Water The Most Anomalous Liquid.
W is the key compound for our existence on this planet and is involved in nearly all biological, geological, and chemical processes. Access to clean water will furthermore become one of the greatest challenges to mankind in the face of global climate change. Although water is the most common liquid, it is also one of the most unusual, with many peculiar properties, such as an increased density upon melting, decreased viscosity under pressure, density maximum at 4 degC, high surface tension, and many more. These anomalous properties of water become more pronounced in the supercooled region below the melting point. In particular, if the thermal expansion coefficient, isothermal compressibility, and heat capacity are extrapolated below the temperature 38 degC) of homogeneous ice nucleation, they appear to diverge at a temperature of 45 degC. However, this regime is experimentally difficult to investigate due to the extremely rapid ice nucleation at this low temperature. Several scenarios have been proposed to account for the anomalies of water in the supercooled regime related to either a stability limit conjecture or to the presence of a second critical point associated with the coexistence curve separating two states of the liquid: low density and high density water. Alternatively, a singularity free scenario based on a percolation picture has been suggested. Other two state models of water have also been proposed based on the coexistence of low density low entropy and high density high entropy forms of water. Although the anomalies are extreme in the supercooled region, they are also present at ambient conditions, where most of water's physical, chemical, and biological processes occur. If we want to understand the properties of water, it is essential that we can create a unifying picture of the structure of water at ambient conditions all the way up to the boiling point, as well as deep into the supercooled regime and in its various crystalline and glassy forms. Since the hydrogen bonds (H bonds) in water fluctuate, it is essential not only to obtain a static picture but also to describe the dynamics, not only in terms of local excitations involving breaking and forming H bonds, but also in the form of collective excitations giving density fluctuations. One of the most essential questions to address for a microscopic understanding of water is What are the structure and dynamics of the hydrogen bonding network in water that give rise to all these unique properties? This question has been discussed intensively for over 100 years and has not yet been resolved. In order to gain new unique information regarding the structure of the H bond network in water, it is essential to develop new techniques, both experimental and theoretical. This requires a close synergy between theory and experiment, as well as cross disciplinary understanding and thinking, which characterize the reviews in the present collection and which are prerequisites for the concerted effort toward a unifying picture of water. Recently, new experimental methods, namely X ray spectroscopy involving electronic transitions between core and valence orbitals, e.g. X ray absorption spectroscopy (XAS) and X ray emission spectroscopy (XES) have been applied to liquid water and ice, resulting in a serious challenge to the traditional nearly tetrahedral picture of the H bond network in liquid water. However, the interpretation of the experimental results in terms of the structure and the picture of the liquid is strongly debated. Both Saykally and co workers and Hamalainen and co workers examined the temperature dependence of the intensity of the pre edge peak by XAS and suggested only minor distortions from the tetrahedral structure. Heske and co workers suggested that one XES peak in the lone pair region is due to emission from dissociated fragments and one from intact molecules. Theoretical simulations of X ray spectra are exploring a multitude of approaches with no consensus on which approach is the most reliable. From a formal perspective of XAS within density functional theory (DFT) time dependent DFT and the BetheSalpeter equation provide formally stringent approaches, but cannot be applied to large models due to computational cost. Similarly, quantum chemical approaches based on coupledcluster methods give high accuracy, but require further development to be applicable to realistic models. The Bethe Salpeter equation reproduces the pre edge structure in calculations of XAS using small models, but it places the postedge in the wrong position. Conversely, the more approximate transition potential method applied to larger models places the post edge in the correct position but does not find significant preedge intensity. A calibration using common benchmark structures and further developments of techniques are sorely needed! Scattering techniques using neutrons and X rays in the wideand small angle regions can now be reliably performed with significantly enhanced statistics. Recent wide angle X ray diffraction to very high q transfers >18 A 1) provides a benchmark oxygen oxygen pair distribution function first peak at a position and height very different from earlier standards to which most theoretical force fields have been calibrated. Here it is important to establish what should be the benchmark for simulators to compare with. At small angles, X ray diffraction provides information on density inhomogeneities due to fluctuations in the liquid with the intercept at q 0 corresponding to the isothermal compressibility. Whether or not this can be interpreted in terms of structural and density inhomogeneities in real water and what spatial extent such inhomogeneities could correspond to is under debate. An important fact is that no simulation model, force field or ab initio based, has been shown to reproduce the temperature dependence of the thermodynamical response functions described above. There is thus room for further developments. Ultrafast, nonlinear vibrational spectroscopy techniques are extremely important in obtaining information on the H bond dynamics in bulk water, ionic solutions, hydrophobic solvation, and confinement. A commonly used technique to simplify the interpretation is to use isotope substitution with dilute HDO in either H2O or D2O. Very recently, however, it has been demonstrated that OH is preferentially exposed at the surface of HDO liquid, i.e. that OD H bonds are stronger than OH H
Chemical reviews