Page 311 - Instant notes

Weak intermolecular interactions 297

through the effect of an electric field. Such a molecule is polarizable and the resulting
dipole is referred to as an induced dipole moment. The strength of the induced dipole is
proportional to the strength of the electric field, E:

α is referred to as the polarizability, ε is the dielectric constant, and N′ the number of
3
dipole moments per cm . Where the induced dipole moment arises as a result of a
molecule, of polarizability α 1, with a permanent electric dipole, µ 2, an attractive potential
is set up between the two molecules whose strength is given by:

Where both molecules have a permanent dipole, the energy of the interaction is given by:

London dispersion interaction

Electrostatic interactions may occur between atoms and molecules which do not contain
polar bonds. Such forces are responsible for the existence of solid and liquid helium and
methane, for example, and are the result of the polarizability of the molecule. At any one
instant, the electron density around a molecule need not be spherically distributed, and
this asymmetry has a dipole associated with it. This spontaneous electrical dipole may in
turn induce an electric dipole moment in a second molecule, to give an attractive
potential (Fig. 1). This phenomenon is referred to as the London interaction or the
dispersion interaction.