COMPLEXING OF THE AMMONIUM ION BY POLYETHERS - COMPARATIVE COMPLEXINGTHERMOCHEMISTRY OF AMMONIUM, HYDRONIUM, AND ALKALI CATIONS

The binding energies of NH4+ to polyethers, and for comparison, to ace tone molecules, were measured by pulsed high-pressure mass spectrometr y. The binding energies in the polydentate complexes increase with inc reasing ligand size and number of available oxygen groups. However, th e binding energies are smaller than in complexes with free Me(2)CO mol ecules, reflecting the geometrical constraints in the polydentate liga nds. The binding energy of NH4+ in various polydentate complexes is si milar to that of K+ and smaller than that of H3O+ by 105 +/- 12 kJ/mol (25 +/- 3 kcal/mol). Based on the H3O+ to crown ether binding energie s, these relations can be used to estimate the binding energy of NH4and K+ to 15-crown-5 of 248 +/- 12 kl/mol (59 +/- 3 kcal/mol) and to 1 8-crown-6 of 296 +/- 12 kJ/mol (71 +/- 3 kcal/mol). The order of bindi ng energies of oxygen-containing ligands to H3O+ > Na+ > NH4+ approxim ate to K+ is reproduced by ab initio calculations on complexes with Me OCH(2)CH(2)OMe. The amount of calculated charge transfer to the ligand s follows the order of binding energies. The difference between the bi nding energies of H3O+ and NH4+ to oxygen-containing ligands in the ga s-phase complexes is similar to the condensed-phase aqueous heats of s olvation of these ions.