Effects of Molecular Identity on Intermolecular Forces

Abstract

The free energy of interaction between two (nonoverlapping) molecules at finite temperature has been treated by quantum-statistical perturbation theory. This has been applied to the interaction between neutral molecules which may be rigid-rotor dipoles. It is shown that, in the absence of retardation, the leading term in the free-energy expansion varies as the inverse sixth power of the intermolecular distance r. This is true in spite of the existence of r−3 corrections to the energies of the pure states for pairs of identical molecules. The leading term in the free energy consists of two parts. One of these is associated with the adiabatic interaction of the two molecules and is expressible in terms of the isolated molecule susceptibilities; the other part is a correction arising from the redistribution of the molecules among the states and is not expressible in terms of the isolated molecule susceptibilities. The adiabatic part is always present and is identical with previously derived expressions for the van der Waals interaction. The second part, the resonance part, is present only when the molecules are identical; it vanishes when the temperature is zero and is negligible in the classical limit. It is concluded that the usual formulations for the dispersion (corresponding to T = 0), induction (T → ∞), and orientation (T → ∞) potentials are essentially correct.