Shape matters

Details of how an ion's reactivity is affected by the size and shape of the water network surrounding it have been elucidated using infrared spectroscopy.

Credit: © 2009 AAAS

Many questions are still to be answered about the solvent-mediated reactions of water. Although we know that many of its properties arise from its ability to form dynamic hydrogen-bonded networks, the intricacies of many reactive processes are still unclear. One such unresolved issue is how water clusters mediate ion chemistry. Now Mark Johnson of Yale University and colleagues from across the Unites States have studied¹ how water clusters surround and react with NO⁺ to form HONO. This reaction involves the transfer of charge from NO⁺ to the surrounding water network and has significance for the understanding of chemistry in the Earth's ionosphere.

The rate of reaction is known to depend on the number of water molecules that surround NO⁺. How the geometry of the water network affects the reaction, however, is less understood. To study this, Johnson and colleagues used infrared spectroscopy to measure the vibrational spectra of clusters of NO⁺(H₂O)_n, and calculations to understand them.

Small clusters with just one or two water molecules were unreactive as expected, but the spectrum of NO⁺(H₂O)₃ revealed three different possible geometries. Only one of them was able to promote charge delocalization over the water network — and thus reactivity. This geometry differed from the others in that only one water molecule was directly interacting with NO⁺, allowing the other two — in the second solvation shell — to accommodate the excess charge on the NO⁺-interacting water molecule.