Acids yielding free protons are common in biological and chemical systems and the measurement of pH to determine acidity of an aqueous solution is a simple, standard procedure. However, it has not been as easy to determine where the liberated protons are located and how they interact with water molecules.
The scientists tackled these questions using infra-red laser light, at much lower energies than were previously accessible, to monitor how the vibration profile changes when a proton is associated with two to eleven water molecules.
The researchers first established a spectral signature for the symmetrically hydrated Eigen cation, which has a minimum energy (H3O)+ ion core and three associated "dangling" water molecules. As they successively added or subtracted water molecules and compared the spectral signatures, they mimicked water fluctuations.
"Surprisingly large spectral shifts are driven by small changes in the hydration environment," said Johnson. "Although previous work anticipated a change from Zundel to Eigen structures as you progress from 8 to 9 water molecules, the change in the low energy bands here is dramatic. The profile for the 9-membered cluster is much like bulk water, but then the 10-membered cluster is again simpler."
The study shows that the proton associated with the Eigen cation undergoes vibrations highest in energy because it supports the greatest distribution of charge, that is, over three H atoms. As different numbers of water molecules surround the H3
Contact: Janet Rettig Emanuel