Discovery of p-hole Boosts Molecular Insights and Nanomaterial Progress

Scientists from the IOCB Prague, the Institute of Physics of the Czech Academy of Sciences, and Palacký University Olomouc have once again successfully uncovered the mysteries of the world of molecules and atoms. 

They have experimentally verified the correctness of a decades-old theory that assumed a non-even distribution of electron density present in aromatic molecules.

This phenomenon considerably impacts the physicochemical properties of molecules and their interactions. This study expands the possibilities for developing new nanomaterials and is the theme of a study that has just been reported in the journal Nature Communications.

The same team of authors who previously conducted a groundbreaking study published in Science, explaining the uneven distribution of electrons within an atom known as the σ-hole, have now confirmed the presence of another phenomenon called the π-hole.

This new research focuses on aromatic hydrocarbons, where electrons are found in clouds above and below the carbon atom plane. By substituting the peripheral hydrogen atoms with more electronegative atoms or groups, which attract electrons, these initially negatively charged clouds transform into positively charged electron holes.

Researchers have taken the sophisticated method of scanning electron microscopy and pushed its abilities further. The method functions at subatomic resolution and could thus image atoms in molecules as well as the structure of the electron shell of an atom.

As one of the researchers involved, Bruno de la Torre from the Czech Advanced Technology and Research Institute (CATRIN) of Palacký University Olomouc, points out, the success of the experiment described here is mainly due to the excellent facilities at his institution and the participation of excellent Ph.D. students.

Thanks to our previous experience with the Kelvin Probe Force Microscopy (KPFM) technique, we have been able to refine our measurements and acquire very complete data sets that have helped us to deepen our understanding not only of how the charge is distributed in the molecules but also of what observables are obtained with the technique.

Bruno de la Torre, Czech Advanced Technology and Research Institute, Palacký University Olomouc

Modern force microscopy has long been the domain of researchers at the Institute of Physics. The team has not only leveraged this exceptional spatial resolution for molecular structures. Some time ago, they also validated the presence of a non-uniform electron density distribution around halogen atoms, referred to as σ-holes.

This achievement was reported in 2021 by Science. One of the most cited Czech researchers of today, Professor Pavel Hobza, from the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences (IOCB Prague), contributed to both the former, as well as present research.

The confirmation of the existence of the π-hole, as well as the σ-hole before it, fully demonstrates the quality of the theoretical predictions of quantum chemistry, which have accounted for both phenomena for decades. It shows that they can be relied upon even in the absence of available experiments.

Pavel Hobza, Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, IOCB Prague

The outcomes of the current study at the subatomic and submolecular levels could be compared to the breakthrough of cosmic black holes. They had also been theorized for decades before experiments confirmed their existence.

Enhancing our understanding of electron charge distribution is key to unraveling numerous chemical and biological mechanisms within the scientific realm. Beyond theoretical implications, this knowledge bears practical significance, enabling the construction of novel supramolecules and, consequently, driving the advancement of cutting-edge nanomaterials endowed with enhanced properties.

Video Credit: UOCHB IOCB Prague

Journal Reference:

Mallada, B., et al. (2023) Visualization of π-hole in molecules by means of Kelvin probe force microscopy. Nature Communications. doi.org/10.1038/s41467-023-40593-3.