Water‐Exchange Mechanism for Zinc(II), Cadmium(II), and Mercury(II) Ions in Water as Studied by Umbrella‐Sampling Molecular‐Dynamics Simulations

Umbrella‐sampling molecular‐dynamics simulations were performed to investigate the water‐exchange reactions of zinc(II), cadmium(II), and mercury(II) ions in aqueous solution. The dissociation of a coordinating water molecule to the MO distance at 3.34, 3.16, and 3.26 Å for Zn II , Cd II , and Hg II , respectively, leads the system to a transition state. For Zn II , the first hydration shell is occupied by five spectator water molecules in the transition state, indicating that the water‐exchange reaction proceeds via a dissociative mode of activation. In contrast, the number of spectator water molecules of 5.85 and 5.95 for Cd II and Hg II , respectively, suggests an associative exchange for these larger metal ions. The average MO distance of the spectator molecules is shortened by 0.06 Å for the dissociative exchange of Zn II , while it is elongated by 0.04 and 0.03 Å for Cd II and Hg II , respectively. The water‐exchange rate constants of 4.1×10 8 , 6.8×10 8 , and 1.8×10 9 s −1 are estimated for Zn II , Cd II , and Hg II , respectively, at 298 K in terms of the transition‐state theory based on the assumption of a transmission coefficient of unity.