Intermolecular (nonbonded) interactions
Molecules attract each other without forming chemical
bonds. When this attraction is stronger than the molecule’s
average kinetic energy, as it is for most molecules, the molecules
aggregate and form a condensed phase (liquid, solid, etc.).
Most intermolecular forces, or nonbonded forces,
are electrostatic. This means that the electrostatic potentials
around a molecule are a good measure of nonbonded “bond strength”
and we can use potential maps to study nonbonded interactions.
Although most nonbonded forces are electrostatic,
chemists like to distinguish between different types of forces according
to the types of charges involved. The strongest nonbonded forces
occur when both molecules contain permanently charged, or partially
charged, atoms capable of generating large electrostatic potentials.
Forces of this type are called ion-ion, ion-dipole,
and dipole-dipole forces (“dipole” means any
neutral molecule containing partially charged atoms capable
of generating large potentials).
Electrostatic forces are proportional to charge. Therefore,
ion-ion forces are normally the strongest in this group and dipole-dipole
forces are normally the weakest. There are large and unexpected
variations, however, because potential is also affected by atomic
radius and by neighboring atoms. To take all of these factors into
account, one should look at a potential map.
One unexpected variation has already been observed
in the potential maps of the halide anions. These maps show that
fluoride, the smallest halide, generates much stronger electrostatic
forces than iodide, even though both ions have the same charge . The potential map of CH3CH2CH2CO2–
also contains a surprise. The most negative potential is found between
the two oxygens, suggesting a cation would be attracted there most
strongly instead of being attracted to a particular oxygen .
Nonpolar molecules can also generate significant nonbonded
forces, but these forces are much weaker (on a per atom basis) than
the ones listed above. One important force acts between an ion (or
dipole) and a polarizable nonpolar molecule (this means a
molecule whose electron density cloud changes shape in response
to electrostatic forces). The approach of permanently charged ion
(or dipole) induces temporary changes in the electron density
cloud of the polarizable molecule. This leads to forces called ion-induced
dipole and dipole-induced dipole forces. Since these
forces rely on a change in the electron density cloud, they
cannot be assessed using potential maps.
Another type of force, known as the van der Waals
force or London dispersion force, is created whenever
two molecules approach, regardless of their polarity. Van der Waals
forces are weak electrostatic forces created by momentary changes
in a molecule’s charge distribution and they cannot be evaluated
using potential maps. They are the weakest of all nonbonded forces
(on a per atom basis), and they only operate over a very short distance
range, so they are often masked by other forces.
What causes van der Waals forces?
You can picture how van der Waals forces are created
by thinking about two nonbonded hydrogen atoms. Each atom
acts like a moving dipole, with the dipole pointing from the nucleus
to the moving electron. The two dipoles can create a weak attractive
force between the two atoms if the two electrons coordinate their
movements so that favorable dipole-dipole interactions are created.