Monte Carlo simulations of guanidinium acetate and methylammonium acetate ion pairs in water

Free energy profiles of guanidinium acetate and methylammonium acetate ion pairs in water were calculated using Monte Carlo simulations. Both systems display a single shallow free energy minimum at a distance corresponding to a contact ion pair. Hydrogen bonding between the ions is an important feature in the association. The weakness of association is attributed to weaker solute-solvent interaction as the ions approach one another, as reflected in the loss of some of the water molecules hydrating the ions. (C) 1997 Academic Press.