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ScienceWeek
CHEMISTRY: ON PROTONATED WATER CLUSTERS
The following points are made by Timothy S. Zwier (Science 2004 204:1119):
1) Of the chemical reactions that can occur in aqueous solution, acid-base reactions are among the most pervasive and important. Although it is easy to specify the balanced chemical equation for such a reaction [HX(aq) <--> H+(aq) + X-(aq), where H is a proton and X is the conjugate base of the acid HX], it is far more difficult to characterize the structures of these solvated ions, particularly the solvated proton. Shin et al (1) and Miyazaki et al (2) have used the powerful tool of infrared (IR) spectroscopy to probe protonated water clusters H+(H2O)n with n = 6 to 27. By isolating and mass-selecting the large protonated water clusters in the gas phase, they are able to record the IR spectra of each cluster size in this range, and thereby follow the development of the IR spectrum as a function of the number of water molecules in the cluster.
2) There are two major issues at stake here related to our understanding of the proton in water. The first is the structure of the "core ion" in the cluster; that is, whether the proton is strongly bound to a single water (forming H3O+, the hydronium ion) or is shared between two (forming H2O...H+...OH2). These limiting structures are called the Eigen and Zundel models for the proton in water, named after their original proponents (3,4). These differing core ion structures are calculated to have different IR spectroscopic signatures. In liquid water, the large number of water molecules not associated with the proton mask and distort the IR absorptions due to the proton. However, in the gas phase, the H+(H2O)n clusters can be mass-selected and studied individually by photo-fragmenting the ion cluster when IR absorption occurs.
3) The second issue is whether there are particularly stable structures for the entire protonated water cluster that appear as a function of cluster size. Studies aimed at answering the latter question also have a rich and interesting history. Thirty years ago, Searcy and Fenn (5) formed protonated water clusters in a high-pressure ion source and noticed that the H+(H2O)n mass peak with n = 21 was unusually intense relative to those around it, forming a "magic number" in the spectrum. This anomalous intensity was ascribed to an unusual stability for this cluster size, pointing to a structural basis for the stability. The n = 21 cluster bore an uncanny resemblance in size to the pentagonal dodecahedron water clathrate cage that surrounds nonpolar molecules (such as methane) in ice. This aesthetically pleasing structure is one water molecule shy of the n = 21 magic number. This suggested that H3O+ takes up a position either in the center of the cage, or on the surface, thereby displacing a neutral water molecule to the cage center. Subsequent mass spectrometry studies have sought to answer these structural questions, but spectroscopic evidence for the dodecahedral cage has been lacking.
References (abridged):
1. J.-W. Shin et al., Science 304, 1137 (2004)
2. M. Miyazaki, A. Fujii, T. Ebata, N. Mikami, Science 304, 1134 (2004)
3. M. Eigen, Angew. Chem. Int. Ed. 3, 1 (1964)
4. G. Zundel, H. Metzer, Z. Phys. Chem. 58, 225 (1968)
5. J. Q. Searcy, J. B. Fenn, J. Chem. Phys. 61, 5282 (1974)
Science http://www.sciencemag.org
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Related Material:
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.
4) 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.
Chemical Reviews http://pubs.acs.org/journals/chreay/index.html
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Related Material:
LIQUID WATER: CURRENT RESEARCH PROBLEMS
Notes by ScienceWeek
In general, "ab initio" (from first principles) calculations utilize experimental data on atomic systems to facilitate the adjustment of parameters. The excellent performance of ab initio techniques distinguishes them from their predecessors, the "semiempirical" methods, with the quantitative predictions of ab initio techniques usually falling within experimental error when comparisons are made to experimental measurements.
Contemporary molecular dynamics simulations, which are extrapolations of statistical mechanics and which originate in the work of Alder and Wainright in the 1960s, are computer simulations of molecular systems typically involving hundreds or sometimes thousands of idealized particles interacting with physically realistic potentials. Such molecular dynamics simulations can provide time-dependent properties of a liquid, but most commonly they are used to produce a set of configurations and forces which can be averaged to give equilibrium properties of the system.
The following points are made by F.N. Keutsch and R.L. Saykally (Proc. Nat. Acad. Sci. 2001 98:10533):
1) The quest to achieve an accurate description of liquid water has produced major advances in the last two decades, but despite the construction of hundreds of model force fields for use in simulations, the great advances in computational technology, and the development of powerful ab initio molecular dynamics methods, we remain unable to accurately calculate the properties of liquid water (e.g., heat capacity, density, dielectric constant, compressibility) over significant ranges in various conditions. We do not yet have a satisfactory molecular description of how a proton moves in the liquid, we do not fully understand the molecular nature of the surfaces of either ice or liquid water, nor do we understand the origin of the intriguing anomalies and singularities found in the deeply supercooled region.
2) Although it is clear that the hydrogen bond network and its fluctuations and rearrangement dynamics determine the properties of the liquid, no experimental studies exist that reveal detailed information on a molecular level without considerable interpretation. Moreover, the reliability of water models for simulating solvation phenomena and biological processes remains relatively untested.
3) A general obstacle to resolving these issues is that of correctly describing the many-body or cooperative nature of the hydrogen bonding interactions among a collection of water molecules. Theoretical work has demonstrated that the H-bond is dominated by electrostatic interactions, balanced by the repulsive electron exchange, but that dispersion makes an appreciable contribution, whereas induction (polarization) is the dominant many-body effect. It has proven notoriously difficult to accurately parameterize these interactions from ab initio calculations.
Proc. Nat. Acad. Sci. http://www.pnas.org
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