The Internal Molecular Potential Between the Substituent Groups in a Benzene Ring as Derived from the Heats of Combustion

It is shown that differences in the observed heats of combustion of isomeric benzene derivatives can be interpreted as the internal molecular potential existing between their substituent groups. A like interpretation can be given for the differences between the values observed for the heats of combustion of certain nonisomeric benzene derivatives and those calculated by the rule of additivity. This internal potential, to which the attractive and repulsive forces between the groups are due, results from the electrostatic potential of the group moments (dipole effect), the polarization of the substituents and of the ring (induction effect), the dispersion effect, and from steric hindrance. We have, therefore, a new and direct method of measuring the internal potential, which determines both the internal motion of groups within an organic molecule and its most stable configuration. The values thus measured are in good agreement with values theoretically evaluated from the above intermolecular (van der Waals) forces. From the data derived by this method we conclude in the case of o-xylene that valence angles of 120° between the C—C${\mathrm{H}}_3$ bond and the aromatic C—C bonds are extremely stable, for the energy required to distort these angles through 10° is greater than 2 K cal/mole. We find, also, very restricted rotation for the butane molecule, from which it follows that saturated aliphatic hydrocarbons in the gaseous state tend to form zigzag chains. Such restricted rotation is found for the ether molecule as well.