An introductory overview of intermolecular forces

Molecular and supramolecular units

The firm establishment of John Dalton's atomic theory in the early nineteenth century ushered in a long period of preoccupation with the nature of molecules and the bond types responsible for molecule formation. By the mid twentieth century, a molecule was commonly defined in operational terms as “the smallest part of a chemical substance that can exist free in the gaseous state, with retention of the composition and chemical properties that are possessed by the gaseous material in bulk,” or in more theoretical terms as “an aggregate of atoms which is held together by relatively strong (valence) forces, and which therefore acts as a unit.”

Let us first seek to give a more rigorous and operational ab initio characterization of such “units.” The important physical idea underlying the above definitions is that of the connecting covalent bonds that link the nuclei. One can therefore recognize that a molecular unit is equivalently defined by the covalent-bond network that contiguously links the nuclei included in the unit. We can re-state the definition of a “molecular unit” in a way that emphasizes the electronic origin of molecular connectivity.