What characteristic defines a σ-hole?

A σ-hole is defined as a region of positive electrostatic potential that appears on the extension of a covalent bond, particularly in molecules where an atom such as boron or silicon, which has empty orbitals or low electronegativity, is involved. This area is characterized by diminished electron density around the atom's bonded regions, creating a "hole" in the electron cloud. The concept is primarily significant in understanding certain types of non-covalent interactions, such as halogen bonding or interactions involving main group elements.

The idea of a methyl group exhibiting diminished electron density relates to the broader context of how certain molecular geometries and bonding arrangements can influence local charge distributions in various molecules. In this case, a methyl group connected to an atom with a σ-hole would create a situation where the electron density is less prominent in the area surrounding that σ-hole.

Other options discussed, such as the formation of polar covalent bonds or increased electron density, do not align with the concept of a σ-hole, as these characteristics describe electron-rich regions or different types of bonding rather than the defining feature of a σ-hole itself. Similarly, aromatic stacking involves specific π interactions rather than the electrostatic characteristics that define a σ-hole.