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.