Until now, observing subatomic structures was beyond the resolution
capabilities of direct imaging methods, and this seemed unlikely to change.
Czech scientists, however, have presented a method with which they became
the first in the world to observe an inhomogeneous electron charge
distribution around a halogen atom, thus confirming the existence of a
phenomenon that had been theoretically predicted but never directly
observed. Comparable to the first observation of a black hole, the
breakthrough will facilitate understanding of interactions between
individual atoms or molecules as well as of chemical reactions, and it opens
a path to refinement of the material and structural properties of various
physical, biological, and chemical systems. The breakthrough will be
published on Friday in Science.
In an extensive interdisciplinary collaboration, scientists from the Czech
Advanced Technology and Research Institute (CATRIN) of Palacký University
Olomouc, the Institute of Physics of the Czech Academy of Sciences (FZU),
the Institute of Organic Chemistry and Biochemistry of the Czech Academy of
Sciences (IOCB Prague), and the IT4Inovations Supercomputing Center at
VSB—Technical University of Ostrava have succeeded in dramatically
increasing the resolution capabilities of scanning microscopy, which several
years ago enabled humankind to image individual atoms, and have thus moved
beyond the atomic level to subatomic phenomena. The scientists have, for the
very first time, directly observed an asymmetric electron density
distribution on single atoms of halogen elements, the so-called sigma-hole.
In doing so, they have definitively confirmed its existence, theoretically
predicted some 30 years ago, and have overcome one of science's longstanding
challenges.
"Confirming the existence of the theoretically predicted sigma-holes is not
unlike observing black holes, which had never been seen until only two years
ago despite being predicted in 1915 by the general theory of relativity.
Viewed in that sense, it's not much of an exaggeration to say that the
imaging of the sigma-hole represents a similar milestone at the atomic
level," explains Pavel JelÃnek of FZU and CATRIN, a leading expert on the
theoretical and experimental study of the physical and chemical properties
of molecular structures on the surface of solid substances.
Until now, the existence of the phenomenon known as a sigma-hole had been
indirectly demonstrated by X-ray crystal structures with a halogen bond,
which revealed the surprising reality that chemically bonded halogen atoms
of one molecule and nitrogen or oxygen atoms of a second molecule, which
should repel one another, are in proximity and thus attract one another.
This observation was in blatant contradiction with the premise that these
atoms carry a homogenous negative charge and repel each other through
electrostatic force.
This led the scientists to examine the subatomic structure of halogen using
Kelvin probe force microscopy. They began by developing a theory describing
the mechanism of the atomic resolution of the Kelvin probe, which allowed
them to optimize the experimental conditions for imaging sigma-holes. The
subsequent combination of experimental measurements and advanced quantum
chemical methods resulted in a remarkable breakthrough—the first
experimental visualization of an inhomogeneous electron density charge
distribution, i.e. a sigma-hole—and the definitive confirmation of the
concept of halogen bonds.
"We improved the sensitivity of our Kelvin probe force microscopy by
functionalizing the tip probe with a single xenon atom, which allowed us to
visualize the inhomogeneous charge distribution in a bromine atom within a
molecule of brominated tetraphenylmethane, that is, a sigma-hole in real
space, and confirm the theoretical prediction," says Bruno de la Torre of
CATRIN and FZU.
"When I saw the sigma-hole for the first time, I was certainly skeptical,
because it implied that we had overcome the resolution limit of the
microscopes down to the subatomic level. Once I had accepted that, I felt
both proud of our contribution in pushing the limits of the experiment and
pleased to have opened a path for other researchers to go further and apply
this knowledge in discovering new effects at the single-atom level," adds de
la Torre.
According to the scientists, the ability to image an inhomogeneous electron
density charge distribution on individual atoms will, among other things,
lead to a better understanding of the reactivity of individual molecules and
the reason for the arrangement of various molecular structures. "I think
it's safe to say that imaging with subatomic resolution is going to have an
impact on various fields of science, including chemistry, physics, and
biology," says JelÃnek.
"I've studied noncovalent interactions all my life, and it gives me great
satisfaction that we can now observe something that previously we could
"see" only in theory and that the experimental measurements precisely
confirm our theoretical premise of the existence and shape of the
sigma-hole. It will allow us to better understand these interactions and
interpret them," says computational chemist Pavel Hobza of IOCB Prague, who
performed the advanced quantum chemical calculations on the supercomputers
at IT4Inovations in Ostrava. "What we're seeing is that halogen bonds and
noncovalent interactions in general play a dominant role not only in biology
but also in materials science. That makes our current paper in Science all
the more important," adds Hobza.
The characteristic shape of the sigma-hole is formed by a positively charged
crown surrounded by a belt of negative electron density. This inhomogeneous
charge distribution leads to the formation of a halogen bond, which plays a
key role in, among other things, supramolecular chemistry, including
molecular crystal engineering, and in biological systems.
A precise knowledge of the electron charge distribution on atoms is
necessary for an understanding of the interactions between individual atoms
and molecules, including chemical reactions. Thus, the new imaging method
opens the door to refinement of the material and structural properties of
many physical, biological, and chemical systems affecting everyday life.
Reference:
Benjamin Mallada et al, Real-space imaging of anisotropic charge of σ-hole
by means of Kelvin probe force microscopy, Science (2021).
DOI: 10.1126/science.abk1479.
Tags:
Physics