M3+ lanthanide cation solvation by acetonitrile: The role of cation size, counterions, and polarization effects investigated by molecular dynamics and quantum mechanical simulations

We report a molecular dynamics (MD) study on M3+ lanthanide (La3+, Eu3+ and Yb3+) cations in dry acetonitrile solution and in M(MeCN)(n)(3+) clusters (n = 1-15) where two classical force-field representations of the cations a re compared, in conjunction with the OPLS model of acetonitrile. It is show n that a set of van der Waals cation parameters (set2) fitted from free ene rgies of hydration overestimates the cation coordination numbers (CNs). Ano ther set of parameters (set1), where the size of cations is scaled down by 2(1/6) (using the sigma van der Waals value for R*) yields better results. Quantum mechanical calculations performed on M(MeCN)(n)(3+) aggregates (n = 1-9) demonstrate the importance of charge-transfer and polarization effect s. They confirm the preferred coordination number of eight for Yb3+, the Yb (MeCN)(8+1)(3+) species with one MeCN molecule in the outer coordination sp here being somewhat more stable than Yb(MeCN)(9)(3+) D-3h. Adding a polariz ation term for the 1-6-12 OPLS acetonitrile to the force field (set2+pol) i ndeed markedly improves the calculated CNs. In all MD simulations, a remark able dynamic feature is observed in the first solvation shell where the lif etime of acetonitrile molecules increases from Yb3+ to La3+, that is, inver sely to the cation-solvent interaction energies and to the aqueous phase be havior. Rare-earth salts with ClO4- and F3CSO3- anions and the question of ion binding selectivity by L ligands (formation of ML33+ complexes, where L is a pyridine-dicarboxamide ligand) in acetonitrile solution are investiga ted by free-energy perturbation simulations, comparing the set1, set2, and set2+pol models. It is found that selectivities are markedly determined by the change in solvation-free energies of the uncomplexed cations, with pron ounced counterion effects. The two simplest models (set1 or set2 without po larization) predict the correct order of complexation (Yb3+ > Eu3+ > La3+), whereas addition of polarization contribution leads to the inverse order, because of overestimation of the cation-anion interactions in the salt solu tions.