STABILITY OF LIKE AND OPPOSITELY CHARGED ORGANIC ION-PAIRS IN AQUEOUS-SOLUTION

To investigate the stability of ionic pairs in aqueous solution, polar izable continuum-model (PCM) ab initio molecular orbital calculations were carried gut. Ton pairs formed from three organic ions, methylacet ate, methylammonium, and guanidinium, were treated. The methylacetate ion pair forms a stable complex when the two methyl groups are in cont act. When the charged groups of methylacetate and methylammonium are i n contact, this pair does not form a stable complex in aqueous solutio n. When the methyl group of methylacetate is in contact with that of m ethylammonium, the ionic groups are far apart, and a stable complex is not formed, even though the complex involves a strong attractive elec trostatic interaction. The guanidinium pair forms a stable complex in aqueous solution at all conformations investigated in this work even t hough a strong repulsive electrostatic interaction is present in this complex.