Molecular mechanics calculations and the metal ion selective extraction of lanthanoids

A molecular mechanics force field for lanthanoid complexes coordinated to organophosphate and aqua ligands is presented. The ligand-metal-ligand (L-M-L) bending interaction are modeled with 1,3-nonbonded interactions (point on a sphere approach) and a harmonic M-L stretching potential is used for the stretching modes. The force field parameters not yet available in the MOMEC force field were fitted to all relevant X-ray crystal structures available in the literature. There is good overall agreement between the observed and the calculated structural parameters. The calculations also indicate that a quantitative structure property relationship (QSPR) may be obtained, which relates the complexation strain energy difference between the lanthanoid cations and La(III) with their relative extractability. Thus, molecular mechanics is an efficient tool for the design of new organophosphate ligands with metal ion specific lanthanoid(III) extractability properties.