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It is water which matters: Observing the rattling modes of ions in the THz Range

Date Do, 24.04.2014 - Do, 24.04.2014
Time 10.15
Speaker Prof. Dr. Martina Havenith, Laserspektroskopie & Biophotonik, Ruhr-Universität Bochum, DE
Location Universität Bern, Institut für Angewandte Physik, Gebäude exakte Wissenschaften, Hörsaal B116, Sidlerstrasse 5, 3012 Bern
Program The details of ion hydration still raise fundamental questions relevant to a large variety of problems in chemistry and biology. The concept of water "structure breaking" and "structure making" by ions in aqueous solutions has been invoked to explain the Hofmeister series introduced over 100 years ago, which still provides the basis for the interpretation of experimental observations, in particular the stabilization/destabilization of biomolecules. Recent studies, using state-of-the- art experiments and molecular dynamics simulations, either challenge or support some key points of the structure maker/breaker concept, specifically regarding longranged ordering/disordering effects. Terahertz absorption spectroscopy in combination with molecular dynamics simulations is shown to be an important tool to study ion hydration. I will show the results for a series of aqueous solutions of divalent salts, which adds a new piece to the puzzle. The picture that emerges from the concentration dependence and assignment of the observed absorption features is one of a limited range of ion effects that is confined to the first solvation shell. For other salt, like lathanum we could show that the terahertz (THz) absorption spectrum allows to dissect the spectrum of the lanthanum with counter chloride and bromide ions in aqueous phase. When dissecting the spectrum we see the influence on cation due to counter anions ranges from self-confinement of both ions to solvent shared ion-pairing, with increasing salt concentration.

Reference: S. Funkner, G. Niehues, D.A. Schmidt, M. Heyden, G. Schwaab, K.M. Callahan, D.J. Tobias, M. Havenith Watching the low frequency motions in aqueous salt solutions – the terahertz vibrational signatures of hydrated ions JACS 134, 1030-1035 (2012)
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Link www.iap.unibe.ch
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