Noncovalent interactions – nature, types, importance
Attractive forces exist among molecules. These forces, called intermolecular forces (noncovalent interactions, noncovalent forces or noncovalent bonds)
In contrast to intermolecular forces are intramolecular forces, or forces that hold atoms together in a molecule (chemical bonding involves intramolecular forces).
Intramolecular forces are responsible for the stability of individual molecules, whereas intermolecular forces are primarily responsible for the bulk properties of matter (for example, melting point and boiling point).
Generally, intermolecular forces are much weaker than intramolecular forces.
- 1. Dipole - Dipole forces. Many molecules have permanent dipoles that give rise to inter- molecular forces. The positive ends of polar molecules will be attracted to the negative ends of other molecules, but the similarly changed ends will repel each other.
The result is a net attractive force, called a dipole - dipole force (or dipole - dipole interaction), that holds the molecules together.
- 2. Ion - dipole forces. Coulomb’s law also explains ion - dipole forces, which occur between an ion (either a cation or an anion) and a dipole. The strength of this interaction depends onthe charge and size of the ion and the magnitude of the dipole. The charges on cations are generally more concentrated, since cations are usually smaller than anions. Therefore, for equal charges, a cation interacts more strongly with dipoles than an anion does.
Hydration is an example of ion - dipole interaction. In an aqueous NaCl solution, the Na+ and Clions are surrounded by water molecules, which have a large dipole moment (1.87 D). In this way the water molecules act as an electrical insulator that keeps the ions apart. This process ex- plains what happens when an ioni compound dissolves to water.
- 3. Dispersion Forces. So far only ionic species and molecules with dipole moments have been discussed. You may wonder what kind of attractive interaction exists among neutral and non- polar molecules.
a) Spherical charge distribution in a helium atom;
b) Distortion caused by the approach of a cation;
c) Distortion caused by the approach of a dipole.