic features, such as the weak (hydrogen) bonding between water molecules, that give it these properties have yet to be nailed down. The continuing intense interest explains why Science magazine, in its annual “Breakthrough of the Year” feature, recognized water research as one of its top ten breakthroughs of 2004.

Consider the structure of the liquid, which is often visualized as a kind of floppy ice; that is, it consists of a random three-dimensional network of water molecules, each of which is hydrogen bonded to four neighbors. Previously, a group combining theory and experimental measurements based on x-ray spectroscopy proposed that liquid water actually has only about 50 percent as many hydrogen bonds as solid ice, resulting in a mixture of rings and chains rather than a network. However, Smith et al. have now presented new x-ray data for water from a novel liquid microjet apparatus that allowed them to determine the energy needed to break a hydrogen bond and that supports the traditional network model. The debate continues.

The standard description of water structure consists of a random tetrahedral network in which every molecule is H-bonded to its four nearest neighbors, an arrangement similar to that in hexagonal ice. In recent years, a number of x-ray absorption spectroscopy (XAS) and related experiments have begun to characterize the H-bond structure in detail by assigning the observed spectral features to specific H-bonding configurations. However, in order for such an analysis to yield a quantitative description of the H-bond structure, it is necessary to establish an experimental definition of what actually constitutes an H-bond, or in other words, what degree of distortion of the H-bond network leads to a measurable change in the XAS.

A liquid microjet about 30 μm in diameter. The nozzle is formed from a fused silica capillary with an inside diameter of 100 μm, which is elongated by means of a commercial CO2 laser pipette puller in order to obtain the final nozzle diameter.

Previously, only a geometric criterion had been established, which was derived by density functional theory (DFT) calculations of a model cluster [Ph. Wernet et al., Science 304, 995 (2004)]. Applying their theoretical H-bonding criterion to their x-ray Raman measurements, the authors had to invoke a picture of a liquid composed of monocyclic rings and linear chains—radically different from a tetrahedral network—to obtain agreement with theoretical calculations.

This latest experiment begins with injection of a small liquid microjet (10–30 μm diameter) into a high-vacuum chamber. The small size of the volatile sample makes it possible to maintain a relatively good vacuum (10-5 torr), which enables windowless coupling to both Beamlines 8.0.1 and 11.0.2. Furthermore, such low pressures allow for efficient transport of electrons generated by the x-ray absorption process away from the jet surface to a detector placed about 1 mm away from the interaction region.

The Berkeley group measured x-ray absorption by the total electron yield (TEY) method. To obtain the temperature dependence, they recorded successive TEY spectra as a function of distance from the liquid jet nozzle. The microjet undergoes rapid evaporative cooling upon injection into the high-vacuum chamber, and therefore the liquid jet temperature continually decreases with distance away from the nozzle. The cooling rate had been thoroughly characterized in previous experiments by the group.

Comparison of the TEY x-ray absorption spectra taken at two different temperatures. The solid black curve was recorded at 288 K and the gray curve at 254 K.

Upon cooling, a sizeable decrease in the pre-edge intensity, an XAS spectral feature previously assigned to broken or distorted hydrogen bonding configurations in water, was observed. A simultaneous increase in the post-edge intensity, a spectral feature assigned to highly symmetric ice-like configurations, was also observed. A plot of the logarithm of the ratio of peak areas (Ipost-edge/Ipre-edge) vs. inverse temperature should be linear, with a slope proportional to the energy difference between the two configurations (distorted and ice-like). From its temperature dependence measurements, the group found the difference in energy to be 1.5 ± 0.5 kcal/mol. This is the experimentally derived energetic hydrogen bond criterion.

Plots of the log of the ratio of the areas of the post-edge and pre-edge features versus inverse temperature from three separate experiments. The solid lines represent the linear fit with a slope proportional to the difference in energy between the distorted and ice-like H-bonding distributions. The energy difference is determined to be 1.5 ± 0.5 kcal/mol (average of the three measurements).

By this experimental criterion, the x-ray Raman measurements reported by Wernet et al. are consistent with the “standard model” for water after all. The Berkeley group would like to study the nature of the transition from the symmetric ice-like structure to the distorted structure described above via molecular dynamics simulations.


Research conducted by J.D. Smith, C.D. Cappa, B.M. Messer, R.C. Cohen, and R.J. Saykally (University of California, Berkeley, and Berkeley Lab) and K.R. Wilson (Berkeley Lab).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: J.D. Smith, C.D. Cappa, K.R. Wilson, B.M. Messer, R.C. Cohen, and R.J. Saykally, “Energetics of hydrogen bond network rearrangements in liquid water,” Science 306, 851 (2004).