The Kinetics of Water Exchange and the Entropy of Activation Revisited

Abstract

Water exchange reactions have attracted considerable interest due in part to the prominent roles they play in controlling the surface reactivity of minerals and the availability of metal ions in the environment. Experimental investigations have sought to identify and characterize water exchange mechanisms by interpreting activation parameters derived from the temperature and pressure dependencies of the reaction rate constant, which is defined relative to the mean residence time, or lifetime, of a single molecule in the primary coordination shell. Using molecular dynamics simulations to revisit the kinetics of water exchange on Ca2+ and Li+, we show that the reaction period is not strictly equivalent to the residence time, and that conventional kinetic analyses that utilize rate constants defined in terms of the residence time underestimate the entropic contribution to the activation free energy when multiple ligands are exchangeable.