Revealing the mysterious world of molecules – scientists confirm a decades-old theory

The researchers have experimentally verified a long-standing theory that posits that electron density is unevenly distributed in aromatic molecules.

Researchers from IOCB Prague, the Institute of Physics of the Czech Academy of Sciences, and Palatske Olomouc University have once again made major progress in unraveling the secrets of the world of molecules and atoms. They experimentally verified a long-standing theory suggesting that electron density is not uniformly distributed in aromatic molecules.

This phenomenon greatly affects the physical and chemical properties of molecules and their interactions. This research expands the possibilities for designing new nanomaterials and is the subject of a just-published paper Nature Communications.

The same team of authors as their previous pioneering study published in Sciences Describe the irregular distribution of electrons in… corn, the so-called hole σ. Now researchers have confirmed the existence of the so-called π-hole. In aromatic hydrocarbons, we find electrons in clouds above and below the level of carbon atoms. If we replace the surrounding hydrogen atoms with more electronegative atoms or groups of atoms that pull electrons away, the originally negatively charged clouds turn into positively charged electron holes.

Professor Pavel Hobza, Distinguished Chair and Head of the Noncovalent Reactions Group at IOCB Prague. Credit: Thomas Bellon / IOCB Prague

Scientists have taken the advanced method of scanning electron microscopy and pushed its capabilities even further. This method works at subatomic resolution, so it can image not only atoms in molecules, but also the structure of an atom’s electron shell. As one of the co-investigators, Bruno de la Torre from the Czech Institute of Advanced Technology and Research (CATRIN) at Palatske University Olomouc, points out, the success of the experiment described here is mainly due to the excellent facilities at his home institution and the institute’s excellent Ph.D. students.

“Thanks to our previous experience with Kelvin probe force microscopy (KPFM) technology, we were able to improve our measurements and obtain very complete datasets that helped us deepen our understanding not only of how charge is distributed in molecules. But also of what things can observed using this technique.

Experimental measurements confirmed theoretical predictions of the existence of a π-hole. From left to right: chemical structure of the investigated molecule, calculated electrostatic potential map of the molecule, experimental Kelvin probe force micrograph (KPFM), and simulated KPFM image. Credit: IOCB Prague

Modern force microscopy has long been the domain of researchers at the Institute of Physics. Not only in the case of molecular structures, they used unprecedented spatial resolution to the greatest extent. Some time ago they confirmed the existence of a non-uniform distribution of electron density around halogen atoms, so-called σ-holes. This achievement was published in 2021 by Sciences. The previous and current research has been significantly contributed by one of the most cited Czech scientists today, Professor Pavel Hobza from the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences (IOCB Prague).

“The confirmation of the existence of the π-hole, as well as the σ-hole that precedes it, fully demonstrates the quality of theoretical predictions of quantum chemistry, which has been responsible for both phenomena for decades. It shows that they can be relied upon even in the absence of available experiments,” says Pavel Hobza. .

The results of Czech scientists’ research at the subatomic and submolecular levels can be compared to the discovery of cosmic black holes. They were also theorized for decades before their existence was confirmed experimentally.

A better knowledge of the electron charge distribution will help the scientific community understand many chemical and primarily biological processes. On a practical level, this will translate into the ability to construct new supermolecules and subsequently develop advanced nanomaterials with improved properties.

Reference: “Visualization of π-holes in molecules by Kelvin probe force microscopy” by B. Mallada, M. Ondráček, M. Lamanec, A. Gallardo, A. Jiménez-Martín, B. de la Torre, P. Hobza and B. . Jelinek, August 16, 2023, Nature Communications.
doi: 10.1038/s41467-023-40593-3