How water
accommodates acids is a long-standing, centuries-old question in chemistry. The
key to this question is the structure of the hydrated proton, the chemically
active ingredient in acids. Acids are found everywhere and their control in
nature is essential for the well being of our environment as well as our body’s
health.
Prof. Ehud Pines from BGU suggested a way to study the structure of strong acids in water.
Teaming up with a research group at the Max-Born Institute in Berlin headed by
Professor Thomas Elsasser, a clear picture of the structure of the hydrated
proton of strong acids emerged by applying ultrafast vibrational spectroscopy.
The study was published
online by Science through their First Release platform late last week entitled
“Large amplitude of transfer motions the hydrated proton mapped by 2-Dvibrational spectroscopy”.
The study mapped
fluctuating proton transfer motions and provides direct evidence that protons
in liquid water are shared by two water molecules which keep the proton locked
inside a two-atom potential box for
relatively long times that may exceed 1 picosecond. The results imply that the
femtosecond large-amplitude proton movement within a particular box are 10 to
50 times faster than proton hopping to a new hydration site, the elementary
charge transfer step in chemistry.
Pines is a
member of the Department of Chemistry.
Chemical structure of hydrated protons in liquid water.
A. Schematic of the Eigen cation H9O4+ (left)
and the Zundel cation H5O2+ (right). The
arrows indicate the O-H bond coordinate r and the (O…H+…O) proton transfer
coordinate. In the Eigen cation a covalent O-H bond localizes the proton
whereas in the Zundel cation the proton is delocalized between two water
molecules. B. Anharmonic vibrational potential (left)
and double minimum potential of the Zundel cation along z (right, red). Distortions
by the solvent surrounding impose.
