Credit: © 2010 AAAS

Hydrated electrons — free electrons in aqueous solution — have been well studied for the past half a century, not only because they pose many fundamental questions about solute–solvent interactions, but also because they can occur as a result of ionizing radiation and cause damage to biomolecules such as DNA. Much of the evidence previously reported supports the 'cavity model': the idea that a free electron occupies an almost spherical void that excludes water molecules.

Now Ross Larsen, Benjamin Schwartz and William Glover, from the National Renewable Energy Laboratory, UCLA and Stanford University respectively, have used a new model to study the hydrated electron (Science 329, 65–69; 2010). Their results suggest that it inhabits a region that does not exclude water; on the contrary, the hydrated electron effectively attracts water molecules. The researchers' model uses a new potential to describe the interactions between water molecules and a free electron. The potential displays features not previously included in simulations of this type, such as a strong attraction between water's oxygen atoms and the electron.

Inspection of calculated probability distributions for the distance between the hydrogen or oxygen atoms of water and the electron's centre of mass reveal that the electron's charge is spread through a volume that includes, on average, 37 water molecules (pictured) rather than existing in an exclusive cavity. Calculations also show that without the electron the same volume would only contain 30 molecules, indicating that the hydrated electron is attractive. The model agrees well with experimental data from optical absorption spectroscopy and photo-excited state dynamics.

The original version of this story first appeared on the Research Highlights section of the Nature Chemistry website.