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The essential role of water as a solvent in chemistry and biology is closely connected to the chemical interactions between dissolved ions and the water molecules immediately surrounding them (the "first hydration shell").
Combining soft x-ray absorption spectroscopy (XAS) applied to liquids under ambient conditions with density functional theory (DFT) calculations it is possible to gain insight into the local electronic structure of a hydrogen-bonded liquid and we were able to identify the "fingerprint" of the molecules in direct contact with the solvated ion.
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In the case of aqueous solution of transition metal ions we were able to demonstrate that the molecular orbitals of the water molecules in the first hydration shell will mix with the d-orbitals of a dissolved transition-metal ion, resulting in characteristic pre-edge features in the oxygen 1s XAS spectrum. The technique turned to be sensitive enough to also detect an additional chlorine ion in the hydration shell.
Combined XAS/DFT on ionic solutions have thus been demonstrated to provide
unique information on the electronic structure, bonding, and composition in
the first hydration shell.
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| REFERENCES: |
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L.Å. Näslund, M. Cavalleri, H. Ogasawara, A. Nilsson, L.G.M. Pettersson, P. Wernet, D.C. Edwards, M. Sandström, and S. Myneni, J. Phys. Chem. A,107, 6869 (2003) |
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News Highlights November 2003
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Surfaces and interfaces of liquid water and ice play a crucial role in a plethora of different processes but the structure of the H-bonding configurations across the liquid-gas interface is far from be understood.
Evidences for previously unrecognized interfacial water species, the "acceptor only" species, in dynamical equilibrium with the gas, came from our XAS measurements in conjunction with DFT calculations. Pictorially speaking, both OH groups are free, implying that these molecules are anchored to the surface by at most two lone pairs. |
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Recently our findings has been corroborated by an ab-initio molecular dynamics study, showing how XAS in combination with sofisticated electronic structure calculations is an important experimental technique to look into local structure of liquids and interfaces.
With the same technique we have studied also the surface properties of liquid
methanol, further emphasizing the sensitivity of XAS to broken H-bonds already
observed in our study of water. Our analysis leads to the conclusion that the
liquid methanol interface is composed by short linear chains, 2 to 4 molecules
in length, in contrast with the bulk longer chains and rings population.
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| REFERENCES: |
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K.R. Wilson, M. Cavalleri, B. S Rude, R. D. Schaller, T. Catalano, R.J.
Saykally, A. Nilsson and L.G.M. Pettersson, in press on J. Phys. Chem. A (2004)
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K.R. Wilson, M. Cavalleri, B. S Rude, R. D. Schaller, A. Nilsson, L.G.M. Pettersson, N. Goldman, T. Catalano, J.D. Bozek and R.J. Saykally,J. of Physics-CM., 14, 8, L221-L226 (2002).
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