Equilibrium isotope effects as a probe of nonbonding attractions.

The equilibrium isotope effects (EIEs) in the capsule complex (12 + p-xylene + CCl4) were computed with two simple models that mimic two types of contacts, -CH3.Ar and -CH3.CCl4. By using the MP2/6-311++G(d,p) method, the calculated equilibrium constants are consistent with the reported experimental data. Harmonic frequency analyses indicate that vibrational frequency changes caused by nonbonding attractions lead to the observed EIE.