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WIREs Comput Mol Sci
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σ‐Holes

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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.

Figure 1.

Three views of the molecular electrostatic potential projected onto an isodensity (contour level 0.07 e Bohr−3) for CF3Br. (a) View showing all the important features of the MEP; (b) view form a point collinear with and opposite to the CBr bond showing the σ‐hole; (c) view from a point perpendicular to the CBr bond, showing the negative ‘equatorial’ belt. Data calculated at the PW91/PNP DFT level.

[ Normal View | Magnified View ]
Figure 2.

Schematic view of the fluorine atomic orbitals in CF3F.

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Figure 3.

Schematic view of the bromine atomic orbitals in CF3Br. This scheme ignores the fact that the four fluorines are equivalent to make it consistent with Figure 2.

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Figure 4.

Schematic view of the interactions possible for an organic halide RX. The color of the contour around the halogen indicates the sign of the MEP at the surface (red is positive, blue is negative).

[ Normal View | Magnified View ]
Figure 5.

View along one HO bond in water, showing the σ‐hole (Newman projection on the right). Data calculated at the PW91/PNP density functional theory level.

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Figure 6.

Schematic view of the principal polarization mechanism in bonds to hydrogen. The disk represents the electron density that we might assign to the hydrogen.

[ Normal View | Magnified View ]

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