Rapid Water-Ion Dynamics Revealed

New spectroscopic techniques are providing some of the most detailed pictures yet of the behavior of water molecules on femtosecond time­scales (Science 2010, 328, 1003 and 1006). Ubiquitous liquid water, with its networks of evanescent hydrogen bonds and its vital interactions with biological and atmospheric molecules and ions, remains mysterious. In one study, Minbiao Ji of the SLAC National Accelerator Laboratory and colleagues used two-dimensional infrared spectroscopy to verify a recent prediction that when water molecules and perchlorate ions (ClO₄ ^–) exchange hydrogen bonds, the water molecules rotate in quick jumps rather than in smooth motions. In a separate study, Klaas-Jan Tielrooij and colleagues at the Institute for Atomic & Molecular Physics, in Amsterdam, employed terahertz dielectric relaxation spectroscopy and femtosecond infrared pump-probe spectroscopy to discover that some cation-anion combinations in water, such as magnesium and sulfate, can impede the motions of water molecules at relatively long ranges. “These and related studies herald a new era in our understanding of solvation dynamics in aqueous solutions,” writes University of Wisconsin, Madison, theoretical chemist James L. Skinner in a commentary about the two studies.—EKW