Do picrate anions attract each other in solution? Molecular dynamics simulations in water and in acetonitrile solutions

Molecular dynamics simulations performed on the Pic(-)...Pic(-) like-anion pair (Pic(-) = picrate = 2,4,6-trinitrophenoxide) show that its behavior is strongly dependent on the solvent. In water, the intimately stacked pair is stable, while in acetonitrile it dissociates. The stability of the stacked pair in water is confirmed by several methodological tests (choice of atomic charges; role of starting configuration; Ewald/no-Ewald) and by PMF (potentials of mean force) calculations. Simulations on (K(+)Pic(-))(2) and (K(+)Pic(-))(4) in water starting with unstacked anions lead to the formation of stacked dimers. The simulation of the dissolution of a (K(+)Pic(-))(27) fragment of the crystal also reveals a contrast in the behavior in water (formation of diluted stacks) compared to that in acetonitrile (formation of a "molten salt"). In the solid stale, stacking arrangements of Pic(-) anions are common but diverse in form, supporting this theoretical prediction that the stacking of two like Pic(-) anions is common and highly environment dependent. Dimerization can be viewed as the primary stage of crystal nucleation. The results also have implications concerning the behavior of Pic(-) at liquid-liquid interfaces in electrochemical or assisted ion-transfer processes.