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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 [ SHOW ME THESE MAPS AGAIN]. 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 [ SHOW ME THIS MAP AGAIN]. 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.
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