Moleculas De Agua

Following the Motions of Water Molecules in Aqueous Solutions James L. Skinner Science 328, 985 (2010); DOI: 10.1126/science.1190093

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CHEMISTRYFollowing the Motions of Water Molecules in Aqueous Solutions
James L. Skinner

Ultrafast spectroscopy is revealing details of the rotational dynamics of water molecules in salt solutions.

CREDIT: YU-SHAN LIN

ater molecules in an aqueous solution constantly translate and rotate in an intricate and beautiful dance that underlies essential processes in areas ranging from biology toatmospheric chemistry. Recent experimental advances are letting us see some steps of this dance for water molecules interacting with simple molecules and ions. These studies are now possible because of the invention of infrared lasers that fire pulses on ultrashort time scales. In this issue, two examples at the forefront of this new field, involving the dynamics of water around anions and cations, arereported by Ji et al. on page 1003 (1) and Tielrooij et al. on page 1006 (2). The isolated water molecule is very well understood from experiments and quantummechanical calculations. However, when a large number of water molecules form a liquid, our understanding of its structure and dynamics is more limited (3). Indeed, liquid water has a number of unusual properties (such as its high boilingpoint and density maximum at 4°C), which are thought to arise from intermolecular hydrogen bonds. It is especially important to understand the properties of water around ionic or hydrophobic solutes, which are also determined by hydrogen bonds (see the figure). The rotational and translational motion of water molecules in solution can control the rates of important chemical and biochemical processes.These include chemical reactions on the surface of aqueous aerosol particles in the atmosphere and electron-transfer reactions critical for respiration and photosynthesis. Most of the experiments that have been used to measure the molecular dynamics of water involve the absorption or scattering of light with different wavelengths: radio waves in nuclear magnetic resonance studies, microwaves indielectric relaxation, or infrared light in vibrational spectroscopy. Theoretical studies rely mainly on computer simulations in which the interactions among a collection of molecules are modeled with a potential energy function. The equations of
Theoretical Chemistry Institute and Department of Chemistry, University of Wisconsin, Madison, WI 53706, USA. E-mail: skinner@chem.wisc.edu

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