Aqueous mineral reactions typically involve the exchange of a water molecule bound to a metal cation with an unbound water from the bulk solution. The dynamics of these ligand exchange reactions have been extensively studied with transition state theory, using first principles calculations to determine the activation parameters. In particular, the activation volume, ?V�, is calculated as the difference between the partial molar volumes of the transition state and the reactants. However, this approach has had limited success for aqueous beryllium(II) which has an experimental ?V� of -13.6 cm3 mol-1. Such a large negative activation volume implies an associative exchange mechanism with a metastable transition state complex which is not observed in the first principles calculations. In this study we use molecular dynamics simulations to determine the solvent exchange rate of aqueous beryllium(II) using the reactive flux method. The simulations employ an effective three-body potential developed for the beryllium(II)-water interaction from DFT and Hartree-Fock calculations on Be2+(H2O)n clusters using 6-311+G** basis sets. We calculate the effect of pressure on both the barrier height and the transmission coefficient for water exchange around the aqueous beryllium(II) ion. The simulations allow direct calculation of the activation volume as opposed to indirect estimates based on the difference in volumes between the transition state complex and reactants. The mechanisms and stereochemistry for the resulting exchange events are analyzed in terms of the five commonly discussed exchange classes: associative (A), dissociative (D), and interchange (I, Ia, Id).

Molecular dynamics simulations of the ligand exchange reaction for the aqueous beryllium(II) ion

248th National Meeting (2014)

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