Intermolecular force

A force between molecules, including effects of electrostatics, polarizability and London dispersion forces. Hydrogen bonds describe a directional interaction pattern rather than an additional fundamental force to add independently to all those components.

An electrostatic interaction involving an electric quadrupole moment, the second-order term in a multipole expansion. About a specified origin, use the conventionHere $q_a$ is an electric charge, ${\mathbf{r}}_a$ its position, and ${\delta }_{ij}$ the Kronecker delta. Far from the electric charges, the quadrupole contribution to electrostatic potential iswhere ${\epsilon }_0$ is vacuum permittivity. Thus a point electric charge has quadrupole interaction potential energy proportional to $r^{-3}$ at fixed orientation. A second electric quadrupole moment couples to second derivatives of this electrostatic potential, giving quadrupole–quadrupole potential energy proportional to $r^{-5}$ at fixed orientations. These are far-field terms, rather than the whole intermolecular force at close contact.

Benzene has no permanent electric dipole moment by symmetry, but its electric quadrupole moment need not vanish. Its electrostatic interaction therefore depends on the relative molecular orientations even without permanent electric dipole moments. Published electric quadrupole moment conventions can differ by a factor of two; the tensor convention above must be matched before inserting tabulated values.^[ref]

Two benzene molecules associated by dispersion and electrostatic interactions. T-shaped and parallel-displaced geometries are important low-energy arrangements; a single permanent dipole–dipole model is inadequate.

Short-range repulsion associated with the antisymmetry requirement for overlapping electronic states, related to the Pauli exclusion principle. It competes with attractive electrostatic and London dispersion forces at small intermolecular separations.

An existing charge distribution polarizes a neighbouring molecule, changing its electron density and interaction energy. The resulting interaction depends on the molecular polarizability and local electric field.