Charge Distribution in Ions
Electrostatic potential maps can be used to analyze
charge distributions in ions. Although an ion’s Lewis structure
necessarily contains a (formally) charged atom, the actual distribution
of charge may be quite different.
To begin, consider the potential maps of CH3–
and HO– shown below (color scale = –200 to
–120 kcal/mol). The Lewis structures of these ions assign
a –1 formal charge to C and O, respectively. The potential
maps support these charge assignments in that the most negative
potentials appear near the charged atoms. Lewis structures appear
to give accurate charge information for these ions.
maps of CH3– and HO–
(red = –200, dark blue = –120 kcal/mol)
Next, consider the potential maps of NH4+
and H3O+ shown below (color scale = +120 to
+200 kcal/mol). The Lewis structures of these ions assign a +1 formal
charge to N and O, respectively, but the potential maps do not support
these charge assignments. According to the maps, the most positive
potentials appear near the hydrogens. We must conclude that the
hydrogens carry the positive charge, and the charge is spread equally
over all of the hydrogens. Lewis structures do not give accurate
charge information for these ions.
maps of NH4+ and H3O+
(red = +120, dark blue = +200 kcal/mol)
Why do Lewis structures fail to get the charges right
in these cations? The answer to this in the assumption underlying
formal charge assignments: bonded atoms share electrons equally.
The O in H3O+ carries a +1 charge if it shares
bonding electrons equally with the neighboring H. We know, however,
that O is more electronegative than H (and O+ is more
electronegative still), so this assumption cannot be correct. Formal
charges can give the wrong picture.
So far we have focused on charge location, the red
region on an anion’s map and the blue region on a cation’s
map. What about the other colors/regions on these maps? Does the
red region near oxygen in H3O+ indicate that
oxygen is negative?
Before you take the bait and say, “yes, a red
O must be negatively charged”, consider this: a cation repels
a +1 charge no matter how the +1 charge approaches the cation. In
other words, all of the regions on the potential map of H3O+
are positive regardless of color. The red color on the map
corresponds to a positive potential, as do all of the other colors.
We cannot guess the charges of atoms near any of these other colors.
Potential maps are especially valuable tools for identifying
and characterizing ionic resonance hybrids. Partial bonding in an
ionic resonance hybrid can spread the ion’s charge over two
or more atoms. This type of charge is known as a delocalized
charge. Each charged atom carries only a portion of the delocalized
charge, and these atoms create noticeably smaller “local”
potentials than atoms with full charges.
One way to detect delocalized charge is to compare
the potential map of a suspected ionic resonance hybrid with the
map of an ion whose charge is concentrated on a single atom of the
same type (a charge that is concentrated on one atom is known as
a localized charge).
For example, we can tell if the negative charge in
is delocalized by comparing the potential map of this compound with
the potential map of a localized ion like CH3CH2CH2CH2O–
. To make this comparison meaningful, we use the same color scale
for both maps, and we compare charged atoms (oxygens) of similar
maps for oxygen anions
(color scale = –174 to –55
As you can see, the two oxygen atoms in the suspect
compound produce significantly smaller negative potentials (yellow-orange)
than the oxygen atom in the reference ion (red). In addition, both
oxygens in the suspect compound produce similar potentials. These
observations suggest that the charge in CH3CH2CH2C(=O)O–
is delocalized over both oxygens: