DipolDipolWechselwirkungen

Dipole–dipole interaction, or Dipol-Dipol-Wechselwirkung in German, is the electrostatic interaction between two molecules that possess permanent electric dipole moments. When polar molecules approach, the partially positive end of one molecule is attracted to the partially negative end of the other, while opposite orientations can lead to repulsion. The interaction energy scales approximately as 1/r^3, where r is the distance between the dipoles, and it depends on the relative orientation of the dipoles with respect to the line joining them. The end-to-end configuration is typically attractive, whereas side-by-side perpendicular orientations tend to be less favorable.

The interaction can be decomposed into several contributions: the Keesom interaction for permanent–permanent dipoles, the Debye interaction for permanent–induced dipoles, and London dispersion for induced–induced dipoles. In molecular assemblies, dipole–dipole forces influence liquid structure, phase behavior, solvation, and properties such as boiling points and dielectric constants. In gases, these forces are weaker due to larger intermolecular distances, but they still affect transport properties and spectral characteristics.

Magnitude and range depend on the dipole moments and on distance, becoming negligible at large separations. Typical interaction energies at close contact are on the order of a few kilojoules per mole, diminishing rapidly with distance and being overcome by thermal motion at higher temperatures or by stronger competing interactions such as hydrogen bonding in complex systems.

Examples of polar molecules where dipole–dipole interactions are significant include water, hydrogen chloride, hydrogen fluoride, and many organic solvents. These forces play a key role in solvation, molecular recognition, and the behavior of liquids and soft matter.