Aqueous Solvation of SmI₂: A Born-Oppenheimer Molecular Dynamics Density Functional Theory Cluster Approach

We report the results of Born-Oppenheimer molecular dynamics (BOMD) simulations on the aqueous solvation of the SmI₂ molecule at room temperature using the cluster microsolvation approach including 32 water molecules. The electronic structure calculations were done using the M062X hybrid exchange-correlation functional in conjunction with the 6-31G** basis sets for oxygen and hydrogen. For the iodine and samarium atoms the Stuttgart-Köln relativistic effective-core potentials were utilized with their associated valence basis sets. Starting from the optimized geometry of SmI₂ embeded in the microsolvation environment, we find a swift substitution of the iodine ions by eight tightly bound water molecules around Sm(II). Through the Sm-O radial distribution function and the evolution of the Sm-O distances, the present study predicts a first rigid Sm(II) solvation shell from 2.6 to 3.4 Å, whose integration leads to a coordination number of 8.4 water molecules, and a second softer solvation sphere from 3.5 to ca. 6 Å. The Sm(II)-O radial distribution function is in excellent agreement with that reported for Sr²⁺ from EXAFS studies, a fact that can be explained because Sr²⁺ and Sm²⁺ have almost identical ionic radii (ca. 1.26 Å) and coordination numbers: 8 for Sr²⁺ and 8.4 for Sm²⁺. The theoretical EXAFS spectrum was obtained from the BOMD trajectory and is discussed in the light of the experimental spectra for Sm(III). Once microsolvation is achieved, no water exchange events were found to occur around Sm²⁺, in agreement with the experimental data for Eu²⁺ (which has a nearly identical charge-to-ionic radius relation as Sm²⁺), where the mean residence time of a water molecule in [Eu(H₂O)₈]²⁺ is known to be ca. 230 ps.