LAUSR

Description Resolution of the σ-hole Anisotropic distribution of charges on atoms plays an important role in intermolecular interactions, yet direct experimental imaging remains a long-standing challenge. A good example is the σ-hole, an anisotropic charge distribution on a halogen atom covalently linked to carbon atoms. The σ-hole gives rise to the halogen-bonding mechanism, which is well known in supramolecular chemistry despite its existence being confirmed only indirectly. Mallada et al. developed Kelvin probe force microscopy with a specifically functionalized tip and report direct real-space visualization of the σ-hole, revealing its strong anisotropic charge distribution. The authors show that this technique, which relies entirely on electrostatic interactions, may be a powerful tool with which to study anisotropic atomic charge distributions. —YS Kelvin probe force microscopy directly images a σ-hole, revealing its strong anisotropic charge distribution. An anisotropic charge distribution on individual atoms, such as σ-holes, may strongly affect the material and structural properties of systems. However, the spatial resolution of such anisotropic charge distributions on an atom represents a long-standing experimental challenge. In particular, the existence of the σ-hole on halogen atoms has been demonstrated only indirectly through the determination of the crystal structures of organic molecules containing halogens or with theoretical calculations, consequently calling for its direct experimental visualization. We show that Kelvin probe force microscopy with a properly functionalized probe can image the anisotropic charge of the σ-hole and the quadrupolar charge of a carbon monoxide molecule. This opens a new way to characterize biological and chemical systems in which anisotropic atomic charges play a decisive role.