Motion of protons in water

The transport of protons through liquid water forms one of the key processes of chemistry and biology. The mobility of the proton in water is much higher than might be expected based on the size of its solvated structure, indicating that proton transport involves a mechanism other than ionic diffusion. At present, the nature of this mechanism is the subject of extensive theoretical and experimental investigations.

In a cooperation with the FOM Institute for Atomic and Molecular Physics (AMOLF), we study the dynamical behavior of protons in liquid water using femtosecond vibrational pump-probespectroscopy.  The proton inwater is believed to occur mainly in two hydration structures, the [H9O4]+ (Eigen) and the [H5O2]+ (Zundel) structures, shown schematically in the picture below. The Eigen and Zundel structures have OH-stretching modes (indicated by the arrows) with distinctly different frequencies.

Direct observation of Eigen-Zundel interconversion

In the experiments, we excite the OH-stretching mode of the Eigen structure using resonant ultrashort infrared pulses, and monitor the subsequent absorption changes using weak, delayed probing pulses. In this way, the dynamics of the proton can be observed in real time. The experiments are carried out in isotopically diluted acidic water (HDO:D2O) in order to avoid effects of heating and resonant energy transfer.

The vibrational lifetime of the protonic OH-stretching mode is found to be extremely short (~120 fs), several times shorter than that of the OH-stretching mode in pH-neutral  HDO:D2O. We also observe an extremely fast (<100fs) interconversion between the Eigen and Zundel hydration structures of the proton. This constitutes experimental evidence for the Grotthuss mechanism of proton transport in liquid water. In this mechanism, the proton charge rather than the proton mass is transported, by means of a rapid interchange between the Eigen and Zundel solvation structures, see the picture below. Our experiments suggest that the elementary step of proton transport, the Eigen-Zundel interconversion, takes place on a time scale of less than 100 fs. The results have been published in the following article:

Phys. Rev. Lett. article on proton transport in liquid water