Computational and Experimental Evidence of Emergent Equilibrium Isotope Effects in Anion Receptor Complexes

The measurement of a deuterium equilibrium isotope effect (EIE) for the aryl CH···Cl^- interaction of anion receptor 1H/1D is reported. Computations corroborate the results of solution measurements for a small, normal EIE in the full complex (K_a^H/K_a^D = 1.019 ± 0.010). Interestingly, isotope effects involving fragments of the anion receptor (urea, aryl ring, etc.) are predicted to produce an inverse effect. This points to an unusual and subtle structural organization effect of the anion receptor complex that changes the nature of the combined interactions to a normal isotope effect. The reversal of EIE values suggests that overall architecture of the anion receptor can dramatically impact the nature of bonding in these complexes.