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Low-Frequency Modes of Aqueous Alkali Halide Solutions: Glimpsing the Hydrogen Bonding Vibration

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Science  12 Feb 2010:
Vol. 327, Issue 5967, pp. 857-860
DOI: 10.1126/science.1183799

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Salty Stretch

What happens at the molecular level when salt dissolves in water? Much of the data characterizing the geometry and dynamics of ion solvation shells has come from indirect observation of the surrounding water structure. Using a time domain Raman technique based on the interference of four ultrashort polarized light pulses, Heisler and Meech (p. 857) have now mapped directly the stretching vibrations associated with the weak hydrogen bonding interactions between bulk water molecules and chloride, bromide, or iodide ions.

Abstract

The solvation of ions in aqueous media is a fundamental process in biology and chemistry. Here, we report direct time-domain observations of the hydrogen bond vibrational mode formed between a halide ion (chloride, bromide, or iodide) and the surrounding water molecules. The frequency of the hydrogen bond mode is sensitive to both the atomic weight and the concentration of the ion. The peak frequencies fall in the 125 to 175 wave-number range, a spectral region accessed through time-domain polarization-resolved coherent Raman scattering using a diffractive optic method. The polarized Raman response observed is discussed in terms of the structure of the anion’s solvation shell and modeled through calculations on water chloride clusters.

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