Water versus Acetonitrile Coordination to Uranyl. Effect of Chloride Ligands

Optimizations at the BLYP and B3LYP levels are reported for the mixed uranyl chloro/water/acetonitrile complexes [UO₂Cl_n(H₂O)_x(MeCN)_5–n−x]^2–n (n = 1–3) and [UO₂Cl_n(H₂O)_x(MeCN)_4–n−x]^2–n (n = 2–4), in both the gas phase and a polarizable continuum modeling acetonitrile. Car–Parrinello molecular dynamics (CPMD) simulations have been performed for [UO₂Cl₂(H₂O)­(MeCN)₂] in the gas phase and in a periodic box of liquid acetonitrile. According to population analyses and dipole moments evaluated from maximally localized Wannier function centers, uranium is less Lewis acidic in the neutral UO₂Cl₂ than in the UO₂²⁺ moiety. In the gas phase the latter binds acetonitrile ligands more strongly than water, whereas in acetonitrile solution, the trend is reversed due to cooperative polarization effects. In the polarizable continuum the chloro complexes have a slight energetic preference for water over acetonitrile ligands, but several mixed complexes are so close in free energy ΔG that they should exist in equilibrium, in accord with previous interpretations of EXAFS data in solution. The binding strengths of the fifth neutral ligands decrease with increasing chloride content, to the extent that the trichlorides should be formulated as four-coordinate [UO₂Cl₃L]⁻ (L = H₂O, MeCN). Limitations to their accuracy notwithstanding, density functional calculations can offer insights into the speciation of a complex uranyl system in solution, a key feature in the context of nuclear waste partitioning by complexant molecules.