PNNL

Abstract

The rates and mechanisms of water exchange around two aqueous ions, namely, Na+ and Fe2+, have been determined using transition path sampling. In particular, the pressure dependence of the water exchange rates was computed to determine activation volumes. A common approach for calculating water exchange rates, the reactive flux method, was also employed and the two methods were compared. The water exchange rate around Na+ is fast enough to be calculated by direct molecular dynamics simulations, thus providing a reference for comparison. The transition path sampling approach yielded more accurate rates, although both approaches predicted activation volumes of +2.6 cm3·mol-1, in agreement with the direct simulation results. The only previously determined activation volume for Na+ is from a theoretical estimation (Spångberg, D.; Wojcik, M.; Hermansson, K. Chem. Phys. Lett. 1997, 276, 114-121) and differs in sign and magnitude from that calculated in this work. We show that this is due to an overestimation of the sodium hydration energy in the previous model. For water exchange around Fe2+, transition path sampling predicts an activation volume of +3.8 cm3.mol-1, in excellent agreement with available experimental data. The reactive flux approach, however, failed to identify the transition state and predicted the opposite pressure dependence of the rate as a result. Analysis of the reactive trajectories obtained with the transition path sampling approach suggests that the Fe2+ exchange reaction takes place via an associative interchange mechanism, which goes against the conventional mechanistic interpretation of a positive activation volume. Collectively, considerable insight obtains not only for the exchange rates and mechanisms for Na+ and Fe2+, but also for identifying the most robust modeling strategy for these purposes.