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Speciation of La(III) Chloride Complexes in Water and Acetonitrile: A Density Functional Study

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journal contribution
posted on 17.12.2012, 00:00 by Michael Bühl, Nicolas Sieffert, Aurélie Partouche, Alain Chaumont, Georges Wipff
Car–Parrinello molecular dynamics (CMPD) simulations and static computations are reported at the BLYP level of density functional theory (DFT) for mixed [LaClx(H2O)y(MeCN)z]3–x complexes in aqueous and nonaqueous solution (acetonitrile). Both methodologies predict coordination numbers (i.e., x + y + z) that are successively lower than nine as the Cl content increases from x = 0 to 3. While the static DFT method with implicit solvation through a polarizable continuum model overestimates the binding strength of chloride and erroneously predicts [LaCl2(H2O)5]+ as global free-energy minimum, constrained CPMD simulations with explicit solvent and thermodynamic integration reproduce the weak binding of chloride in water reasonably well. Special attention is called to the dipole moments of coordinated water molecules as function of coligands and solvent, evaluated through maximally localized Wannier function centers along the CPMD trajectories. Cooperative polarization of these water ligands by the metal cation and the surrounding solvent is remarkably sensitive to fluctuations of the La–O distances and, to a lesser extent, on the La-water tilt angles. The mean dipole moment of water ligands is rather insensitive to the other coligands, oscillating around 3.2 D, 3.5 D, and 3.3 D in MeCN, water, and [dmim]­Cl solution, respectively, the latter being an archetypical ionic liquid.