Scientists have discovered fundamental steps of charging of
nano-sized water droplets and unveiled the long-sought-after mechanism of
hydrogen emission from irradiated water. Working together at the Georgia
Institute of Technology and Tel Aviv University, scientists have discovered
when the number of water molecules in a cluster exceeds 83, two excess
electrons may attach to it —
forming dielectrons — making it
a doubly negatively charged nano droplet. Furthermore, the scientists found
experimental and theoretical evidence that in droplets comprised of 105
molecules or more, the excess dielectrons participate in a water-splitting
process resulting in the liberation of molecular hydrogen and formation of two
solvated hydroxide anions. The
results appear in the June 30 issue of the Journal of Physical Chemistry A.
It has been known since the early 1980s that while single
electrons may attach to small water clusters containing as few as two molecules,
only much larger clusters may attach more than single electrons. Size-selected,
multiple-electron, negatively-charged water clusters have not been observed — until now.
Understanding the nature of excess electrons in water has captured
the attention of scientists for more than half a century, and the hydrated
electrons are known to appear as important reagents in charge-induced aqueous
reactions and molecular biological processes. Moreover, since the discovery in the early 1960s that the
exposure of water to ionizing radiation causes the emission of gaseous molecular
hydrogen, scientists have been puzzled by the mechanism underlying this
process. After all, the bonds in
the water molecules that hold the hydrogen atoms to the oxygen atoms are very
strong. The dielectron hydrogen-evolution
(DEHE) reaction, which produces hydrogen gas and hydroxide anions, may
play a role in radiation-induced reactions with oxidized DNA that have been
shown to underlie mutagenesis, cancer and other diseases.
“The attachment of multiple electrons
to water droplets is controlled by a fine balancing act between the forces that
bind the electrons to the polar water molecules and the strong repulsion
between the negatively charged electrons,” said Uzi Landman, Regents’ and
Institute Professor of Physics, F.E. Callaway Chair and director of the Center
for Computational Materials Science (CCMS) at Georgia Tech.
“Additionally, the binding of an
electron to the cluster disturbs the equilibrium arrangements between the
hydrogen-bonded water molecules and this too has to be counterbalanced by the
attractive binding forces. To
calculate the pattern and strength of single and two-electron charging of
nano-size water droplets, we developed and employed first-principles quantum mechanical
molecular dynamics simulations that go well beyond any ones that have been used
in this field,” he added.
Investigations on controlled size-selected clusters allow
explorations of intrinsic properties of finite-sized material aggregates, as
well as probing of the size-dependent evolution of materials properties from
the molecular nano-scale to the condensed phase regime.
In the 1980s Landman, together
with senior research scientists in the CCMS Robert Barnett, the late Charles
Cleveland and Joshua Jortner, professor of chemistry at Tel Aviv University,
discovered that there are two ways that single excess electrons can attach to
water clusters – one in which they bind to the surface of the water droplet,
and the other where they localize in a cavity in the interior of the droplet,
as in the case of bulk water. Subsequently, Landman, Barnett and graduate
student Harri-Pekka Kaukonen reported in 1992 on theoretical investigations
concerning the attachment of two excess electrons to water clusters. They
predicted that such double charging would occur only for sufficiently large nano-droplets.
They also commented on the possible hydrogen evolution reaction. No other work
on dielectron charging of water droplets has followed since.
That is until recently, when Landman, now one of the world leaders in the area of cluster and nano
science, and Barnett teamed up with Ori Chesnovsky, professor of
chemistry, and research associate Rina Giniger at Tel
Aviv University, in a joint project aimed at understanding the process
of dielectron charging of water clusters and the mechanism of the ensuing
reaction - which has not been observed previously in experiments on water
droplets. Using large-scale, state-of-the-art
first-principles dynamic simulations, developed at the CCMS, with all valence
and excess electrons treated quantum mechanically and equipped with a newly
constructed high-resolution time-of-flight mass spectrometer, the researchers
unveiled the intricate physical processes that govern the fundamental dielectron
charging processes of microscopic water droplets and the detailed mechanism of
the water-splitting reaction induced by double charging.
The mass
spectrometric measurements, performed at Tel Aviv, revealed that singly charged
clusters were formed in the size range of six to more than a couple of hundred
water molecules. However, for clusters containing more than a critical size of
83 molecules, doubly charged clusters with two attached excess electrons were
detected for the first time. Most significantly, for clusters with 105 or more
water molecules, the mass spectra provided direct evidence for the loss of a
single hydrogen molecule from the doubly charged clusters.
The theoretical
analysis demonstrated two dominant attachment modes of dielectrons to water
clusters. The first is a surface mode (SS’), where the two repelling electrons
reside in antipodal sites on the surface of the cluster (see the two wave
functions, depicted in green and blue, in Figure 1). The second is another
attachment mode with both electrons occupying a wave function localized in a
hydration cavity in the interior of the cluster — the so-called II binding mode
(see wave function depicted in pink in Figure 2). While both dielectron
attachment modes may be found for clusters with 105 molecules and larger ones,
only the SS’ mode is stable for doubly charged smaller clusters.
“Moreover, starting
from the II, internal cavity attachment mode in a cluster comprised of 105
water molecules, our quantum dynamical simulations showed that the concerted
approach of two protons from two neighboring water molecules located on the
first shell of the internal hydration cavity, leads, in association with the
cavity-localized excess dielectron (see Figure 2), to the formation of a
hydrogen molecule. The two remnant hydroxide anions diffuse away via a sequence
of proton shuttle processes, ultimately solvating near the surface region of
the cluster, while the hydrogen molecule evaporates,” said Landman.
“What’s more, in
addition to uncovering the microscopic reaction pathway, the mechanism which we
discovered requires initial proximity of the two reacting water molecules and
the excess dielectron. This can happen only for the II internal cavity
attachment mode. Consequently, the theory predicts, in agreement with the
experiments, that the reaction would be impeded in clusters with less than 105
molecules where the II mode is energetically highly improbable. Now, that’s a
nice consistency check on the theory,” he added.
As for future plans,
Landman remarked, “While I believe that our work sets methodological and
conceptual benchmarks for studies in this area, there is a lot left to be done.
For example, while our calculated values for the excess single electron
detachment energies are found to be in quantitative agreement with
photoelectron measurements in a broad range of water cluster sizes — containing
from 15 to 105 molecules — providing a consistent interpretation of these
measurements, we would like to obtain experimental data on excess dielectron
detachment energies to compare with our predicted values,” he said.
“Additionally, we
would like to know more about the effects of preparation conditions on the
properties of multiply charged water clusters. We also need to understand the
temperature dependence of the dielectron attachment modes, the influence of
metal impurities, and possibly get data from time-resolved measurements. The understanding
that we gained in this experiment about charge-induced water splitting may
guide our research into artificial photosynthetic systems, as well as the
mechanisms of certain bio-molecular processes and perhaps some atmospheric phenomena.”
“You know,” he added. “We started
working on excess electrons in water clusters quite early, in the 1980s — close
to 25 years ago. If we are to make future progress in this area, it will have
to happen faster than that.”
This research was funded by the U.S. Office of Basic Energy Sciences and the Israel Science Foundation.
