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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.
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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
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