INTERMOLECULAR INTERACTIONS
Molecules can interact among one another in two ways. Reactive way results in formation or breaking of covalent bonds (chemical, covalent interaction). Non-reactive way results in formation of intermolecular complexes, when properties of the parent molecules remain unchanged to a considerable extent. Interactions of the second kind are called weak, non-covalent, non-chemical, physical interactions.
Existence of all liquids, complexes with transfer of charges, and complexes with hydrogen bonds is conditioned by weak interactions. In is almost impossible to find an important process in biology, where intermolecular forces do not play an important part. In particular, they determine consistency of the DNA and RNA.
Coulomb forces of interaction between electrons and nuclei of one molecule and electrons and nuclei of another molecule form the basis of intermolecular interactions. When the distance (R) between the molecules is large and exceeds significantly the size (l) of molecules themselves, when electron sheaths of molecules do not overlap, the forces of intermolecular interactions can be divided into three kinds, that is, electrostatic forces, polarizing (induction) forces, and dispersion forces.
When the distances are small, when R ≈ l, different kinds of intermolecular interactions can be distinguished only approximately. Anyway, in this case two more kinds of intermolecular interactions are distinguished. They are associated with overlapping of electron sheaths. They are: exchange interaction and interactions, which appear owing to transfer of electron charges.
The energy of intermolecular interaction can be viewed as the sum of two components, that is, attraction energy and repulsion energy. This energy is often approximately described by the Lennard-Jones formula.