Conformation of 18-Crown-5 and Its Influence on Complexation with Alkali and Ammonium Cations: Why 18-Crown-5 Binds More Than 1000 Times Weaker Than 18C6

Abstract

Stability constants of potassium, sodium, and benzylammonium salts with 18C5 are determined in water, methanol, and acetonitrile by potentiometric titrations. The corresponding free energies DeltaG agree within the error with those obtained from calorimetric titrations. In comparison to 18C6 the DeltaG values are lower by 14 to 16 kJ/mol, with methanol or acetonitrile as solvent and K(+) or benzylammonium salts. Differences in the calorimetrically determined binding enthalpies DeltaH between 18C6 and 18C5 are usually even larger. In water, however, the DeltaG differences between the 18C5 and 18C6 complexes become almost negligible. The D(3)d-like conformation of such crown ethers can be evaluated for the first time by NOE methods using the less symmmetrical 18C5. The NMR data indicate also the absence of significant conformational changes upon complexation, in line with molecular mechanics calculations (CHARMm). These show that the low binding constants of K(+) with 18C5 are due to the expulsion of the cation due to one C-H bond pointing toward the cavity, leading to larger K(+).O distances. The CHARMm calculated gas phase energy difference between the K(+) crown complexes of 26 kJ/mol agrees approximately with experimental differences.