FLUORIDE-FLUORIDE ASSOCIATION IN WATER FROM MOLECULAR-DYNAMICS SIMULATIONS

The potential of mean force (PMF) for an F--F- ion pair at room temperature was calculated using a rigid simple point charge (SPC/E) water model and molecular dynamics techniques. The PMF displayed a minimum at an ion-ion separation of 4.3 angstrom with a barrier height of approximately 1.5 kcal/mol. The barrier height is a result, in part, of the presence of two water molecules that simultaneously hydrogen bond to both fluoride ions. To investigate the effect of the ionic charge on the hydration properties of an ion pair, we also computed the PMF for the F+-F+ ion pair. Our calculations indicated that the barrier height of this PMF is less significant than the corresponding barrier height observed in the computed PMF of the F--F- ion pair. This result is reasonable because of the absence of the highly favorable hydrogen bonding between water molecules and the ion pair. When comparing the present calculation with those reported in previous works on similar systems using integral equation techniques, our simulated F--F- PMF displays more structure and the minimum well depth is shallower; however, the barrier to dissociation is comparable.