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Lone pairs

Lone pairs compose a beautiful example of the close relationship between the chemical notions and the density deformations.  In the upper figures of the following table, the Lewis structure ofammonia (left plate) is compared with the deformations of nitrogen (middle plate) and the total molecular deformations (right plate)  for contour values of ±0.05au. As it can be seen, beside the three charge accumulations corresponding to the sigma N--H bonds, there is a large, and nearly spherical, charge accumulation in the opposite part of the N atom. This structure is reproduced practically unchanged in amines. A diferent type of charge accumulation appears in nitriles, as it can be observed in case of ethanonitrile (lower figures).

Lewis structure
Nitrogen deformation (±0.05au) Molecular deformation (±0.05au)
Lewis structure NH3 Nitrogen in NH3 Ammonia Contour 0.05
Lewis structure CH3CN
N in CH3CN Contours (+,-)0.05
CH3CN Contours (+,-)0.05

Click on the figures to enlarge, click here for further pictures.

Another interesting case is the atom of oxygen. Depending on the molecular environment, different types of deformations associated to lone pairs can be observed. The following figures show the Lewis structures and the deformations in water,formaldehyde and cabon monoxide for a contour value of ±0.05.

Lewis structure Oxygen deformation (±0.05au) Molecular deformation (±0.05au)
Lewis structure H2O
O in H2O H2O Contour 0.05
Lewis struct H2CO
O in formaldehyde Formaldehyde
Lewis structure CO
O in CO CO

Halogens have three lone pairs, with a deformation pattern different from the previous ones. The following pictures illustrate this type of deformation in fluoromethane and fluorobenzene.

Lewis structure Oxygen deformation (±0.05au) Molecular deformation (±0.05au)
Lewis structure CH3F
Deformation of F Contour 0.05
Deformations of CH3F Contour 0.05
Lewis structure C6H5F
Deformations of F in C6H5F Contours (+,-)0.05
Deformations of C6H5F Contours (+,-)0.05