Weak Interactions

Abstract

Interactions between atoms, ions and molecules leading to the formation (and breaking) of chemical bonds are of particular importance in chemistry. However, weak and very weak interactions between systems with closed electronic shells have already been studied for more than a century. No reaction in the chemical sense occurs between these systems under “normal” laboratory conditions. The existence of the liquid state and of molecular crystals is a consequence of the existence of attractive inter­molecular forces. The equilibrium distance between molecules forming associates in the liquid and solid phases is dependent on the compensation of the attractive and repulsive forces. Repulsive forces have been shown to decrease very sharply with increasing intermolecular distances (ap­proximately with the twelfth power of this distance); the increase in attractive forces, on the other hand, is not as rapid, as the distance decreases (it is roughly proportional to its sixth power). This has important consequences: whereas repulsive forces practically cease to be effective at distances greater than the length of chemical bonds, attractive forces are not negligible even at distances of about 0.4 nm; we therefore speak of long-range forces. A very important place is occupied by dispersion forces; in Section 17.2 the quantum mechanical derivation will therefore be discussed using a simple model. Expressions, resulting from the perturbation calculation, suitable for description of the intermolecular effects, will be introduced. Initially a few words will be said on the origin of Coulomb, induction and dispersion forces.