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

Stability constants of potassium, sodium, and benzylammonium salts wit h 18C5 are determined in water, methanol, and acetonitrile by potentio metric titrations. The corresponding free energies Delta G agree withi n the error with those obtained from calorimetric titrations. In compa rison to 18C6 the Delta G values are lower by 14 to 16 kJ/mol, with me thanol or acetonitrile as solvent and K+ or benzylammonium salts. Diff erences in the calorimetrically determined binding enthalpies Delta H between 18C6 and 18C5 are usually even larger. In water, however, the Delta G differences between the 18C5 and 18C6 complexes become almost negligible. The D(3)d-like conformation of such crown ethers can be ev aluated for the first time by NOE methods using the less symmmetrical 18C5. The NMR data indicate also the absence of significant conformati onal changes upon complexation, in line with molecular mechanics calcu lations (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 point ing toward the cavity, leading to larger K+.... O distances. The CHARM m calculated gas phase energy difference between the K+ crown complexe s of 26 kJ/mol agrees approximately with experimental differences.