The London equation for dispersion energy between nonpolar molecules is used to calculate boiling point as a function of polarizability, ionization energy, and size, provided the shape of the molecule is taken into account. For spherical molecules, density is proportional to molecular weight M divided by the cube of the radius. For the zero-point energy hv0, the ionization potential I is used. For polarizability, the molar refraction RM is used and the boiling point Tb, is assumed proportional to the energy of vaporization (Trouton's rule). The result is that Tb1/2 is proportional to RMI1/2/ Vb, where Vb is the molar volume at Tb. The data for rare gases and for group 4A tetrahalides give good straight lines. The method is then extended to cylindrical and flat molecules, in which case Tb1/2 is proportional to RMI1/2L3/2/Vb3/2, where L is the length of the molecule, and RMI1/2A3/Vb3, where A is the area of the molecule. Good straight lines are obtained for normal hydrocarbons (cylindrical) and aromatic compounds (flat). © 1979 American Chemical Society.
