Dispersion

Dispersion is a type of intermolecular attraction that exists between all atoms and molecules, even those that lack permanent dipole moments or the ability to form hydrogen bonds. This force plays a crucial role in non-covalent binding and becomes the dominant attractive force for molecules containing more than five atoms.

Quantum mechanical origin

The dispersion force originates from the quantum mechanical nature of electrons. In atoms and molecules, electrons are constantly in motion, and their positions are described by wavefunctions. These fluctuations in the electron distribution can create instantaneous dipoles, even in non-polar molecules. An instantaneous dipole occurs when the electron distribution momentarily becomes asymmetric, with more electrons on one side of the molecule than the other. Although the average dipole moment of a non-polar molecule is zero, these instantaneous dipoles can still interact with neighboring molecules.

When an instantaneous dipole forms in one molecule, it can induce a dipole in a nearby molecule through electrostatic interactions. The induced dipole is aligned in such a way that it is attracted to the original instantaneous dipole. This attraction between the instantaneous dipole and the induced dipole gives rise to the dispersion force. The interaction between the molecules leads to a correlation between the motion of their electrons, resulting in a lower overall energy state.

Effect on non-covalent binding

Dispersion forces contribute to the overall stability of non-covalent binding between molecules. In the absence of other strong intermolecular forces, such as dipole-dipole interactions or hydrogen bonding, dispersion forces can be the primary source of attraction.

The strength of the dispersion force depends on several factors, including the size and polarizability of the molecules involved. Larger molecules and those with more easily polarizable electron clouds tend to have stronger dispersion forces. This is because larger molecules have more electrons that can participate in the formation of instantaneous dipoles, and more polarizable electron clouds are more easily distorted by nearby instantaneous dipoles.

As the distance between molecules increases, the magnitude of the dispersion force decreases. At very large separations, the finite speed of light becomes a factor in the interaction between molecules. The correlation between the fluctuations in the electron distributions of the two molecules becomes less effective because the information about a fluctuation in one molecule takes time to reach the other molecule. By the time the second molecule responds and the information about its response reaches the first molecule, the electrons have already moved, and the fluctuations are no longer in phase. This retardation effect leads to a weaker dispersion force at very large separations.