Stability of like and oppositely charged organic ion pairs in aqueous solution

To investigate the stability of ionic pairs in aqueous solution, polarizable continuum-model (PCM) ab initio molecular orbital calculations were carried out. Ion pairs formed from three organic ions, methylacetate, methylammonium, and guanidinium, were treated. The methylacetate ion pair forms a stable complex when the two methyl groups are in contact. When the charged groups of methylacetate and methylammonium are in contact, this pair does not form a stable complex in aqueous solution. When the methyl group of methylacetate is in contact with that of methylammonium, the ionic groups are far apart, and a stable complex is not formed, even though the complex involves a strong attractive electrostatic interaction. The guanidinium pair forms a stable complex in aqueous solution at all conformations investigated in this work even though a strong repulsive electrostatic interaction is present in this complex.