The sizes of molecules - Revisited

The size of molecules, be it their volume, surface area or linear extent, is an important quantity in many fields of chemistry. Molecules are often tacitly or explicitly treated as if they were spheres, so that a diameter can be assigned to them. However, the molecules of many kinds of substances are rather cylindrical, being rod- or disk-shaped, so that two linear dimensions are needed for their description. The collision diameters obtainable from experimental data on gases and vapors or diameters obtained from the molar volume or the application of an expression for the interaction potential in liquids, e.g. the Lennard-Jones potential, are explored as ways to yield information on molecular sizes. The ratio of the van der Waals volume and surface area is related to a simple expression derived solely from the composition of the molecules for various types of molecular shapes. These approaches were applied to a database of 350 mainly liquid (but also some gaseous and solid) organic (and a few inorganic) substances.