The MP2 QM/MM MD simulation revealed that Cd²⁺ is coordinated with mostly six water molecules in the first hydration shell with numerous ligand exchange events during the simulation time of 200 ps.

Abstract

The spin component scale MP2/molecular mechanics molecular dynamics simulation investigated the hydration shell formation and hydrated Cd²⁺ dynamics in the water environment. At the first hydration shell, six water molecules with 2.27 Å for the average distance between water and Cd²⁺. Dynamical properties were analyzed by computing the water molecule's mean residence time (MRT) in its first and second hydration shells. The MRT of each shell was determined to be 31.8 and 1.92 ps, suggesting the strong influence of Cd²⁺ in the first hydration shell. The second shell was labile, with an average number of water molecules being 18. Despite the strong interaction between Cd²⁺ and water molecules in the first shell, the influence of ions in the second hydration shell remained weak.