Aqueous microsolvation of CdCl₂: density functional theory and Born-Oppenheimer molecular dynamics studies

We report a systematic study of aqueous microsolvation of CdCl2. The optimized structures and binding energies of the CdCl2-(H2O)n clusters with n = 1-24 have been computed at the B3PW91/6-31G** level. The solvation patterns obtained at the DFT level are verified at the MP2/AVTZ level for n < 6. Unlike HgCl2-(H2O)n case, where there are at most three Hg-O(w) orbital interactions, Cd also establishes four equatorial orbital interactions with water for n > 6 leading to a planar square bipyramid hexacoordination around Cd. The first solvation shell is fully attained with 12 water molecules. At the same level of theory the water binding energies are much larger than those previously found for HgCl2 due to the stronger Cd-O(w) interactions arising from the smaller core of Cd. For the largest system studied, CdCl2-(H2O)24, both penta- and hexa-coordination stable patterns around Cd are found. However, Born-Opphenheimer molecular dynamics simulations starting from these optimized geometries at 700 K reveal the greater stability of the Cd-pentacoordinated species, where a CdCl2-(H2O)3 trigonal bipyramid effective solute appears. The Cd-O(water) radial distribution function shows a bimodal distribution with two maxima at 2.4 Å and 4.2 Å, revealing the different coordination spheres, even with such a small number of solvating water molecules.