The Physical Nature of van der Waals Interactions

Let us deal with two mutually interacting species and let us call them subsystems. A chemical bond is formed between the subsystems when their electron clouds overlap. In general, however, overlap is not necessary in molecular interactions (molecules interact even at very large distance where overlap is zero) and the reason for mutual attraction must be sought in the electrical properties of the subsystems.

The nonuniform distribution of charge throughout the molecule gives rise to an electric multipole; this may be a dipole, quadrupole, octapole, etc. Interaction of these multipoles leads to the most prominent energy contribution, the electrostatic (cou-

lombic) contribution. An induction contribution appears between molecules with permanent multipoles and nonpolar molecules. In this case the attraction originates from the electrostatic interaction between permanent and induced multipoles. And, finally, also the dispersion interaction taking place between nonpolar molecules originates from the electrostatic interaction between a time-variable multipole and an induced multipole.

In addition to the attractive contributions considered, there must be another force preventing subsystems from approaching one another too closely. This force, termed exchange-repulsion, originates similarly as chemical covalent forces, from the overlap of electron clouds.

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