Polar H–X bonds
The previous chapter showed how isodensity surfaces
can be used to get information about H-X bond polarity. This information
can be obtained much more easily from a potential map.
The following figure shows potential maps for the
same XHn molecules discussed in the previous chapter.
The maps are colored using the standard color scale, and the molecules
are oriented so that one H is always on the left side of the map.
As you can see, the potential near hydrogen is very negative
in LiH, close to neutral in BH3 and CH4, and
increasingly positive in NH3, H2O, and HF.
As we expect the potentials closely track the charge on hydrogen.
maps (standard color scale) of LiH, BH3, CH4,
NH3, H2O, HF
Although the potential near hydrogen varies systematically
with the electronegativity of X, the potential near X does not.
N and O are surrounded by more negative potentials than F, an unexpected
result since F is more electronegative.
This strange behavior is due to a combination of factors,
one of which is the non-spherical shape of X (in the previous chapter
we pointed out that all atoms are spherical, but they lose this
shape when they make bonds).
It is not hard to see how a non-spherical electron
density cloud might create unusual electrostatic potentials. Consider
the cartoon atoms shown below. If the nucleus is at the center of
a spherical cloud (pink region), the potential is zero everywhere
outside the atom (A). However, if the nucleus is set to one side
of the cloud (B), or the cloud is non-spherical (C), unusual potentials
Chemists refer to the regions of positive and negative
potential as electron-poor and electron-rich, respectively.
Thus, the red regions in the NH3 and H2O maps
appear to be electron-rich. As we will see later, these regions
contain nonbonding electron density.