Hydrogen ions caught in the act of wandering
Researchers report for the first time on capturing snapshots of protons hopping through
water bridges between acid and bases
An international team of scientists headed by Erik T. J. Nibbering of the Max Born Institute
for Nonlinear Optics and Short Pulse Spectroscopy (MBI) and Ehud Pines of the Chemistry
Department in the Ben-Gurion University of the Negev (BGU) report for the first time
experimental evidence of the motions of hydrogen ions (protons, H+) from acids via water to
bases. Until now this has only been estimated as a possible reaction mechanism with
theoretical calculations. The results provide insight into fundamental processes in nature (acidbase neutralization, proton transmission through water and through biomembranes), that may
well become relevant for technological applications, e.g. in fuel cells. The scientists (coming
from Egypt, Germany, Israel and the Netherlands) report on these findings in the October 7
issue of Science.
For an extended period of time it was not clear how water accommodates the proton and how
the transfer of protons in aqueous solutions occur. The proton is the nucleus of the hydrogen
atom and is only about 1/10000 the size of the hydrogen atom. Protons are the fundamental
carriers of the positive electrostatic charge in nature and are unusually mobile in water.
Remarkably, the high mobility of the proton in water occurs in spite of the fact that the proton
does not move freely in water as the bare proton, H+, but always reside on water (H2O)
molecules. The simplest of the proton-water ion is the hydronium (H3O+) ion that forms larger
complexes through hydrogen-bonds with nearby water molecules, in continuously exchanging
configurations. The most important of such water-proton hydrogen-bonding complexes are the
so-called Zundel (H5O2+) and Eigen (H9O4+) cations, named after which are the proton
complexed with two and four water molecules respectively.
Erik Nibbering, together with his team members Omar F. Mohammed (a Ph. D. student from
Egypt) and the theoretician Jens Dreyer, and in collaboration with Ehud Pines and his team
member Dina Pines, succeeded to make snapshots of the proton motions with ultrashort laser
flashes. It turned out that by using novel experimental techniques hydrogen ions can be
‘captured in the act’ in the form of H3O+ while being transmitted from acid to base by water
molecules.
Water has many outstanding properties which have made them the solvent of choice for
supporting life on earth. The ability of water to transmit the proton much more efficiently than
any other common cation is essential for many life processes and the elucidation of this
remarkable property of water has captured the imagination of many generations of chemists
and biologists. First theoretical considerations on this extraordinary property of water were
made exactly 200 years ago by Theodor von Grotthuss, and since exactly 100 years scientists
use the phrase “von Grotthuss mechanism” to indicate the jump-like transmission of protons to
neighbouring water molecules (see Fig. 1). “One can use the picture of the improving a dike
with sandbags”, says Nibbering. “A chain of people will transport the sandbags more
efficiently and faster towards the dike, than when everybody on their own would bring the
sandbags to the dike.” Only recently around the millennium change numerous theoretical
refinements have become available, e.g. from detailed calculations it has become clear that
proton transmission becomes possible when the surrounding water rearranges its extensive
hydrogen-bonding network at particular points in time to enable the proton to transfer so
efficiently while appearing momentarily as the Zundel-cation and at other times as the Eigencation. “Never keeping still the proton constantly changes between appearances like a
chameleon” says Ehud Pines. It continuously adapts to its environment while flying through
ever-changing water molecule configurations.
+
+
+
Fig. 1: Von Grotthuss mechanism for proton conductivity in water. The blue area indicates the
hydronium ion.
From contributions by chemistry Nobel prize laureate Manfred Eigen and by Albert Weller in
the middle of the 20th century, theoriticians have derived that the exchange of protons
between simple mineral acids and bases in aqueous solution should occur stepwise in a similar
fashion to the von Grotthuss mechanism. Now follows the experimental confirmation of the
sequential hopping model by the current report in Science (Fig. 2). Here Nibbering, Pines and
their co-workers describe how the proton transfer reaction proceeds from an organic acid
towards a carboxylic base via an intermediate hydronium stage that according to its
spectroscopic signature has a symmetric configuration similar to the Eigen cation.
_
_
_
+
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Fig. 2: Suggested mechanism in the proton exchange between the organic photoacid and the
chloroacetate base. The location of the proton while transmitting between the acid and base is
indicated by blue.
This has become possible by a technique implemented by the Nibbering team in MBI that
enables the determination of the reaction progress in time steps of 150 femtoseconds. This is
extremely fast. For comparison: when one turns on a laser beam, and lets it on for one second,
the beam will have reached the moon. In 100 femtoseconds on the other hand a laser beam
will only have reached a distance equivalent to the diameter of a human hair. The scientists
have used in their experiments an aqueous acid-base mixture with which they already
performed proton transfer studies two years ago, a study also reported in Science (Science 301,
349 (2003)). “Then we were not able to observe the intermediate steps, but only the beginning
and the end of the proton transfer reaction”, mentions Nibbering. By a change of the proton
acceptor the reaction has been slowed down in such a fashion, that now the sequential proton
hopping via water molecules could be captured and recorded.
Further information can be obtained from:
Dr Erik T. J. Nibbering (tel.: 00-49-30-6392-1477; e-mail: [email protected])
Prof. Dr. Ehud Pines (tel: 00-972-8-6461640; e-mail: [email protected])