HOST-GUEST SUPRAMOLECULAR CHEMISTRY .34. THE INCREMENTAL APPROACH TO NONCOVALENT INTERACTIONS - COULOMB AND VANDERWAALS EFFECTS IN ORGANIC ION-PAIRS

The equilibria of 10 ion pairs with aromatic units in one or both ions are determined in water by UV/vis and partially by NMR and conductivity experiments, showing in most cases satisfactory agreement. UV/vis measurements at different wavelengths usually exhibit weaker 1:2, in addition to 1:1, complexes. On the basis of a generalized principle of additive binding increments, the results from about 40 ion pairs-many from the literature-show a surprisingly constant Coulomb increment of DELTA-G = 5 +/- 1 kJ/mol per single salt bridge. The van der Waals forces in the aromatic ion pairs also lead to a linear increase of the complexation strength with the number of aryl groups in the complexes. A corresponding two-term correlation yields, in agreement with other studies, a vdW increment of 1-2 kJ/mol per aryl unit, which is shown to be due to ion-induced dipoles in the pi-system. Conformations were partially modeled with force field calculations (CHARMm); they are in qualitative agreement with complexation-induced shifts obtained from NMR titrations. The CIS-NMR values explain deviations from the correlations for three ion pairs due to steric hindrance and contradict edge-to-face arrangements of benzene rings. Charge-transfer (CT) interactions, although visible in typical UV/vis absorptions, do not contribute significantly (<0.5 kJ/mol) to the associations. Selected measurements with less polar solvents added show the expected binding increase but no correlation with the bulk dielectric constant.