Intermolecular force

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Intermolecular forces are weak forces as compared to the intramolecular forces. For example, the covalent bond present within HCl molecules is much stronger then the forces present between the neighbouring molecules. These forces are believed to exist between molecules when they are sufficiently close to each other. They are believed to consist of four types:

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London dispersion forces

Otherwise known as quantum-induced instantaneous polarization or instantaneous dipole-induced dipole forces , the London dispersion force is caused by correlated movements of the electrons in interacting molecules. The electrons, which belong to different molecules, start "feeling" and avoiding each other at the short intermolecular distances, which is frequently described as formation of "instantaneous dipoles" that attract each other.

Debye (induced dipole) force

The induced dipole forces appear from the induction (also known as polarization), which is the attractive interaction between a permanent multipole on one molecule with an induced multipole on another. This interaction is called Debye force after Peter J.W. Debye.

Dipole-dipole interactions

Dipole-dipole interactions are electrostatic interactions of permanent dipoles in molecules. These interactions tend to align the molecules to increase the attraction (reducing potential energy). An example of dipole-dipole interactions can be seen in hydrogen chloride (HCl): The positive end of a polar molecule will attract the negative end of the other molecule and cause them to be arranged in a specific arrangement. Polar molecules have a net attraction between them. For example HCl and chloroform (CHCl3)

Dipole-dipole-interaction-in-HCl-2D.png

Keesom interactions (named after Willem Hendrik Keesom) are attractive interactions of dipoles that are Boltzmann-averaged over different rotational orientations of the dipoles. The energy of a Keesom interaction depends on the inverse sixth power of the distance, unlike the interaction energy of two spatially fixed dipoles, which depends on the inverse third power of the distance.

Often, molecules have dipolar groups within them, but have no overall dipole moment. This occurs if there is symmetry within the molecule, causing the dipoles to cancel each other out. This occurs in molecules such as tetrachloromethane. Note that the dipole-dipole interaction between two atoms is usually zero, because atoms rarely carry a permanent dipole, see atomic dipoles.

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