Effective Intermolecular Pair Potentials in Nonpolar Media

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Abstract
Using third‐order perturbation theory, effective London dispersion forces between two molecules in a medium are evaluated. The reduction in the pair interaction is calculated for spherical and polymeric bodies at various separations. For nearest neighbors in pure liquids, pair potentials decrease by 1.8% in helium, and 32% in carbon tetrachloride. The contribution to the total energy of the liquid is one‐third of this. The dispersion forces between lateral base pairs in a single DNA double helix decrease by about 28% due to third‐ and fourth‐order perturbations involving the four neighboring bases of each pair. Solvent effects cause an additional decrease of 14% (in water) to 18% (in formamide). The guanine‐cytosine and adenine‐thymine pairs change similarly. In general between segments of any two typical linear polymer chains (ionization potential of 10 V), dispersion forces decrease between 17% and 22% in water. In the theoretical sections, operator and diagrammatic techniques are presented which simplify the calculation of higher‐order effects. We also show that dispersion forces do not decrease as the square of the high‐frequency dielectric constant of the solvent, since this does not include the attractive many‐body correlations which reduce the effect of the medium on the pair potential.