σ-Holes

σ‐Holes are regions of positive molecular electrostatic potential collinear with and opposite to covalent bonds to atoms of Groups IV–VII. They are responsible for many noncovalent bonding interactions, such as halogen bonding. σ‐Holes make ‘negatively charged’ atoms act as if they were ‘positively charged’. The existence of σ‐hole bonding emphasizes what has been called ‘the fallacy of net atomic charges’, which means that many covalently bonded atoms cannot be represented adequately by a single charge because they look negative from some directions and positive from others. Hydrogen bonding can also be regarded as a special case of σ‐hole bonding, although in this case the origin of the σ‐hole is rationalized differently than in the heavier elements. Phenomena such as the directionality of hydrogen bonds and ‘blue‐shifted’ hydrogen bonds can be explained very simply using the σ‐hole concept.