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) |
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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) |
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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) |
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